DOT HS 812 159 June 2015

Review and Analysis of Potential 

Safety Impacts of and Regulatory 

Barriers to Fuel Efficiency 

Technologies and Alternative Fuels 

In Medium- and Heavy-Duty Vehicles
DISCLAIMER

This publication is distributed by the U.S. Department of Transportation, National 

Highway Traffic Safety Administration, in the interest of information exchange. 

The opinions, findings, and conclusions expressed in this publication are those of 

the authors and not necessarily those of the Department of Transportation or the 

National Highway Traffic Safety Administration. The content is not intended to be 

used for determination of federal grant programs. The United States Government 

assumes no liability for its contents or use thereof. If trade or manufacturers’ 

names or products are mentioned, it is because they are considered essential to 

the object of the publication and should not be construed as an endorsement. The 

United States Government does not endorse products or manufacturers.

Suggested APA Format Citation:

Brecher, A., Epstein, A. K., & Breck, A. (2015, June). Review and analysis of 

potential safety impacts of and regulatory barriers to fuel efficiency technologies 

and alternative fuels in medium- and heavy-duty vehicles. (Report No. DOT HS 

812 159). Washington, DC: National Highway Traffic Safety Administration.
i

TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

DOT HS 812 159

2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle

Review and Analysis of Potential Safety Impacts of and Regulatory 

Barriers to Fuel Efficiency Technologies and Alternative Fuels in 

Medium- and Heavy-Duty Vehicles

5. Report Date

June 2015

6. Performing Organization Code

7. Author(s)

Aviva Brecher, Alexander K. Epstein, & Andrew Breck

8. Performing Organization Report No.

DOT-Volpe-NHTSA-14-

01

9. Performing Organization Name and Address

U.S. Department of Transportation 

Research and Innovative Technology Administration 

John A. Volpe National Transportation Systems Center

Cambridge, MA 02142

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

Intra-Agency Agreement

HS9AA1

12. Sponsoring Agency Name and Address

National Highway Traffic Safety Administration

1200 New Jersey Avenue SE.

Washington, DC 20590

13. Type of Report and Period Covered

Technical Report

14. Sponsoring Agency Code

NHTSA/NVS-132 

15. Supplementary Notes

Prepared for: 

James MacIsaac - Contracting Officer’s Representative (COR)

NHTSA Fuel Economy Division

16. Abstract 

This report summarizes a safety analysis of medium- and heavy-duty vehicles (MD/HDVs) equipped 

with fuel efficiency (FE) technologies and/or using alternative fuels (natural gas-CNG and LNG, 

propane, biodiesel and power train electrification). The study included a comprehensive literature 

review, complemented with inputs from subject matter experts (SMEs), and a scenario-based hazard 

analysis. Specific FE technologies examined include: Intelligent Transportation Systems (ITS) and 

telematics, speed limiters, idle reduction devices, tire technologies (single-wide tires, tire pressure 

monitoring systems-TPMS and Automated Tire Inflation Systems-ATIS), aerodynamic components, 

long combination vehicles (LCVs) and lightweighting materials. Federal and State safety regulations, 

and voluntary technical standards affecting MD/HDV fleet adoption of FE technologies and alternative 

fuels were discussed, and potential regulatory barriers identified. The findings suggest that the potential 

safety hazards identified can be prevented or mitigated by complying with safety regulations and 

voluntary standards and industry best practices. The study did not identify any major regulatory 

barriers to rapid adoption of FE technologies and alternative fuels by the MD/HDV fleet.

Note: This report was subjected to external peer review per OMB guidelines for a Highly Influential Scientific 

Assessment (HISA). Materials from the peer review process are publicly available in accompanying documents.

17. Key Words

Safety impacts, fuel Efficiency technologies; alternative 

fuels; Medium and Heavy-duty Vehicles (MD/HDVs); 

truck fleet; bus fleet; idle reduction; intelligent 

transportation systems (ITS); aerodynamics; tire 

technologies; speed limiters; Long combination vehicles 

(LCV); lightweighting materials; hazard scenarios; subject 

matter experts (SMEs); crash rates; driver training; 

scenario hazard analysis.

18. Distribution Statement

Document is available to the public 

from the National Technical 

Information Service www.ntis.gov

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

 193

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
ii

TABLE OF CONTENTS

List of Tables ................................................................................................................................. vi

List of Figures............................................................................................................................... vii

Acronyms and Selected Glossary ................................................................................................viii

Acknowledgments.......................................................................................................................... xi

Executive Summary........................................................................................................................ 1

1 Introduction: Background and Objectives............................................................................... 6

1.1 Safety Performance of the National Medium Duty/Heavy-Duty Vehicle Fleet and 

Technical Approach........................................................................................................ 6

1.2 National Heavy-Duty Fleet Crash Statistics and Trends................................................ 9

2 Review of Literature on Adoption of FE Technology Clusters, Alternative Fuels, and 

Operational Safety of MD/HDV Fleets................................................................................. 16

2.1 Introduction................................................................................................................... 16

2.2 Adoption of Alternative Clean Fuels............................................................................ 16

2.2.1 Natural Gas Benefits and Safety Issues.................................................................. 16

2.2.1.1 Penetration/Adoption ....................................................................................... 17

2.2.1.2 Benefits ............................................................................................................ 21

2.2.1.3 Safety Considerations of CNG-Fueled MD/HDVs.......................................... 22

2.2.1.4 Safety Considerations for LNG-Fueled Vehicles ............................................ 28

2.2.1.5 Findings and Conclusions................................................................................ 31

2.2.2 Biodiesel ................................................................................................................. 33

2.2.2.1 Penetration/Adoption ....................................................................................... 33

2.2.2.2 Benefits ............................................................................................................ 34

2.2.2.3 Safety Considerations ...................................................................................... 35

2.2.2.4 Findings and Conclusions................................................................................ 36

2.2.3 Liquefied Petroleum Gas (Propane) ....................................................................... 36

2.2.3.1 Penetration/Adoption ....................................................................................... 37

2.2.3.2 Benefits ............................................................................................................ 38

2.2.3.3 Safety Considerations ...................................................................................... 39

2.2.3.4 Conclusions and Recommendations ................................................................ 40

2.3 Drivetrain Hybridization and Electrification ................................................................ 41

2.3.1 Penetration/Adoption.............................................................................................. 43

2.3.2 Benefits................................................................................................................... 45

2.3.3 Safety Considerations............................................................................................. 47

2.3.4 Findings and Conclusions....................................................................................... 51

2.4 ITS and Telematics for Fuel Efficiency and Operational Safety.................................. 51
iii

2.4.1 Driver and Vehicle Monitoring .............................................................................. 51

2.4.1.1 Penetration/Adoption ....................................................................................... 52

2.4.1.2 Benefits ............................................................................................................ 53

2.4.1.3 Safety Considerations ...................................................................................... 53

2.4.1.4 Findings and Conclusions................................................................................ 54

2.5 FE Technologies Affecting Operations ........................................................................ 54

2.5.1 Speed Limiters........................................................................................................ 54

2.5.1.1 Penetration and Adoption................................................................................. 55

2.5.1.2 Benefits ............................................................................................................ 55

2.5.1.3 Safety Considerations ...................................................................................... 56

2.5.1.4 Findings and Conclusions................................................................................ 59

2.5.2 Idle Reduction......................................................................................................... 59

2.5.2.1 Penetration/Adoption ....................................................................................... 60

2.5.2.2 Benefits ............................................................................................................ 60

2.5.2.3 Safety Considerations ...................................................................................... 61

2.5.2.4 Findings and Conclusions................................................................................ 61

2.6 Longer Combination Vehicles...................................................................................... 61

2.6.1 Penetration/Adoption.............................................................................................. 62

2.6.2 Benefits................................................................................................................... 62

2.6.3 Safety Considerations............................................................................................. 63

2.6.4 Literature Findings ................................................................................................. 65

2.7 Tire Technologies......................................................................................................... 65

2.7.1 Tire Pressure Monitoring Systems and Automatic Tire Inflation Systems............ 65

2.7.1.1 Penetration/Adoption ....................................................................................... 66

2.7.1.2 Benefits ............................................................................................................ 66

2.7.1.3 Safety Considerations ...................................................................................... 67

2.7.1.4 Literature Findings........................................................................................... 67

2.7.2 Single Wide Tires................................................................................................... 67

2.7.2.1 Penetration/Adoption ....................................................................................... 68

2.7.2.2 Benefits ............................................................................................................ 68

2.7.2.3 Safety Considerations ...................................................................................... 69

2.7.2.4 Findings and Conclusions................................................................................ 69

2.7.3 Low Rolling Resistance Tires................................................................................ 69

2.7.3.1 Penetration/Adoption ....................................................................................... 70

2.7.3.2 Benefits ............................................................................................................ 70

2.7.3.3 Safety Considerations ...................................................................................... 70

2.7.3.4 Findings and Conclusions................................................................................ 71

2.8 Aerodynamic Components ........................................................................................... 71
iv

2.8.1 Penetration/Adoption.............................................................................................. 71

2.8.2 Benefits................................................................................................................... 73

2.8.3 Safety Considerations............................................................................................. 74

2.8.3.1 Side Skirts........................................................................................................ 75

2.8.4 Findings and Conclusions....................................................................................... 76

2.9 Lightweight Materials................................................................................................... 77

2.9.1 Penetration/Adoption.............................................................................................. 78

2.9.2 Benefits................................................................................................................... 79

2.9.3 Safety Considerations............................................................................................. 79

2.9.4 Findings and Conclusions....................................................................................... 80

3 Summary of Subject Matter Expert Inputs on MD/HD Safety Impacts of FE 

Technologies.......................................................................................................................... 81

3.1 Introduction: Respondents and Fleet Profiles............................................................... 81

3.2 Fuel Efficiency Technology Clusters and Safety Concerns Reported.......................... 83

3.2.1 Alternative Clean Fuels.......................................................................................... 83

3.2.2 Drivetrain Hybridization and Electrification Safety............................................... 85

3.2.3 Intelligent Transportation Systems and Telematics............................................... 87

3.2.4 Speed Limiters........................................................................................................ 87

3.2.5 Idle Reduction (Auto Stop-Start)............................................................................ 88

3.2.6 Driver Training: Human Factors and Operation Issues.......................................... 88

3.2.7 Tire Systems: Wide Base, LRR, and TPMS Safety Issues..................................... 89

3.2.8 Aerodynamic Components and Safety ................................................................... 90

3.2.9 Lightweighting Materials ....................................................................................... 90

3.3 Conclusions Regarding Key Safety Issues ................................................................... 91

4 Scenario Hazard Analysis and Prevention/Mitigation Strategies.......................................... 94

4.1 Technical Approach to Hazard Analysis of FE Technologies and Fuels in 

MD/HDVs..................................................................................................................... 94

4.2 Scenario Analysis of Safety Hazards identified by SMEs............................................ 97

4.2.1 Hazard Analysis of CNG-Fueled Trucks and Buses.............................................. 97

4.2.1.1 Incidents of Vehicle Fires or Explosion Due to CNG Tank Rupture .............. 98

4.2.1.2 Rupture of Degraded or Expired CNG Tanks.................................................. 99

4.2.1.3 Prevention and Mitigation of CNG Tank Failure and Refueling Hazards..... 100
v

4.2.2 Safety of LNG-Fueled MD/HDVs and Refueling Systems.................................. 101

4.2.3 Flammability of Lightweighting Materials and Toxicity of Burning 

Byproducts............................................................................................................ 103

4.2.4 Safety Performance of Single Wide Base Tires and Low-Rolling Resistance 

Tires...................................................................................................................... 104

4.2.5 Longer Combination Trucks . .............................................................................. 105

4.2.6 Aerodynamic Components................................................................................... 108

4.2.7 Potential Fires from Overheated EV/PHEV Battery ............................................ 111

4.2.7.1 Electrical Hazards Prevention and Mitigation ............................................... 113

5 Regulatory Barriers Analysis............................................................................................... 115

5.1 Introduction................................................................................................................. 115

5.2 Adoption of Alternative Clean Fuels.......................................................................... 116

5.3 Hybridization and Electrification................................................................................ 117

5.3.1 Electric Drivetrain ................................................................................................ 117

5.3.2 Hybrid Hydraulic Drivetrain ................................................................................ 117

5.4 ITS and Telematics..................................................................................................... 117

5.5 Human Factors and Operations................................................................................... 118

5.5.1 Idle Reduction....................................................................................................... 118

5.5.2 Speed limiters....................................................................................................... 118

5.6 Longer Combination Vehicles and Truck Size and Weight Limits............................ 118

5.7 Tire Technologies....................................................................................................... 120

5.7.1 Tire Pressure Management Systems and Automatic Tire Inflation Systems ....... 120

5.7.2 Single Wide Tires................................................................................................. 121

5.7.3 Low Rolling Resistance Tires.............................................................................. 121

5.8 Aerodynamic Components ......................................................................................... 122

5.9 Lightweight Components............................................................................................ 122

5.10 Findings and Conclusions Regarding Regulatory Barriers......................................... 123

6 Conclusions and Recommendations.................................................................................... 129

7 Appendices .......................................................................................................................... 133

7.1 Operations/Human Factors Bibliography ................................................................... 133

7.2 Regulatory Bibliography............................................................................................. 139

7.3 Green Vehicle Deployment Bibliography .................................................................. 142

7.4 Safety Bibliography .................................................................................................... 147

7.5 Advanced Fuels Bibliography .................................................................................... 159

7.6 Fuel Efficiency Technology Bibliography ................................................................. 169
vi

LIST OF TABLES

Table 1-1: FMCSA Safety Statistics 2009-2011. ......................................................................... 11

Table 1-2: Truck Fatalities in 2010-11 ......................................................................................... 12

Table 1-3: Estimated Numbers of Trucks in All Crashes, by Critical Reasons............................ 12

Table 1-4: Alternative-Fuel Vehicles in Use by Fleet Operators: 2010 ....................................... 13

Table 1-5: Alternative-Fuel Vehicles in Use 2011 by Weight Class and Fuel Type.................... 13

Table 2-1: Alternative Fuel Vehicles in Use in the United States by Fleet Operators: 2010 ....... 38

Table 3-1: Broad Cross-Section of Subject Matter Experts Interviewed ..................................... 81

Table 5-1: Summary of Potential Regulatory Barriers for the Adoption of MD/HDV Fuel 

Efficiency Technologies ........................................................................................... 125
vii

LIST OF FIGURES

Figure 1-1: Medium and Heavy-Duty Vehicles Types by Gross Vehicle Weight Rating 

(GVWR). ..................................................................................................................... 6

Figure 1-2: Heavy-duty vehicles share of national fleet, fuel consumption and GHG emissions.. 7

Figure 1-3: Relative potential fuel savings from six FE technologies applied to: tractor-trailers 

(TT), Class 3-6 box and bucket vehicles, transit buses and motor coaches; Class 8-

refuse trucks; and Class 2b pickups and vans. ............................................................ 8

Figure 2-1: Estimated Consumption of Alternative Fuel by AFVs in the United States 

(Ref. 15)..................................................................................................................... 18

Figure 2-2: Alternative Fueled Vehicles in Use in the United States........................................... 19

Figure 2-3: Carbon Intensity of Various Fuels in California HDVs. ........................................... 22

Figure 2-4: CNG Tank Failures Frequencies by Cause ................................................................ 25

Figure 2-5: Prevalence of Aftermarket Conversions in CNG Failures......................................... 28

Figure 2-6: U.S. Biodiesel Production and Consumption............................................................. 34

Figure 2-7: Schematic of a Parallel Hybrid-Electric Utility Truck............................................... 43

Figure 2-8: Hydraulic Hybrid Refuse Truck................................................................................. 46

Figure 2-9: Smith Electric Truck .................................................................................................. 47

Figure 2-10: GreenRoad On-Board Driver Behavior and Telematics.......................................... 53

Figure 2-11: Single Wide Base Tires Replacing Duals at Each Wheel Site................................. 67

Figure 2-12: FE Technology Adoption Across Eight Major Fleets.............................................. 72

Figure 2-13: Aerodynamic Trailer Skirt Survivability to Impacts................................................ 75

Figure 4-1: A Summary Risk Assessment Matrix Ranking and Prioritizing Hazards ................. 94

Figure 4-2: A Hazard Level Matrix Showing Ranked Event Probability and Consequence 

Severity...................................................................................................................... 95

Figure 4-3: Refueling a Waste Management LNG Truck .......................................................... 102

Figure 4-4: Three Types of LCV Configurations....................................................................... 106

Figure 4-5: Trailer Skirt Made From Resilient Composite Material .......................................... 109

Figure 5-1: Regulatory Environment for Longer Combination Vehicles in the United States .. 120
viii

ACRONYMS AND SELECTED GLOSSARY

ABS anti-lock brake system

AFDC Alternative Fuels Data Center

AFV alternative fueled vehicle 

AHSS advanced high strength steel

ANL Argonne National Laboratory

ANSI American National Standards Institute

APTA American Public Transportation Association

APU auxiliary power unit

ATIS automatic tire inflation system

ATRI American Transportation Research Institute

AVTA advanced vehicle testing activity

BASIC Behavior Analysis and Safety Improvement Categories; a relative ranking of 

carriers among their peers according to safety performance, as measured by the 

SMS

BCCS Bus Crash Causation Study

BP best practice

CAFE Corporate Average Fuel Economy

CARB California Air Resources Board

CC clean carrier; a carrier that is a member of the DOE National Clean Fleets 

Partnership

CF conventional fleet

CDL commercial driver license

CFRC carbon fiber reinforced composite

CFRP carbon fiber reinforced plastics

CMV commercial motor vehicle

CNG compressed natural gas

CO carbon monoxide

CO2 carbon dioxide

CSA Compliance, Safety, Accountability; an FMCSA initiative to improve truck and 

bus carriers’ safety

CTBSSP Commercial Truck and Bus Safety Synthesis Program

DOC diesel oxidation catalysts

DOE Department of Energy

DPF diesel particulate filter

DTNA Daimler Truck North America

EC European Commission

ECM electronic control module

EGR exhaust gas recirculation 

EIA Energy Information Administration

EMT Emergency Management Technician

EPA Environmental Protection Agency

ESC electronic stability control

ESS energy storage system

EV electric vehicle
ix

FARS Fatality Analysis Reporting System; a NHTSA database of fatal crashes

FE fuel efficiency

FEI fuel efficiency improvement

FHWA Federal Highway Administration

FMCSA Federal Motor Carrier Safety Administration

FMCSR Federal Motor Carrier Safety Regulations

FMVSS Federal Motor Vehicle Safety Standards

FRP fiber reinforced plastics

GEG gasoline-equivalent gallon (GEG); the amount of alternative fuel with equal the 

energy content of a gallon of gasoline

GBO green bus operators; these have signed the APTA sustainability commitment

GES General Estimates System, a NHTSA crash database

GGE gasoline gallon equivalent (GGE); the amount of alternative fuel that contains the

energy of one gallon of gasoline

GHG greenhouse gases

GPS Global Positioning System 

GVWR gross vehicle weight rating

H2 hydrogen

HDV heavy-duty vehicle

HEV hybrid electric vehicle

HHV hybrid hydraulic vehicle

ICE internal combustion engine

IIHS Insurance Institute for Highway Safety

ISTEA Intermodal Surface Transportation Efficiency Act (1991)

ITS Intelligent Transportation Systems

LBT Long Beach Transit

LCV longer combination vehicle

LDWS lane departure warning system

L&I licensing and insurance; a data set in MCMIS

LNG liquefied natural gas

LPG liquefied petroleum gas, propane, or autogas

lpm liters per minute

LRR low rolling resistance

LTCCS Large Truck Crash Causation Study

MAP-21 Moving Ahead for Progress in the 21st Century Act (2012)

MCMIS Motor Carrier Management Information System, an FMCSA database 

MD/HDV medium- and heavy-duty vehicles

MDV medium-duty vehicle

mph miles per hour

MSDS Material Safety Data Sheet

MTBF mean time between failures

MTBRC mean time between road calls

MY model year(s)

NACFE North American Council for Freight Efficiency

NFPA National Fire Protection Association

NGV natural gas vehicle
x

NHTSA National Highway Traffic Safety Administration

NOx nitrogen oxides

NREL National Renewable Energy Laboratory

NTEA National Truck Equipment Association

OEM original equipment manufacturer

O&M operations and maintenance

OOIDA Owner Operator Independent Driver Association

PCIV plastics and composites intensive vehicles

PHMSA Pipelines and Hazardous Materials Administration

PM particulate matter

PRD pressure relief device

psi pounds per square inch

PU power unit; trucks or buses, excluding trailers

RESS Rechargeable Energy Storage System 

ROI return on investment

RSC roll stability control

SAE Society of Automotive Engineers

SBMTD Santa Barbara Municipal Transit District

SCR selective catalytic reduction

SME subject matter expert

SMS Safety Measurement System; a process for assigning BASIC scores to carriers 

under the CSA initiative

SOP standard operating procedure

TARDEC United States Army Tank Automotive Research, Development And Engineering 

Center

TPMS tire pressure maintenance system

UMTRI University of Michigan Transportation Research Institute

VMT vehicle miles traveled

VRU vulnerable road user

WMATA Washington Metropolitan Area Transit Authority
xi

ACKNOWLEDGMENTS

The principal authors of the report are Dr. Aviva Brecher, principal technical advisor, and Dr. 

Alexander Epstein of the Energy Analysis and Sustainability Division in the Policy, Planning 

and Environment Technical Center at the USDOT Volpe National Transportation Systems 

Center, part of the Office of the Assistant Secretary for Research and Technology. Andrew Breck 

of the Organizational Performance Division contributed to Chapter 5 on regulatory analysis. The 

authors are also grateful for the valuable contributions and support from Volpe Energy and 

Sustainability Analysis Division colleagues, Scott Lian and Coralie Cooper, and Kevin Green, 

chief of the Volpe CAFE Program Office. The authors thank the NHTSA Fuel Economy 

Division (NVS-132) sponsors, James MacIsaac and James Tamm, chief, for their guidance and 

feedback throughout the project, and Ken Katz, Lily Smith, and Ryan Hagen for their helpful 

reviews. We also acknowledge the Volpe Center editorial support team and our IAA officer 

Brunilda Santos.
1

EXECUTIVE SUMMARY

The 2010 National Research Council report Technologies and Approaches to Reducing the Fuel 

Consumption of Medium- and Heavy-Duty Vehicles evaluated technology options for improving 

the fuel efficiency (FE) of Class 2-8 medium and heavy-duty vehicles and recommended that 

NHTSA perform a thorough safety analysis to identify and evaluate potential safety issues. In 

2011, NHTSA and EPA issued joint rulemaking for fuel efficiency and Greenhouse Gas (GHG) 

emissions of model year (MY) 2014-2018 MD/HDVs. 

This report summarizes research and analysis findings on potential safety issues associated with 

both the diverse alternative fuels (natural gas-CNG and LNG, propane, biodiesel, and power 

train electrification), and the specific FE technologies recently adopted by the MD/HDV fleets. 

These include Intelligent Transportation Systems (ITS) and telematics, speed limiters, idle 

reduction devices, tire technologies (single-wide tires, and tire pressure monitoring systemsTPMS and automated tire inflation systems-ATIS), aerodynamic components, vehicle lightweighting materials, and long combination vehicles (LCVs).

Chapter 1 provides an overview of the study rationale, background, and key objective, namely to 

identify the technical and operational/behavioral safety benefits and disbenefits of MD/HDVs 

equipped with FE technologies and using emerging alternative fuels. Recent MD/HDV national 

fleet crash safety statistical averages are also provided for context, although no information 

exists in crash reports relating to specific vehicle FE technologies and fuels. Chapters 2 and 3 are 

organized by clusters of functionally-related FE technologies (e.g., tire systems, ITS, lightweighting materials, and aerodynamic systems) and alternative fuels, which are described and 

their respective associated potential safety issues are discussed.

Chapter 2 summarizes the findings from a comprehensive review of available technical and trade 

literature and Internet sources regarding the benefits, potential safety hazards, and the applicable 

safety regulations and standards for deployed FE technologies and alternative fuels. Chapter 2 

safety-relevant fuel-specific findings include:

• Both CNG- and LNG-powered vehicles present potential hazards, and call for wellknown engineering and process controls to assure safe operability and crashworthiness. 

However, based on the reported incident rates of NGVs and the experiences of adopting 

fleets, it appears that NGVs can be operated at least as safely as diesel MD/HDVs. Using 

natural gas instead of diesel fuel helps fleets comply with the MD/HD greenhouse gas 

rules that require up to 20 percent emissions reduction by 2018.

• There are no safety contraindications to the large scale fleet adoption of CNG or LNG 

fueled heavy-duty trucks and buses, and there is ample experience with the safe operation 

of large public transit fleets. Voluntary industry standards and best practices suffice for 

safety assurance, though improved training of CMV operators and maintenance staff in 

natural gas safety of equipment and operating procedures is needed.

• Observing CNG and LNG fuel system and maintenance facility standards, coupled with 

sound design, manufacture and inspection of natural gas storage tanks will further reduce 

the potential for leaks, tank ruptures, fires, and explosions.
2

• Biodiesel blends used as drop-in fuels have presented some operational safety concerns 

dependent on blending fraction, such as material compatibility, bio-fouling sludge 

accumulation, or cold-weather gelling. However, best practices for biodiesel storage, and 

improved gaskets and seals that are biodiesel resistant, combined with regular 

maintenance and leak inspection schedule for the fuel lines and components enable the 

safe use of biodiesel in newer MD/HDVs.

• Propane (LPG, or autogas) presents well-known hazards including ignition (due to leaks 

or crash) that are preventable by using overfill prevention devices (OPDs), which

supplement the automatic stop-fill system on the fueling station side, and pressure release 

devices (PRDs). Established best practices and safety codes (e.g., NFPA) have proven 

that propane fueled MD/HDVs can be as operationally safe as the conventionally-fueled 

counterparts. 

• As the market penetration of hybrid and electric drivetrain accelerates, and as the 

capacity and reliability of lithium ion batteries used in Rechargeable Energy Storage 

Systems (RESS) improve, associated potential safety hazards (e.g., electrocution from

stranded energy, thermal runaway leading to battery fire) have become well understood, 

preventable and manageable. Existing and emerging industry technical and safety 

voluntary standards, applicable NHTSA regulations and guidance, and the growing 

experience with the operation of hybrid and electric MD/HDVs will enable the safe 

operation and large-scale adoption of safer and more efficient power-train electrification 

technologies. 

The safety findings from literature review pertaining to the specific FE technologies 

implemented to date in the MD/HDV fleet include:

• Telematics—integrating on-board sensors, video, and audio alerts for MD/HDV 

drivers—offer potential improvements in both driver safety performance and fuel 

efficiency. Both camera and non-camera based telematics setups are currently integrated 

with available crash avoidance systems (such as ESC, RSC, LDWS, etc.) and appear to 

be well accepted by MD/HDV fleet drivers. 

• Both experience abroad and the cited U.S. studies of trucks equipped with active speed 

limiters indicated a safety benefit, as measured by up to 50 percent reduced crash rates, in 

addition to fuel savings and other benefits, with good CMV driver acceptance. Any 

negative aspects were small and avoidable if all the speed limitation devices were set to 

the same speed, so there would be less need for overtaking at highway speeds.

• No literature reports of adverse safety impacts were found regarding implementation of 

on-board idle-reduction technologies in MD/HDVs (such as automatic start-stop, directfired heaters, and APUs).

• There was no clear consensus from the literature regarding the relative crash rates and 

highway safety impacts of LCVs, due to lack of sufficient data and controls and 

inconsistent study methodologies. Recent safety evaluations of LCVs and ongoing MAP21 mandated studies will clarify and quantify this issue.

• Tire technologies for FE (including ATIS, TPMS, LRR and single-wide tires) literature 

raised potential safety concerns regarding lower stability or loss of control, e.g., when tire 

pressure is uneven or a single wide tire blows out on the highway. However, systems 
3

such as automated tire monitoring systems and stability enhancing electronic systems

(ABS, ESC, RSC) may compensate and mitigate any adverse safety impacts.

• Aerodynamic technologies that offer significant fuel savings have raised potential 

concerns about vehicle damage or injury in case of detached fairings or skirts, although 

there were no documented incidents of this type in the literature. 

• Some light weighting materials may pose some fire safety and crashworthiness hazards, 

depending on their performance in structural or other vehicle subsystem applications 

(chassis, power-train, crash box or safety cage). Some composites (fiberglass, plastics, 

CFRC, foams) may become brittle on impact or due to weathering from UV exposure or 

extreme cold. Industry has developed advanced, high performance lightweight material 

options tailored to their automotive applications, e.g., thermoplastics resistant to UV and 

weathering. No examples of such lightweight material failures on MD/HDVs were 

identified in the literature. 

Chapter 3 provides complementary inputs on the potential safety issues associated with FE 

technologies and alternative fuels obtained from Subject Matter Experts (SMEs). The broad 

cross-section of SMEs consulted had experience with the operation of “green” truck and bus 

fleets, were Federal program managers, or were industry developers of FE systems for 

MD/HDVs. Safety concerns raised by the SMEs can be prevented or mitigated by complying 

with applicable regulations and safety standards and best practices, and are being addressed by 

evolving technologies, such as electronic collision prevention devices. Although SMEs raised 

some safety concerns, their experience indicates that system- or fuel-specific hazards can be 

prevented or mitigated by observing applicable industry standards, and by training managers, 

operators and maintenance staff in safety best practices. Specific safety concerns raised by SMEs 

based on their experience included:

• Alternative fuels did not raise major safety concerns, but generally required better 

education and training of staff and operators. There was a concern expressed regarding 

high pressure (4,000 psi) CNG cylinders that could potentially explode in a crash 

scenario or if otherwise ruptured. However, aging CNG fuel tank safety can be assured 

by enforcing regulations such as FMVSS 304, and by periodic inspection and end-of-life 

disposal and replacement. A propane truck fleet manager found the fuel to be as safe as 

or safer than gasoline, and reported no safety issues with the company’s propane, nor 

with hybrid gasoline-electric trucks. OEMs of drivetrain hybridization and electrification 

systems, including advanced Lithium Ion batteries for RESS, indicated that they undergo 

multiple safety tests and are designed with fail-safes for various misuse and abuse 

scenarios. Integration of hybrid components downstream by bodybuilders in retrofits, as 

opposed to new vehicles was deemed a potential safety risk. Another potential safety 

concern raised was the uncertain battery lifetime due to variability of climate, and dutycycles and aging. Without state-of-charge indicators, this could conceivably leave 

vehicles underpowered or stranded if the battery degrades and is not serviced or replaced 

in a timely manner. 

• ITS and telematics raised no safety concerns; on the contrary, fleet managers stated that

“efficient drivers are safer drivers.” Monitoring and recording of driver behavior, 

combined with coaching, appeared to reduce distracted and aggressive driving and 

provided significant FE and safety benefits. 
4

• A wide-base single tire safety concern was the decrease in tire redundancy in case of a 

tire blowout at highway speeds. For LRRs, a concern was that they could negatively 

affect truck stopping distance and stability control.

• A speed-limiter safety concern was related to scenarios when such trucks pass other 

vehicles on the highway instead of staying in the right-hand lane behind other vehicles. 

By combining speed limiters with driver training programs, overall truck safety could 

actually improve, as shown by international practice. 

• Aerodynamic systems safety performance to date was satisfactory, with no instances of 

on-road detaching. However, covering underside or other components with aerodynamic 

fairings can make them harder to inspect, such as worn lugs, CNG relief valve shrouds, 

wheel covers, and certain fairings. Drivers and inspectors need to be able to see through 

wheel covers and to be able to access lug nuts through them. These covers must also be 

durable to withstand frequent road abuse. 

• For lightweighting materials, the safety concern raised was lower crashworthiness 

(debonding or brittle fracture on impact) and the potential for decreased survivability in 

vehicle fires depending on the specific material choice and its application. 

The key finding from the literature review and SME interviews is that there are no major safety 

hazards preventing the adoption of FE technologies or the increased use of alternative fuels and 

vehicle electrification. In view of the scarcity of hard data currently available on actual highway 

crashes that can be directly or causally attributed to adoption of FE technologies and/or 

alternative fuels by MD/HDVs, and the limited experience with commercial truck and transit bus 

fleets operations equipped with these technologies, it was not possible to perform a quantitative, 

probabilistic risk assessment, or even a semi-quantitative preliminary hazard analysis (PHA). 

Thus, Chapter 4 employs a deterministic scenario-based hazard analysis of potential crash or 

other safety concerns identified from the literature review or raised by subject matter experts 

(SMEs) interviewed (e.g., interfaces with charging or refueling infrastructure). For each specific 

hazard scenario discussed, the recommended prevention or mitigation options, including 

compliance with applicable NHTSA or FMCSA regulations, and voluntary industry standards 

and best practices are identified, along with FE technology or fuel-specific operator training.

SMEs safety concerns identified in Sec 3.3 were complemented with actual incidents, and

developed into the hazard scenarios analyzed in Chapter 4.

The scenario-based deterministic hazard analysis reflected not only the literature findings and 

SME’s safety concerns, but also real truck or bus mishaps that have occurred in the past. Key 

hazard analysis scenarios included: CNG-fueled truck and bus vehicle fires or explosions due to 

tank rupture, when pressurized fuel tanks were degraded due to aging or when PRDs failed; LNG 

truck crashes leading to fires, or LNG refueling-related mishaps; the flammability or brittle 

fracture issues related to lightweighting materials in crashes; reduced safety performance for 

either LRR or wide-base tires; highway pile-ups when LCVs attempt to pass at highways speeds; 

aerodynamic components detaching while the vehicle traveled on a busy highway or urban 

roadway; and fires resulting in overheated lithium ion batteries in electric or hybrid buses. These 

hypothetical worst case scenarios appear to be preventable or can be mitigated by observing 

safety regulations and voluntary standards, or with engineering and operational best practices. 

Chapter 5 reviews and discusses the existing Federal and State regulatory framework for safely 

operating MD/HDVs equipped with FE technologies or powered by alternative fuels. The review
5

identifies potential regulatory barriers to their large-scale deployment in the national fleet that

could delay achievement of desired fuel consumption and environmental benefits, while ensuring 

equal or better safety performance.

Chapter 6 summarizes the major findings and recommendations of this preliminary safety 

analysis of fuel efficiency technologies and alternative fuels adopted by MD/HDVs. The

scenario-based hazard analysis, based on the literature review and experts’ inputs, indicates that 

MD/HDVs equipped with advanced FE technologies and/or using alternative fuels have 

manageable potentially adverse safety impacts. However, an in depth comparative statistical 

analysis of truck and bus crash databases over a longer time period (at least five years) is needed 

to understand quantitatively the safety implications of the rapid adoption of ”green” technologies 

and fuels in the Nation’s heavy-duty fleet. 
6

1 INTRODUCTION: BACKGROUND AND OBJECTIVES

1.1 Safety Performance of the National Medium Duty/Heavy-Duty Vehicle Fleet and 

Technical Approach

The adoption and deployment of new fuel-efficiency (FE)-improving technologies is accelerating

across Class 2b-8 MD/HDVs (Figure 1-1). Manufacturers, commercial truck and bus, and public 

transit bus fleets are implementing a diverse mix of FE technologies and alternative fuels so as to 

improve fuel efficiency in order to comply with Phase I of the NHTSA and EPA fuel efficiency 

and GHG emissions regulations for model years 2014-2018.

1

Figure 1-1: Medium and Heavy-Duty Vehicles Types by Gross Vehicle Weight Rating 

(GVWR).

2

 1 See posted Sept 15, 2011 NHTSA and EPA final rule at CFR Volume 76, Number 179, Pages 57105-57513,

www.gpo.gov/fdsys/pkg/FR-2011-09-15/html/2011-20740.htm 2 Figure 1-1a from EPA/NHTSA Final Rulemaking to Establish Greenhouse Gas Emissions Standards and Fuel 

Efficiency Standards for Medium- and Heavy-Duty Engines and Vehicles Regulatory Impact Analysis
7

In 2010-2011, the baseline years of NHTSA/EPA rulemaking, heavy-duty vehicles represented 

only 4 percent of the national fleet, but consumed 20 percent of transportation fuel, and produced 

a similar fraction of GHG emissions as shown below in Figure 1-23. 

Figure 1-2: Heavy-duty vehicles share of national fleet, fuel consumption and GHG emissions 

According to the DOT/BTS National Transportation Statistics4, in 2011 the heavy-duty fleet 

consisted of over 10 million trucks, more than 666,000 commercial buses, and about 63,000

transit buses. According to the APTA2011 Public Transportation Factbook5 over 35 percent of 

public transit buses used alternative power, including hybrid, electric, CNG, LNG and biodiesel 

blends.

A 2010 National Research Council report examined the technology options to improve fuel 

efficiency for all eight classes of MD/HD vehicles, covering a broad range of functions and duty 

cycles.6

 The report evaluated and ranked the FE technologies, also described in Chapter 2 of this 

report, by their potential fuel savings and associated emissions reduction for vehicle classes 

(Figure 1-3). It examined the costs and benefits of hybrid power trains (hydraulic, parallel, or 

series electric), advanced lithium-ion battery packs, power management electronics and software, 

hydrogen fuel cell auxiliary power units (APUs) used to extend range, electric motors, 

regenerative braking, idle reduction and automated start-stop, lightweight materials, fairings for 

improved aerodynamics, and single wide tires. The report also noted, but did not discuss in 

detail, the potential for safety impacts of these fuel efficiency-improving technologies for 

MD/HD vehicles when deployed either singly or in synergistic clusters. The fuel efficiency-

 3 Figure 1-1b source is the Feb. 2014 White House report: “Improving the Fuel Efficiency of American Trucks,” at 

www.whitehouse.gov/sites/default/files/docs/finaltrucksreport.pdf

4 See NTS Table 1-11 for 2010 and 2011 number of trucks and commercial and transit buses at 

www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/national_transportation_statistics/html/table_01_11.htm

l

5 See www.apta.com/resources/statistics/Documents/FactBook/APTA_2011_Fact_Book.pdf 6 See March 2010 NRC report “Technologies and Approaches to Reducing the Fuel Consumption of MDHD 

Vehicles.”
8

improving technologies considered in the NRC report include EPA SmartWay tractor trailer 

technologies.7

The report also provided the technical underpinning for the 2011 medium- and heavy-duty 

regulation, and recommended that the agencies conduct a thorough evaluation of safety impacts 

associated with MD/HD efficiency technologies. Finding 6-15 of the NRC report stated that: 

“There are potential safety issues associated with particular fuel reduction technologies. 

Examples are hybrids that use high-voltage batteries, or aerodynamic fairings that may 

detach from trucks on the road.” 

Figure 1-3: Relative potential fuel savings from six FE technologies applied to: tractor-trailers

(TT), Class 3-6 box and bucket vehicles, transit buses and motor coaches; Class 8-refuse trucks; 

and Class 2b pickups and vans.8

An in-depth safety analysis of fuel efficiency-improving technologies for MD/HDV fleet-wide

deployment can help inform future FE rulemaking for the medium- and heavy-duty fleet. In 

addition, it may help to identify and address potential gaps in safety regulations and standards, 

and enable the identification and prioritization of safety-focused research, while promoting fuel 

economy for environmental and economic benefits. The present study implements the NRC 

recommendation.

 7

 See www.epa.gov/smartway/transport/what-smartway/verified-technologies.htm and 

www.epa.gov/smartway/technology/designated-tractors-trailers.htm 8 Ibid.
9

The technical approach was to organize the literature review so as to facilitate a focused analysis 

of potential FE technology and alternative fuel safety impacts, and compile topically organized 

bibliographies extracted from an Excel database. This process facilitated the identification and 

analysis of potential safety issues related to specific FE technologies and alternative fuels 

adopted by vehicle fleets, and shed light on potential operator training and human factors safety 

issues. Over 500 references were reviewed and organized by topic (FE technology, alternative 

fuel, MD/HDV fleet employment and experience, regulations, safety, operational and human 

factors issues, etc.). The references were organized in a sortable Excel database, and are both 

footnoted and listed in Appendices 7.1-7.6 as topical Bibliographies: Safety; Fuel Efficiency 

Technology; Advanced Fuels; Operations/Human Factors; Regulatory Barriers; and Green 

Vehicle Deployment.

Relevant safety findings from the literature review are summarized in Chapter 2, and were 

complemented with inputs from focused interviews with a representative and diverse set of 

Subject Matter Experts (SMEs) with fleet management or engineering experience in the use of 

FE technologies and alternative fuels, as discussed in Chapter 3. Safety issues identified from 

Chapters 2 and 3 were then developed into the deterministic hazard scenarios described and 

analyzed in Chapter 4, including respective prevention and mitigation options. Chapter 5 then 

focused on the Federal and State regulatory framework affecting MD/HDV adoption of FEs and 

alternative fuels, with the goal of identifying residual regulatory barriers.

The key safety finding from this comprehensive review and analysis is that there are no major 

safety hazards preventing the adoption of FE technologies, nor from increased use of alternative 

fuels and vehicle electrification. Any identified safety concerns can be prevented or mitigated by 

complying with applicable regulations and safety standards and best practices and are being 

addressed by evolving technologies, such as electronic collision prevention devices. 

1.2 National Heavy-Duty Fleet Crash Statistics and Trends

Approximately 5.7 million commercial motor vehicle (CMV) drivers were licensed in 2010. The 

FMCSA CMV fleet and crash statistics9 show that in 2010 the national fleet was operated by 

498,144 interstate freight carriers and included 846,051 buses operated by 11,701 interstate 

passenger carriers. In addition, public transit authorities operated over 63,000 buses in 2010. The 

public bus transit fleet used about 35 percent alternative power in 2010 and offered valuable 

experience with their safe operability. As illustrated below, the total number of alternative-fueled 

heavy-duty vehicles in the Nation’s fleet in 2010 was under 70,000 -- a very small fraction of the 

conventional fleet. 

Crash fatalities and injury totals and rates for trucks and buses, and other relevant safety 

performance annual statistics and trends are analyzed and reported annually by FMCSA and 

NHTSA, using data collected from States and analyzed by NHTSA’s National Center for 

Statistics and Analysis (NCSA). Crash data provides safety performance trends over time in 

comparison with the Nation’s entire vehicle fleet. The NCSA annual summaries,

10 and the

 9 See Commercial Motor Vehicle Facts (March 2013) at www.fmcsa.dot.gov/safety/data-and-statistics/commercialmotor-vehicle-facts-%E2%80%93-march-2013 10 See NCSA Large Trucks 2012 data at www-nrd.nhtsa.dot.gov/Pubs/811868.pdf
10

FMCSA Motor Carrier Safety Progress Reports11 (MCSPR) offer fleet-wide safety statistics that 

provide a useful context for examining the relative safety performance of the small fraction of 

“green fleets” of MD/HDVs so equipped. Filed field crash reports and analyses to date have not 

included any information on whether specific fuel efficiency technologies, or alternative fuels 

were involved as causal crash factors.

The NHTSA Fatality Analysis Reporting System (FARS) contains only records of fatal crashes 

and does not include census data for motor carriers. The FMCSA’s MCMIS includes records of 

all crashes involving interstate and intrastate hazmat motor carriers, both fatal and non-fatal. 

Another difference is that the FMCSA Motor Carrier Management Information System 

(MCMIS) the Crash Rate is defined as to the Crash Count/Power Unit (PU)12 Count * 1000,

whereas in the FARS the fatality rate is defined as the number of fatalities normalized to 100 

million vehicle miles travelled (VMT).

Crash figures and trends summarized by FMCSA13 indicate a long term safety improvement 

trend : the total number of large trucks involved in fatal crashes decreased over a decade by 25

percent, and by 30 percent for crash injuries. Similarly, the number of buses involved in fatal 

crashes decreased by 16 percent. However, the rate of large truck and bus fatalities per 100 

million VMT actually increased by 2 percent from 2010 to 2011. 

Since the NHTSA/EPA MD/HDV fuel efficiency regulations have used 2010-11 as the 

regulatory fuel efficiency baseline years, this safety analysis also focused on crash safety 

performance data for the same period. Table 1-1 shows the 2009-2011 FMCSA number of truck 

crashes, fatalities and injuries. In 2009, NHTSA reported 3,380 fatalities from large truck crashes 

and only 254 from bus crashes, with 93,000 injured from both truck and bus crashes. Overall, the 

HDV fleet VMT in 2011 decreased from 2,966,506 million to 2,930,654 million compared to

2010, a decrease of 1.2 percent. Large truck crash fatalities and injuries, and the respective rates 

per 100 M VMT in 2010 and 201114 were compared to the national overall fleet rates: while the 

overall fleet fatality rate for the entire fleet fell to a historic low of 1.10 in 2011, the injury rate 

increased by 1.3 percent from 2010 to 2011. For large truck crashes however15, there was a 1.9

percent increase in the number of people killed (from 3.686 in 2010 to 3,757 in 2011), with the 

corresponding fatality rate increase from 1.22 to 1.35.

Although there has been an increase in fatalities from large truck crashes from 2009 to 2011, large 

trucks continue to have a better relative safety record than the light-duty fleet: Total fatalities from 

large truck crashes rose from 3,380 in 2009 to 3,686 in 2010 (+9%), and 3,757 (+2%) in 2011. 

But the 2009 fatality rate for large trucks and buses was only 0.123 per 100 million VMT, 

compared to 1.14 for the entire fleet, including cars and light trucks. Similarly, the crash injury 

rate in 2009 for large trucks and buses was 3.15 per 100 million VMT, compared to 75.1 for the 

entire U.S. fleet. This superior CMV safety performance relative to privately owned light duty 

 11 See www.fmcsa.dot.gov/sites/fmcsa.dot.gov/files/docs/MCSPR-09-30-13.pdf 12 A power unit (PU) as defined by FMCSA is a truck or a bus, excluding trailers. 13 See FMCSA “Large Truck and Bus Crash Facts 2011 at www.fmcsa.dot.gov/safety/research-and-analysis/largetruck-and-bus-crash-facts-2011 14 2011 Motor vehicle crashes-an overview at www-nrd.nhtsa.dot.gov/Pubs/811701.pdf 15 See NHTSA Traffic Safety Facts 2011 Data-Large Trucks at www-nrd.nhtsa.dot.gov/Pubs/811752.pdf
11

vehicles may be due to well-trained professional CMV drivers, and to frequently inspected and 

well-maintained vehicles.

In 2010, there were 3,341 fatalities and 56,000 injuries from large truck crashes (an increase of 

about 9 % relative to the 2009); and 247 fatalities and 12,000 injuries for buses. These 2010 

NHTSA large truck crash statistics16 (see also Table 1-1) show that, for the 10,770,054 large 

trucks registered and their risk exposure from 286,585 million VMTs, the large truck crash 

vehicle involvement rate was:

• 1.22 for 3,484 large trucks fatal crashes; and

• 20 for 58,000 trucks involved in injury crashes.

An updated FMCSA17 analysis also summarized and compared 2009 through 2011 large truck 

crash totals and rates normalized to 100 million VMT. Although truck crash fatalities (Table 1-2)

totals increased from 3,686 in 2010 to 3,757 in 2011 (by 1.9%) and so did injuries (by 15%), the 

associated fatality rate decreased modestly by 0.9 percent, while the injury rate increased by 1.3

percent. Large truck crashes resulted in 23,000 injuries in 2011, versus 20,000 in 2010, or a 15 

percent increase. From 2011 to 2012, the number of large trucks involved in fatal crashes again 

increased by 5 percent, and the fatality rate per 100 million VMT increased by 4 percent.

18

Table 1-1: FMCSA Safety Statistics 2009-2011.19

 16 See Table 36 in NHTSA “2010 Motor Vehicle Crash Data from FARS and GES” at wwwnrd.nhtsa.dot.gov/Pubs/811659.pdf

17 See “Commercial Motor Vehicle (CMV) Facts- March 2013” at www.fmcsa.dot.gov/safety/data-andstatistics/commercial-motor-vehicle-facts-%E2%80%93-march-2013 18 See www.fmcsa.dot.gov/safety/data-and-statistics/large-truck-and-bus-crash-facts-2012 19 See March 2012 Motor Carrier Safety Progress Report (MCSPR) at http://ebookbrowse.com/mcspr-03-31-12-

pdf-d389813594
12

Table 1-2: Truck Fatalities in 2010-11

This report focuses on the relative safety performance of the currently very small fraction of 

heavy-duty vehicles in the fleet that have adopted fuel efficiency technologies, or alternative 

fuels (CNG, LPG, biodiesel, hybrid-electric and electric). In attempting to link FE technologies 

and driver training to the safety of advanced vehicles relative to the overall fleet, it is important 

to examine the causes contributing to large trucks crashes. Table 1-3 summarizes the findings 

from the 2007 FMCSA Large Trucks Crash Causation Study (LTCCS). Causative factor analysis 

showed that over 90 percent of crashes are due to human factors, such as driver distraction or 

error, highlighting the importance of driver fitness and training. Only 10 percent of crashes were 

vehicle-related, so any safety incidents attributable to FE technologies and/or alternative fuels 

would be included in this category.

Table 1-3: Estimated Numbers of Trucks in All Crashes, by Critical Reasons20

Critical Reasons Number of Trucks Percent of Total

Driver 68,000 87%

Non-Performance 9,000 12%

Recognition 22,000 28%

Decision 30,000 38%

Performance 7,000 9%

Vehicle 8,000 10%

Environment 2,000 3%

Total Number of Large Trucks Coded with Critical Reason 78,000 100%

Total Number of Large Trucks Not Coded with Critical Reason 63,000 —

Total Number of Large Trucks Involved in Crashes 141,000 —

 

20 See Analysis Brief of FMCSA Large Truck Crash Causation Study (2007) at www.fmcsa.dot.gov/factsresearch/research-technology/analysis/FMCSA-RRA-07-017.htm; LTCCS Table Notes: Results shown are national 

estimates for the 141,000 large trucks estimated to have been involved in fatal and injury crashes during the study 

period. The estimates may differ from true values, because they are based on a probability sample of crashes and not 

a census of all crashes. Estimates are rounded to the nearest 1,000 large trucks.
13

The total number of alternatively-fueled trucks and buses in the U.S. fleet, and their current fuel 

consumption and fuel efficiency were extracted from annual Department of Energy (DOE) 

publications.

21 Table 1-4 and Table 1-5 provide Energy Information Administration (EIA) 

statistics22 on the number of MD/HD Alternative Fuel Vehicles (AFVs) in use for 2010 and 

2011, respectively, by fuel type and weight class.

Table 1-4: Alternative-Fuel Vehicles in Use by Fleet Operators: 201023

Light Duty Medium Duty Heavy Duty Total

Ethanol — Flex Fuel 592,231 26,255 18 618,504

Natural Gas — Total 67,296 22,575 28,976 118,847

Compressed Natural Gas 67,296 22,521 26,000 115,817

Liquefied Natural Gas 0 54 2,976 3,030

LPG — Propane 76,694 27,771 36,057 140,522

Electric — Battery 56,566 86 779 57,451

Total 792,787 76,687 65,850 935,324

Table 1-5: Alternative-Fuel Vehicles in Use 2011 by Weight Class and Fuel Type24

Light 

Duty

Medium 

Duty

Heavy 

Duty Total

Ethanol — Flex Fuel 819,133 43,387 317 862,837

Liquid Petroleum Gas (Propane) 76,647 26,855 35,975 139,477

Natural Gas 66,147 23,473 32,030 121,650

Compressed Natural Gas 65,980 23,343 28,891 118,214

Liquefied Natural Gas 167 130 3,139 3,436

Electric — Battery 66,409 87 779 67,295

Hydrogen 425 1 101 527

Total 1,028,761 93,803 69,222 1,191,786

 21 Current Annual Transportation Energy Data Book posted at www.cta.ornl.gov/data 22 See Alternatives to Traditional Transportation Fuels 2010 Report Table at 

www.eia.gov/tools/faqs/faq.cfm?id=93&t=4

23 See www.eia.gov/renewable/afv/index.cfm 24 See Alternatives to Traditional Transportation Fuels 2011 report at www.eia.gov/renewable/afv/index.cfm
14

An APTA study found that as of January 2011 over 35 percent of public transit buses used 

alternative fuels (such as CNG or hybrid electric technology), and the adoption rate of fuel 

efficiency technologies has been accelerating.25 Recent DOE data26 on MD/HDVs show that 

trucks consumed 2,901,000 barrels per day or 21.4 percent of total transportation fuel use, while 

buses consumed 95.0 bblpd, representing only 0.7 percent of total transportation fuel use. The 

Class 8 trucks had the highest annual fuel use of about 13,000 GGE/year, followed by transit 

buses with 10,800 GGE/year, and refuse trucks with 10,000 GGE/year. The total for delivery 

trucks and school buses combined was 2,000 GGE/year. The DOE number of MD/HDVs by 

type, and fuel efficiency included:

• 8,356 registered single unit trucks with an average FE of 7.4 mpg;

• 2,617 combination trucks with average FE of 6 mpg; and

• 850,000 buses in the United States, of which 650,000 are school buses and the rest are

intercity buses and 65,363 transit buses averaging 7.1 mpg.27

The DOE Alternative Fuels Data Center (AFDC) case studies and the “Clean Cities Guide to 

Alternative Fuel and Advanced Medium and Heavy-Duty Vehicles28”discuss truck and bus fleets 

that successfully converted to, or adopted alternative fuels (CNG, LNG, biofuels) or hybridelectric propulsion.29 The DOE National Clean Fleets Partnership enlisted 28 of the largest 

corporate commercial fleet owners/operators, that successfully adopted FE technologies and/or 

alternative fuels, as discussed in a safety context in Sections 2 and 3.30

The rate of adoption over time for various FE technologies by truck fleets was studied by 

NACFE in annual benchmark reports31 focused primarily on cost recovery through fuel savings 

and operational efficiency gains, along with barriers to increased fleet penetration, but did not

address potential safety issues.32 NACFE annual surveys of “green” freight truck fleet managers 

were focused only on benchmarking large fleets adoption rates of, and payback time for FE 

technologies. NACFE adoption trends reported for specific FE technologies are also shown in 

Figure 2-12 and further discussed in Chapter 2. Similarly, CalHEAT, the California truck 

research center, conducted recent surveys of truck fleets regarding the market barriers to 

adoption of FE technologies and alternative fuels,33 again without addressing any associated 

potential safety concerns. Literature resources reviewed and findings on potential safety issues 

 25 See www.apta.com/mediacenter/pressreleases/2013/Pages/130422_Earth-Day.aspx 26 Posted DOE AFV data at: www.afdc.energy.gov/data/tab/all/data_set/10308 27 See BTS table 

www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/national_transportation_statistics/html/table_04_15.htm

l

28 See www.afdc.energy.gov/uploads/publication/medium_heavy_duty_guide.pdf 29 See www.afdc.energy.gov/case 30 See www.afdc.energy.gov/uploads/publication/60619.pdf 31 See NACFE-ICCT 2013 report at www.theicct.org/sites/default/files/publications/ICCT-NACFECSS_Barriers_Report_Final_20130722.pdf

32 See http://nacfe.org/ postings 33 See CalHEAT “Market Barriers And Opportunities For Alternative Fuel-Hybrid Systems” June 2013 at 

www.calstart.org/Libraries/CalHEAT_2013_Documents_Presentations/CalHEAT_Market_Barriers_and_Opportunit

ies_for_Alternative_Fuel-Hybrid_Systems.sflb.ashx
15

associated with MD/HDV fleet adoption of FE technologies and alt-fuels are discussed below in 

Chapter 2. 

In conclusion, although the national heavy-duty motor vehicles crash statistics for 2010-2011 

provide average baseline for fleet safety performance, these statistics could not be used for a 

safety analysis of specific FE technologies due to the lack of such information in crash field 

reports, and due to the very small number of such vehicles compared to the conventional 

MD/HDV fleet. 

A preliminary analysis of large trucks and bus crashes in 2010-2011 was conducted as part of 

this study to compare crash rates of conventional CMVs to the few FE–equipped “green” fleets. 

However, the small percentage of hybrid, electric, and alternative-fuels vehicles in the overall 

fleet resulted in an analysis that would rely on the statistics of small numbers. Therefore, 

comparisons between the crash rates of conventional fleets and green fleets was deemed 

inadvisable. Analysis over a longer time period, or after a larger penetration of FE technologies 

and/or alternative fuels occurs in the MD/HDV fleet, may allow for statistically meaningful 

comparisons.
16

2 REVIEW OF LITERATURE ON ADOPTION OF FE TECHNOLOGY CLUSTERS, 

ALTERNATIVE FUELS, AND OPERATIONAL SAFETY OF MD/HDV FLEETS

2.1 Introduction

A comprehensive literature survey was conducted to identify any MD/HDV safety issues 

regarding existing and emerging FE technologies and alternative fuels. Resources reviewed 

included research reports, technical articles, and news items posted on the Internet; conference 

proceedings and presentations; annual statistical data on truck and bus safety and energy 

consumption; Congressional hearings, and trade association surveys and testimonies, etc. The 

report focus was on extracting any MD/HDV safety information specific to the NHTSA and 

EPA-approved FE technologies from key sources (e.g., EPA SmartWay, DOE Advanced Vehicle 

Technologies- AVT and 21st Century Truck, DOD/TARDEC, CALSTART, and NRC FE 

technology reports); crash safety from IIHS, NHTSA, and FMCSA MCMIS databases; FHWA 

reports on extra-heavy trucks and trailers; regulatory information from the NHTSA R&D and 

regulatory plan, commercial fleet truck and bus trade associations; Congressional testimony;

newsletters, etc.).

The bibliography database, comprising over 500 references, was organized as an Excel 

spreadsheet, which can be “filtered” by category to facilitate data mining and retrieval by topic 

(vehicle safety, FE technology, alt-fuels, operational best practices, regulatory documents). 

Separate bibliographies were compiled by topic, i.e., specific FE technology applied to 

MD/HDVs, safety statistics and analyses, alternative fuels, operational and behavioral issues, 

and green fleet vehicle deployment. (See Appendices 7.1 to 7.6.) Sections 2.2-2.9 describe the 

literature findings on MD/HDV safety issues, organized by the specific FE technology and/or 

alternative fuel wherever possible. References cited relating to MD/HDV fleet deployment of 

specific FE technologies and alternative fuels provide the general overview of their current 

adoption and penetration. However, most references offer little to no quantitative statistical data 

regarding potential hazards, and only sparse evidence regarding the safety impacts of individual 

FE technologies, alternative fuels, and related driver operation factors.

2.2 Adoption of Alternative Clean Fuels 

2.2.1 Natural Gas Benefits and Safety Issues

Natural gas is a domestically produced gaseous fuel readily available through the utility 

infrastructure. It is an odorless, nontoxic, gaseous mixture of hydrocarbons—predominantly 

methane (CH4), which must be odorized for leaks to be detectable. At high concentrations it is a 

potential asphyxiant, as well as a fire or explosion hazard; methane is also a powerful 

Greenhouse Gas (GHG). Because of the gaseous nature of this fuel, it must be stored onboard a 

vehicle either as compressed natural gas (CNG) or liquefied natural gas (LNG).34

 34 DOE EERE. 2012d. National Clean Fleets Partnership 

www1.eere.energy.gov/cleancities/national_partnership.html
17

CNG is used in light-, medium-, and heavy-duty applications. To provide adequate driving 

range, it is stored onboard a vehicle in cylinders at a pressure of 3,000 to 5,000 pounds per 

square inch (psi). The AFDC states that a CNG-powered vehicle has approximately the same 

fuel economy as a conventional gasoline vehicle on a gasoline-gallon-equivalent (GGE) basis.35

LNG is produced by purifying natural gas and super-cooling it to -260°F to turn it into a liquid. 

Because it must be kept at cold temperatures, LNG is stored in double-walled, vacuum-insulated 

pressure vessels at up to 230 psi. LNG is good for trucks needing a longer range because liquid is 

more dense than gas (CNG) and more energy can be stored by volume in a given tank. LNG is 

typically used in medium- and heavy-duty vehicles. A GGE equals about 1.5 gallons of LNG.

Propane is another hydrocarbon alternative fuel (C3H8) fuel sometimes referred to as liquefied 

petroleum gas (LPG) or autogas. It is produced domestically from both natural gas processing 

and crude oil refining. It is also nontoxic, colorless, and virtually odorless. As with natural gas, 

an identifying odor is added so the gas can be readily detected.

2.2.1.1 Penetration/Adoption

CNG vehicles are currently more commonplace than LNG vehicles. Using CNG as fuel is 

currently best suited for vehicles that are housed and fueled at a common location, such as public 

transit vehicles, refuse trucks, and delivery fleets because of refueling infrastructure and special 

handling requirements. Modest growth of CNG fueling stations has been spurred by the Energy 

Policy Act of 1992, and demand is expected to rise further.36 However, even though the United 

States has about 180,000 gasoline stations, there are only about 1,000 CNG stations, with only 

half open to the public.37 

Large vehicles are necessary to accommodate the cryogenic fuel tanks used to store LNG, which 

are 70 percent larger by volume than a comparable diesel tank. (Compressed gas needs about six 

times as much space as diesel, even when squeezed down to 3,000 psi.) LNG systems are 

typically more expensive than CNG systems, as is the fuel itself, but LNG vehicles can exceed

300 miles before needing to be refueled, in contrast to CNG vehicles. Therefore, LNG is 

considered an alternative for both long-haul (thousands of miles) and short-haul (hundreds of 

mile) trucking. However, until LNG infrastructure is commonplace, it is unlikely to become a 

widespread alternative fuel.38

 35 See Natural Gas Fuel Basics: 1 GGE equals about 5.66 pounds of CNG, at 

www.afdc.energy.gov/fuels/natural_gas_basics.html

36 Carbon War Room Research & Intelligence Group. 2012. Unlocking fuel-saving technologies in trucking and 

fleets

http://carbonwarroom.com/sites/default/files/reports/Unlocking%20Fuel%20Saving%20Technologies%20in%20Tru

cking%20and%20Fleets%20(Carbon%20War%20Room).pdf

37 Garthwaite, J. 2011. Trading Oil for Natural Gas in the Truck Lane 

http://news.nationalgeographic.com/news/energy/2011/09/110902-natural-gas-fuel-for-trucks 38 See Carbon War Room Research & Intelligence Group. 2012. Unlocking fuel-saving technologies in trucking and 

fleets 

http://carbonwarroom.com/sites/default/files/reports/Unlocking%20Fuel%20Saving%20Technologies%20in%20Tru

cking%20and%20Fleets%20(Carbon%20War%20Room).pdf
18

Even though natural gas has long been used to power vehicles, only about 0.1 percent of current 

U.S. natural gas production is currently used for transportation fuel.39 According to NGV 

America, a trade association, the amount of diesel fuel currently used annually for highway 

travel is equivalent to six trillion cubic feet of natural gas. Current national natural gas demand is 

in the range of 22 trillion cubic feet a year, suggesting that the diesel use could in principle be 

displaced by natural gas. 

Compressed natural gas (CNG) consumption for transportation has increased steadily since 1995, 

as shown in Figure 2-1. As discussed in Section 2.2.3, the use of propane, which was the most 

common alternative vehicle fuel in the early 1990s, has trended downward as CNG (and to a 

lesser extent LNG) has become more popular.

Figure 2-1: Estimated Consumption of Alternative Fuel by AFVs in the United States (Ref. 15)

Natural gas vehicles have been in use in the United States since the early 1970s, especially by 

urban transit bus “clean” fleets, and, to a lower extent, by personal or fleet dual-fueled vehicles, 

i.e., vehicles that can operate on two different fuels. By 2010, there were 115,863 CNG and 

3,354 LNG vehicles across all weight classes in the United States (Figure 2-2), which amounts to 

less than one percent of the 15 million NGVs globally, the vast majority of which are light-duty 

vehicles.40,41 A total of 43,088 commercial vehicles were using natural gas in 2010, of which 

approximately 35,000 were medium or heavy duty. 

 39 DOE/EERE 2012, Alternative Fuels and Advanced Vehicles Data Center (AFDC), 

www.afdc.energy.gov/afdc/fuels/natural_gas.html

40 Ibid. 

0

50

100

150

200

250

Million GGEs

CNG

Propane

E85

LNG

Electric

Hydrogen

E95

Methanol

(M85)

Methanol

M100

www.afdc.energy.gov/afdc/data
19

Figure 2-2: Alternative Fueled Vehicles in Use in the United States42

Adoption of natural gas in transit bus fleets increased rapidly in the late 1990s, growing from 2.8 

percent of buses in 1996 to 18.6 percent in 2011. More than 40 North American transit agencies 

now use buses powered by CNG. In 2011, the Los Angeles County Metropolitan Transportation 

Authority became the first major U.S. transit agency with a fleet 100 percent equipped with 

alternative fuel technologiesalmost all CNG. 43

The 21,510 natural gas-powered transit buses comprise almost two-thirds of U.S. MD/HDV 

NGVs.44 In heavy-duty vehicles, both dedicated natural gas and dual-fuel, compression-ignited 

engines are available. The dual-fuel engines are slightly more fuel-efficient than the sparkignited dedicated engines, but they also increase the complexity of the fuel storage system by 

requiring storage of both types of fuel. 45

 41 Carbon War Room Research & Intelligence Group. 2012. Unlocking fuel-saving technologies in trucking and 

fleets 

http://carbonwarroom.com/sites/default/files/reports/Unlocking%20Fuel%20Saving%20Technologies%20in%20Tru

cking%20and%20Fleets%20(Carbon%20War%20Room).pdf

42 DOE EERE. 2012a. Alternative Fuels and Advanced Vehicles Data Center

www.afdc.energy.gov/afdc/fuels/natural_gas.html

43 Gentry, W. 2012. Statement of William Gentry, President, Gentry Trailways and on behalf of American Bus 

Association and United Motorcoach Association Subcommittee on Highways and Transit, House Transportation and 

infrastructure Committee, U.S. House of Representatives 

http://republicans.transportation.house.gov/Media/file/TestimonyHighways/2012-09-13-Gentry.pdf 44 Kwan, Q. 2012. Safety Considerations Related to Commercial Vehicles Using Natural Gas 

www1.eere.energy.gov/cleancities/pdfs/ngvtf12_kwan.pdf

45 DOE EERE. 2012a. Alternative Fuels and Advanced Vehicles Data Center 

www.afdc.energy.gov/afdc/fuels/natural_gas.html

0

50

100

150

200

250

300

350

400

Thousand Vehicles

Electric

LNG

CNG

Propane
20

LNG has not been widely deployed in transit systems to date,46 although over 1,000 transit buses 

were operational in 2003 with seven public transit agencies operating LNG powered buses.47 

According to the DOE Alternative Fuels and AFDC, there were 836 public and private CNG 

stations and 38 LNG stations in the United States as of June 2, 2010.

Examples of significant adoption of natural gas by truck fleets include those below:

• United Parcel Service (UPS) operates about 1,100 vehicles running on LNG or 

compressed natural gas (CNG in its global fleet, including about 60 LNG heavy tractor 

trucks.48 These new vehicles will reduce greenhouse gas emissions by 25 percent 

compared to the older generation diesel trucks they replaced. The original demonstration 

fleet of 11 vehicles shuttling between California and Las Vegas showed UPS that LNG 

trucks could handle demanding situations like hauling multiple trailers over mountain 

ranges.49 Out of its 17,000 tractor-trailers, UPS would like to convert 1,000 to liquefied 

natural gas, depending on the availability of refueling infrastructure. UPS is also using 

propane, electric and hybrid electric vehicles to help the company reach its 20 percent 

mpg improvement goal for the entire U.S. package delivery fleet between 2000 and 

2020.50

• Waste Management has over 1,000 LNG trucks in its fleet. A third of its California 

fleet is fueled by LNG derived from the decomposition of organic waste in the 

company’s Altamont Landfill in Livermore, California. Waste Management has CNG 

and LNG fueling stations at 17 of its facilities throughout North America with more 

under development.51 Waste Management began operating a fleet of heavy-duty refuse 

trucks powered by LNG in Pennsylvania as early as 1997. This implementation was 

considered successful, although the LNG trucks had a 9 percent to 12 percent lower fuel 

economy versus diesel. 52

• South Coast Air Quality Management District (SCAQMD) Heavy-Duty Natural Gas 

Drayage Truck Replacement Program in California purchased 219 LNG trucks in 2008, 

 46 Friedman, D. M., & Malcosky, N. D. 1996. Liquefied Natural Gas Safety in Transit Operations: Final Report

http://trid.trb.org/view.aspx?id=713273

47 See www.afdc.energy.gov/pdfs/mod04_lng.pdf and Gentry, W. 2012. Statement of William Gentry, President, 

Gentry Trailways and on behalf of American Bus Association and United Motorcoach Association Subcommittee on 

Highways and Transit, House Transportation and infrastructure Committee, U.S. House of Representative

http://republicans.transportation.house.gov/Media/file/TestimonyHighways/2012-09-13-Gentry.pdf 48 “UPS press release Feb.22, 2011 “UPS Clearing the air with new LNG tractors” at www.pressroom.ups.com 49 Wald, M. 2011. U.P.S. Finds a Substitute for Diesel: Natural Gas, at 260 Degrees Below Zero 

http://green.blogs.nytimes.com/2011/02/22/u-p-s-finds-a-substitute-for-diesel-natural-gas-at-260-degrees-belowzero/

50 Environmental Leader. 2011. UPS Adds to its Natural Gas Truck Fleet 

www.environmentalleader.com/2011/02/25/ups-adds-to-its-natural-gas-truck-fleet 51 Work Truck Magazine. 2011. Waste Management Adds 1,000th Natural Gas Truck to Fleet 

www.worktruckonline.com/Channel/Fuel-Management/News/Story/2011/07/Waste-Management-Adds-1-000thNatural-Gas-Truck-to-Fleet.aspx 52 Clark, K. C. P. N. N. 2001. Waste Management's LNG Truck Fleet Final Results 

www.afdc.energy.gov/pdfs/waste_lng_final.pdf stated: “This fuel economy result shows considerable improvement 

compared to other heavy-duty natural gas engine products.”
21

all of which were scheduled to be deployed by the end of 2012. These heavy-duty trucks 

completed 5.3 million LNG-powered miles by the end of 2011.53

• Green Energy Oilfield Services operates 60 LNG-powered vacuum and winch trucks 

supporting its gas drilling operations.54

• AT&T’s fleet of 73,500 vehicles includes 3,000 NGVs, and by 2013, the company 

reportedly plans to increase the number of natural gas vehicles to 8,000.55

According to a recent economic report, the widespread fleet adoption of LNG heavy-duty trucks 

still faces hurdles, including the scarcity of LNG retail outlets, refueling infrastructure stations, 

uncertainty over future governmental support, and the need for new training and logistics. 

Further study in the areas of hardware maintenance, availability of replacement parts, and the 

long-term durability of cryogenic fuel tanks would enable better long-term assessments of 

potential safety effects.

56

2.2.1.2 Benefits

Natural gas vehicles (NGVs) have similar power, acceleration, and cruising speed to gasoline or 

diesel vehicles. The driving range of NGVs is generally lower than that of comparable gasoline 

and diesel vehicles because less energy content can be stored in a natural gas tank of the same 

size as that of gasoline or diesel fuels. Extra natural gas storage tanks, or the use of LNG instead 

of CNG, can increase the range for large vehicles. However, this imposes a fuel economy penalty 

due to the added weight and reduces the amount of useable space in a vehicle.57 A bus with an 

LNG fuel system weighs approximately 1,000 pounds less than one with a CNG fuel system and 

has a longer range. One gallon of LNG contains about 60 percent of the energy in a gallon of 

diesel fuel, while CNG at 3,600 pounds per square inch (psi) pressure contains only about 30 

percent. 

Natural gas burns cleaner than gasoline or diesel, and it can help ensure compliance with the 

EPA Clean Air and the recent NHTSA/EPA MD/HDV fuel efficiency and GHG emissions rules. 

Natural gas offers life cycle GHG emissions benefits over conventional fuels, as well as a 

reduction in EPA regulated tailpipe emissions.58

Assessments of the greenhouse gas benefits of natural gas fuels vary, but they generally show 

that on an energy basis (grams per million BTU or grams per megajoule) CNG has 15-20 percent

 53 DOE EERE. 2012c. Largest U.S. Port Complex Embraces LNG for Heavy-Duty Trucks 

www.afdc.energy.gov/case/1203

54 Work Truck Magazine. 2012. Green Energy Oilfield Services Builds Green Truck Fleet 

www.worktruckonline.com/News/Story/2012/02/Green-Energy-Oilfield-Services-Builds-Green-Truck-Fleet.aspx 55 Garthwaite, J. 2011. Trading Oil for Natural Gas in the Truck Lane 

http://news.nationalgeographic.com/news/energy/2011/09/110902-natural-gas-fuel-for-trucks/ 56 Oil and Gas Journal. 2012. LNG seen viable fuel for heavy-duty trucks

www.ogj.com/articles/2012/06/lng-seen-viable-fuel-for-heavy-duty-trucks.html 57 Friedman, D. M., & Malcosky, N. D. 1996. Liquefied Natural Gas Safety in Transit Operations: Final Report 

http://trid.trb.org/view.aspx?id=713273

58 DOE EERE. 2012a. Alternative Fuels and Advanced Vehicles Data Center

www.afdc.energy.gov/afdc/fuels/natural_gas.html
22

lower GHG emissions than diesel fuel, while LNG has 3-9 percent lower GHG emissions.59 The 

lower benefits from LNG are due to a higher energy requirement for liquefaction of LNG 

compared to compression of CNG. However, natural gas engines typically have lower efficiency 

than diesel engines, so some of the GHG benefit of natural gas fuels is reduced in practice, i.e., 

per-mile GHG reductions from the use of natural gas fuels instead of diesel will be lower due to 

greater fuel use per mile. In the future, these benefits may increase if natural gas supplies are 

blended with renewable natural gas captured from the natural decay of organic materials, e.g., in 

landfills. This biomethane could reduce life cycle carbon emissions by 85-90 percent when 

compared with diesel and gasoline, as shown in Figure 2-3.

60

Figure 2-3: Carbon Intensity of Various Fuels in California HDVs. 61

2.2.1.3 Safety Considerations of CNG-Fueled MD/HDVs

Since LNG and CNG tanker trucks are considered hazardous material transport, they are 

regulated by PHMSA and were not considered in this literature review. Only the crash rates of 

vehicles propelled by natural gas were considered.

Natural gas has certain safety advantages compared to gasoline and diesel: it is non-toxic, lighter 

than air, and dissipates rapidly when released. An odorant is added to provide a distinctive smell 

that is easy to recognize, and it is detectable at 20 percent of the lower flammability limit.62 

Unlike diesel and gasoline leaks, which puddle on the ground and can create an on-going hazard 

 59 Argonne National Laboratory Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation 

(GREET) Model, https://greet.es.anl.gov/ 60 Natural Gas Vehicles for America. 2012. NGVs and the Environment

www.ngvc.org/about_ngv/ngv_environ.html

61 DOE EERE. 2012a. Alternative Fuels and Advanced Vehicles Data Center 

www.afdc.energy.gov/afdc/fuels/natural_gas.html

62 Clean Vehicle Education Foundation. 2010. How Safe are Natural Gas Vehicles?

www.cleanvehicle.org/committee/technical/PDFs/Web-TC-TechBul2-Safety.pdf

95 92

83 82

75 75

46

39

16 14 13

0

10

20

30

40

50

60

70

80

90

100

ULSD LNG Biodiesel

(Soy)

Renew.

Diesel

(Soy)

CNG Hydrogen

(NG)

Electricity Renew.

Diesel

(Tallow)

Biodiesel

(Grease)

Dairy CNG Landfill

CNG

Carbon Intensity (adjusted gCO2e/MJ)

www.afdc.energy.gov/afdc/data/
23

over a wide area, leaking natural gas tends to rise and dissipate to non-hazardous levels quickly, 

with only a short, vertical column directly above the leak in which the gas mixture is flammable. 

Natural gas leaks therefore pose little fire or explosion risk if the gas is leaking into open air. 

However, if the leak is into an enclosed space (either a building or an enclosed space on the 

vehicle) the resulting fire and explosion hazard can be significant, depending on the size of the 

leak.

While natural gas is non-toxic, it can displace the air in a pit, and/or reduce oxygen concentration 

levels, and potentially pose an asphyxiation hazard to workers. Hazards relating to CNG also 

include fire, thermal explosion if release is into an enclosed space, and mechanical rupture of 

pressure vessel. Gas release can occur from a fuel system leak or from activation of a PRD. 

Ignition can also result from contact with hot surfaces, open flames, and sparks, including static 

electricity.

63 There are four basic types of CNG tank designs: Type 1- all steel, Type 2- metal liner 

with hoop wrapped composite; Type 3 metal liner with fully wrapped composite; and Type 4- plastic 

liner with fully wrapped composite (glass fiber or carbon fiber may be used). Onboard CNG storage 

tank pressures range from 250 bars, 350 bars, to 700 bars, but all must have pressure relief valves. 

At a CNG fueling station, the gas is compressed before being supplied to vehicles at 3,000 to 

3,600 psi. This stored energy represents a potential hazard not found in unpressurized diesel or 

gasoline fuel tanks. CNG fuel tanks can release significant energy if the tank suddenly fails. 

Such failures are rare, but have occurred. According to one database, between 2000 and 2008 

there were only 26 CNG cylinder failures worldwide, which is a very small dataset for predictive 

failure analysis.64 Figure 2-4 lists the causes of these failure incidents. The leading causes of 

catastrophic cylinder ruptures were vehicle fires and environmental damage, such as corrosion. 

Several ruptures occurred when exposure to a fire on board the vehicle failed to activate the 

cylinder Pressure Relief Devices (PRDs), and the cylinder pressure increased until explosion.

Safety in the design and operation of the CNG refueling infrastructure is partly addressed by 

National Fire Protection Association (NFPA) Code 52, Vehicular Gas Systems Code, which 

requires minimum venting rates for indoor facilities as well as pressure relief devices on all 

storage systems that discharge to a sufficiently vented indoor or to an outdoor area. There is also 

a recommended practice from the Society of Automotive Engineers, SAE J2406, Recommended 

Practices for CNG Powered Medium and Heavy-Duty Trucks. Both of the above are voluntary.

A 2005 probabilistic risk assessment study of CNG bus safety65 concluded that CNG buses were 

2.5 times more prone to fire fatality risk than diesel buses. The study also estimated that CNG 

bus passengers were at up to two orders of magnitude increased fire fatality risk: an estimated 

0.16 fatalities per 100-million miles compared to (based on historical data) 0.0007 per 100-

million miles for diesel school buses. This study used historical component failure data from the 

 63 NHTSA. 2010a. Study of Heavy Truck S-Cam, Enhanced S-Cam, and Air Disc Brake Models Using NADS 

www.nhtsa.gov/DOT/NHTSA/NVS/Vehicle%20Research%20&%20Test%20Center%20(VRTC)/ca/capubs/811314

.pdf

64 Wong, J. 2009. CNG & Hydrogen Tank Safety, R&D, and Testing 

www1.eere.energy.gov/hydrogenandfuelcells/pdfs/cng_h2_workshop_8_wong.pdf

65 “Compressed natural gas bus safety: a quantitative risk assessment,” by S. Chamberlain and M. Modarres, in Risk 

Analysis, April 2005, vol25(2) 377-387, at www.ncbi.nlm.nih.gov/pubmed/15876211
24

aerospace and process industries to estimate the frequency and consequences of different fire and 

explosion scenarios. One major limitation of this analysis was its reliance on generic component 

failure data rather than on CNG-specific hardware failure data. The analysis also did not attempt 

to model the physics of fatigue and corrosion failure modes of these components, which would 

be needed to increase the accuracy of the estimates.

A 2010 Clean Vehicle Foundation survey of 8,331 natural gas utility, school bus, municipal, and 

business fleet NGVs that traveled 178.3 million miles66 indicated that the injury rate was 37 

percent lower than the gasoline fleet vehicle injury rate. The study also reported no fatalities 

compared with 1.28 deaths per 100 million miles for gasoline fleet vehicles. The reported 

collision rate for NGV fleet vehicles was 31 percent lower. NGVs were involved in seven fire 

incidents, with only one incident directly attributable to failure of the natural gas fuel system.67

This study indicates that NGVs may operate more safely than gasoline or diesel vehicles.

Ten transit agencies, representing 4,071 CNG buses, reported that the mean time between 

failures (MTBF) for their CNG buses ranged from 58 percent to 80 percent of diesel buses, 

indicating lower reliability, although not necessarily lower safety.68 The potential safety issues 

that respondents raised were fire safety, explosion, and toxicity and health. However, most 

respondents did not consider the fire risk of CNG higher than that of diesel. 

Another survey of recent transit industry practice drew on seven transit agencies, which stated 

that their CNG buses performed well and have achieved broad public acceptance. Differences 

between agencies in hazard and consequence mitigation measures were noted, such as 

inconsistent implementation of methane monitoring in CNG bus facilities, procedures and 

actions to detect methane, and comprehensive control of strong ignition sources.69

 66 Adams, R. and D.B Horne, 2010b. “CNG Transit Experience Survey 2009-2010” (Clean Vehicle Education 

Foundation and NREL report NREL/SR-7A2-48814

September 2010) at www.afdc.energy.gov/pdfs/48814.pdf

Adams, R. 2010b. CNG Transit Experience Survey (Clean Vehicle Education Foundation) 

www1.eere.energy.gov/cleancities/pdfs/ngvtf10_trans_lessons_learned.pdf

68 Adams, R. 2010b. CNG Transit Experience Survey (Clean Vehicle Education Foundation)

www1.eere.energy.gov/cleancities/pdfs/ngvtf10_trans_lessons_learned.pdf 69 Friedman, D. N., & Malcosky, N. D. 1995. Compressed Natural Gas Safety in Transit Operations: Clean Air 

Program http://144.171.11.39/view.aspx?id=573745
25

Figure 2-4: CNG Tank Failures Frequencies by Cause70

To date, there have been relatively few CNG cylinder failure incidents, including only two 

leading to fatalities71,72 that have occurred on the approximately 115,000 CNG vehicles in the 

United States There were eight cylinder ruptures in the nine-year period between 1993 and 2001; 

about one rupture per 56,000 cylinder-years. Three of the eight CNG cylinder ruptures prior to 

2002 were caused by glass fiber stress corrosion cracking due to battery acid exposure. Four 

other tanks ruptured due to severe physical tank damage, and one tank failed due to 

overpressurization. However, another CNG bus destroyed by an engine fire did not have CNG 

cylinders explode. In this case, their PRDs opened and released the gas, which was then ignited 

by the primary fire and produced a jet flame.73 There have been similar cases of vehicle fires 

resulting in CNG cylinder PRD actuation and subsequent cylinder depressurization.74 Other 

 70 Wong, J. 2009. CNG & Hydrogen Tank Safety, R&D, and Testing 

www1.eere.energy.gov/hydrogenandfuelcells/pdfs/cng_h2_workshop_8_wong.pdf 71 Clean Vehicle Education Foundation. 2010. How Safe are Natural Gas Vehicles? 

www.cleanvehicle.org/committee/technical/PDFs/Web-TC-TechBul2-Safety.pdf 72 Clean Vehicle Education Foundation. 2012. Fatal accident removing cylinder solenoid valve 

www.cleanvehicle.org/technology/ValveFatality.pdf

73 Gambone, L. 2005. CNG Cylinders 101

www.cleanvehicle.org/technology/CNGCylinderDesignandSafety.pdf

74 Zalosh, R. 2009. CNG and Hydrogen Vehicle Fuel Tank Failure Incidents, Testing, and Preventive Measures 

www.mvfri.org/Contracts/Final%20Reports/CNGandH2VehicleFuelTankPaper.pdf
26

cylinder failures were attributable to a less catastrophic leakage event, including fewer than 50 

Type 1 steel cylinder pinhole leaks and hundreds of Type 4 plastic liner leaks.75

The CNG safety incident reports provided by the Clean Vehicle Education Foundation and other 

cited sources show that in most cases incidents occur during or shortly after refueling, or in a 

CNG vehicle fire.76

CNG cylinder safety depends on a wide range of factors including: 

• Handling procedures;

• Installation design and procedure;

• Operational and maintenance procedures;

• Inspection program and procedures and inspection personnel qualifications;

• Safety management systems for refueling facilities, vehicle workshops, and parking 

facilities; and

• Emergency response program and procedures.

For example, unauthorized drilling or grinding of cylinder surfaces has been reported in some 

CNG vehicle workshops. There are also reports of cylinder inspectors walking on roof-mounted 

composite cylinders on top of a CNG bus during in-situ inspection. Such unsafe work practices 

may indicate a gap in training for workshop staff and their supervisors, or gaps in the work 

procedures.77

The most recent CNG failure occurred as a result of human error when the owner attempted to 

remove a valve from a cylinder that had not been properly vented. 78 The cylinder exploded, 

killing the owner.79

A recent, non-fatal cylinder failure caused a delivery truck explosion in Los Angeles while

employees of California Linen Services were refueling, causing serious injury.80,81 In another 

incident, high outdoor temperature caused pressure to build up inside the fuel tank of a propanepowered truck, which activated the PRD. When the driver of the vehicle tried to stop the leaking

 75 Wong, J. 2009. CNG & Hydrogen Tank Safety, R&D, and Testing 

www1.eere.energy.gov/hydrogenandfuelcells/pdfs/cng_h2_workshop_8_wong.pdf 76 Hien Ly. 2011. Keeping Up the Good Safety Record of CNG Cylinders

www.ngvglobal.com/keeping-up-the-good-safety-record-of-cng-cylinders-0803 77 Ibid. 78 Clean Vehicle Education Foundation. 2012. Fatal accident removing cylinder solenoid valve 

www.cleanvehicle.org/technology/ValveFatality.pdf

79 News On 6. 2012. Broken Arrow Police: Business Owner Killed When CNG Tank Explodes 

www.newson6.com/story/17393515/broken-arrow-police-call-mans-death-accidential-death 80 NHTSA. 2006. Class 8 Truck Tractor Braking Performance Improvement Study – Low Coefficient of Friction 

Performance and Stability Plus Parking Brake Evaluations of Four Foundation Brake Configurations 

www.nhtsa.gov/DOT/NHTSA/NRD/Multimedia/PDFs/VRTC/ca/capubs/DOTHS809753.pdf 81 CNG Chat. 2009. CNG Tank Failure in Los Angeles http://cngchat.com/forum/archive/index.php/t-4581.html
27

gas, static electricity caused gas ignition.82 Both LPG and CNG cylinders have the potential for 

ignition near a venting PRD.

There have been reported complaints about fumes due to vented fuel inside a CNG vehicle. In 

Tulsa, Oklahoma, 140 public school buses that had used CNG since 1988 were checked 

following driver complaints. The school district considered installing carbon monoxide detectors 

in the buses83 (although the reference noted that NGVs produce 70-90 percent less carbon 

monoxide in the exhaust than conventional vehicles).84

The literature indicates that some road and environmental conditions pose severe operating 

environments for pressure vessels: Vehicles may operate in temperature extremes that affect the 

mechanical properties of the cylinder. Multiple fills result in repetitive pressure changes that can 

lead to fatigue cracking over thousands of cycles. Exposure to road environments, including road 

salt, can lead to corrosion or abrasion, and there is vibration whenever the vehicle is in motion. 

Incidents such as vehicle fires and collisions can also thermally and/or mechanically damage 

CNG cylinders. 

The NHTSA regulations that apply to CNG vehicles are FMVSS 301 (Fuel System Integrity), 

FMVSS 303 (Fuel System Integrity of Compressed Natural Gas Vehicles) and FMVSS 304 

(CNG Fuel Container Integrity).85 Of these regulations, only FMVSS 301 applies to MD/HDVs, 

while the other two are only applicable to light-duty vehicles. These regulations specify tests or 

installation requirements for CNG tanks and vehicles to account for each of these potential 

conditions.86 Testing of CNG cylinders for in-service safety certification includes damage 

scenarios such as impact, abrasion, gunshot, and bonfire. Cylinders are designed not to rupture 

when fully fueled over six times a day, 365 days a year; far beyond what they will see in service. 

CNG cylinders (Types 1-4) are designed with a pressure safety factor of two, i.e., they should be 

able to contain a pressure up to twice the maximum fill pressure.87 The cylinders are designed 

for a specific lifetime from 15 up to 25 years, and are required to be permanently labeled with 

the statement “This container should be visually inspected after a motor vehicle accident or fire 

and at least every 36 months or 36,000 miles, whichever comes first, for damage and 

deterioration.” (FVMSS 304, § 571.304, S7.4 (g))88

 82 Morgenstern, M. 2012. Propane tank explodes in McDonald's drive-through www.theblaze.com/stories/propanetank-explodes-in-mcdonalds-drive-through/ 83 Mills, R. 2011. TPS investigates claims that CNG-fueled buses made bus drivers sick; The district says it checked 

out all 140 CNG buses in its fleet www.krmg.com/news/news/local/tps-investigates-claims-cng-fueled-buses-madebus-/nFhNq/ 84 Natural Gas Vehicles for America. 2012. NGVs and the Environment 

www.ngvc.org/about_ngv/ngv_environ.html

85 NHTSA. 2009. FMVSS 304 CNG fuel container integrity 

www.nhtsa.gov/DOT/NHTSA/NVS/Crashworthiness/Alternative%20Energy%20Vehicle%20Systems%20Safety%2

0Research/811150.pdf

86 Wong, J. 2009. CNG & Hydrogen Tank Safety, R&D, and Testing 

www1.eere.energy.gov/hydrogenandfuelcells/pdfs/cng_h2_workshop_8_wong.pdf

87 Ibid. 88 See FMVSS 304 and ANSI CSA NGV2 standard: “Periodic In-Service Inspection Requirements (Sec. 4.1.4, 

NGV2):“Each container shall be visually inspected at least every 36 months, or at the time of any re-installation, for 

external damage and deterioration...” CNG cylinder inspection guidelines are posted at 
28

The Clean Vehicle Education Foundation (CVEF) sponsored by the DOE National Energy 

Technology Lab (NETL) developed a “CNG Fuel System Inspector Study Guide”89 with 

industry stakeholders, which is required for certification of CNG fuel system and cylinder 

inspectors by CSA America. The CVEF also posted and maintains a comprehensive list of 

“Codes, Standards and Advisories Applicable to Natural Gas Vehicles and Infrastructure.”90

The literature indicates that aftermarket vehicle conversions to CNG, which are not within 

NHTSA’s scope, represent over 50 percent of failure incidents, as shown in Figure 2-5. 

Figure 2-5: Prevalence of Aftermarket Conversions in CNG Failures91

2.2.1.4 Safety Considerations for LNG-Fueled Vehicles

From a material properties perspective, LNG poses different potential hazards than compressed 

natural gas (CNG). The energy density of LNG is greater than for CNG so more fuel can be 

stored onboard. This makes LNG well suited for Class 7 and 8 trucks that need a greater range.

Because it is a cryogenic liquid, spills of LNG can lead to a heavier-than-air vapor cloud that can 

travel horizontally from the spill site and then ignite some distance away.92 In contrast, CNG is 

 www.transecoenergy.com/pages/CNG_Tank.htm; and www.cleanvehicle.org/committee/technical/PDFs/Web-TCTechBul1-ContainerInspection.pdf 89 See http://cleanvehicle.org/technology/Study_Guide_Final.pdf

90 See Codes, Standards and Advisories Applicable to Natural Gas: 

www.cleanvehicle.org/technology/image/code.pdf 91 Wong, J. 2009. CNG & Hydrogen Tank Safety, R&D, and Testing 

www1.eere.energy.gov/hydrogenandfuelcells/pdfs/cng_h2_workshop_8_wong.pdf
29

lighter than air, and rises and dissipates when leakage occurs. Because of LNG's extremely low 

temperature, odorants cannot be added, resulting in a potential increased hazard in the event of a 

leak, as workers cannot detect an odor.93 There is a safety advantage of LNG over CNG: since 

the tanks are only pressurized to 150-230 psi, LNG systems are not exposed to the high-pressure 

explosion hazard of CNG systems.94

An LNG refueling station has a higher inventory of cryogenic fuel onsite, and therefore of stored 

energy, than a pipeline-fed CNG refueling station. Hazards relating to LNG include: fire, 

cryogenic burns, changes in properties of contacted materials (e.g., cryogenic embrittlement), 

and asphyxiation (by displacing oxygen). Ignition can result from contact with hot surfaces, open 

flames, and sparks, including static electricity.95

The low number of LNG vehicles currently in service in the United States means there is 

relatively little statistical safety data to draw on. Approximately 20-30 public safety incidents 

involving LNG vehicles have been reported since 1971 in the United States and Europe.96

Almost all incidents were vehicle crashes. Fewer than half of the incidents resulted in a loss of 

cargo and only two of the incidents led to fires. Notably, there have been no fatalities reported 

from LNG vehicle crashes, and only the drivers of the LNG vehicles have been seriously injured. 

While the research literature indicate that the overall safety record of LNG trucks is good, in 

2002 an LNG truck in Spain exploded in a boiling liquid, expanding-vapor type explosion.97 

Penetration of an LNG vehicle fuel tank below the liquid level could result in rapid ejection of 

LNG and formation of an ignitable vapor cloud. However, LNG tanks are designed to be more 

robust pressure vessels than conventional diesel fuel tanks, making a penetration of an LNG tank 

less likely than for a similarly mounted diesel-fuel tank. The National Petroleum Council cites 

several severe crashes or fires of LNG-powered trucks and buses in which the LNG tanks were 

not penetrated.98 Some have suggested that the long-term durability of cryogenic fuel tanks still 

warrants further study.99

Venting from pressure relief devices on LNG vehicles parked for prolonged periods is expected 

as the liquefied gas evaporates. As a result, LNG vehicles must be stored outside or in facilities 

designed to collect and discharge methane vapor releases, so that gas releases cannot build up in 

enclosed spaces. 

 92 Whyatt, G. 2010. Issues Affecting Adoption of Natural Gas Fuel in Light- and Heavy-Duty Vehicles 

http://s3.amazonaws.com/zanran_storage/www.pnl.gov/ContentPages/184758856.pdf

93 Friedman, D. M., & Malcosky, N. D. 1996. Liquefied Natural Gas Safety in Transit Operations: Final Report 

http://trid.trb.org/view.aspx?id=713273

94 Westport HD. 2012. Servicing Heavy-Duty Natural Gas Trucks: Preparation And Shop Requirements 

www.automotiveworld.com/webinars/servicing-heavy-duty-natural-gas-trucks-preparation-and-shop-requirements/ 95 Friedman, D. M., & Malcosky, N. D. 1996. Liquefied Natural Gas Safety in Transit Operations: Final Report 

http://trid.trb.org/view.aspx?id=713273

96 National Petroleum Council. 2012. An Initial Qualitative Discussion on Safety Considerations for LNG Use in 

Transportation www.npc.org/FTF_Topic_papers/19LNG_Transportation.pdf 97 Ibid. 98 Ibid. 99 Oil and Gas Journal. 2012. LNG seen viable fuel for heavy-duty trucks www.ogj.com/articles/2012/06/lng-seenviable-fuel-for-heavy-duty-trucks.html
30

A methane explosion occurred inside an LNG-powered 60-foot articulated bus during servicing 

in 1992. The bus had just been delivered and was being readied for LNG operation. Although 

such repairs are supposed to be performed outdoors, because of inclement weather, the mechanic 

performed the work in a conventional bus bay. After a gas detector alarm went off, the mechanic 

overrode it and started the bus engine to move it outside; this ignited the flammable natural gasair mixture inside the bus and blew out all of the windows and roof hatches. 100

Waste Management, Inc. provided a positive case study with its fleet of heavy-duty refuse trucks 

powered by LNG that have been based at a Pennsylvania facility since 1997.101 This 

implementation was considered successful by both Mack Truck and Waste Management, with no 

safety incidents reported. Drivers and refuse workers reported that there was no difference in 

operation between the propulsion systems. The refuse workers preferred the LNG trucks because 

of their lack of diesel odor and quieter operation. The LNG fuel station was installed with the 

storage tank underground and was reported to operate well, except for minor problems with the 

fuel nozzle. Waste Management deployed approximately 200 LNG refuse collection trucks in 

locations throughout California based on the outcome of the pilot fleet.

Safety in the design and operation of the LNG refueling infrastructure is partly addressed by 

National Fire Protection Association (NFPA) Code 57, which requires a drainage containment 

system to catch any liquid draining from the LNG tank and establishes a minimum distance 

between the edge of the drainage system and any buildings or property lines. The NFPA codes 

and standards are voluntary. Regulatory bodies such as the Federal Energy Regulatory 

Commission (FERC) may mandate compliance with NFPA standards in safety reviews of LNG 

facilities. 

Recent efforts, such the Shell collaboration with Mack and Volvo to develop LNG-fueled trucks, 

and the expansion of the LNG Flying J network of LNG refueling stations at truck stops in the 

Great Lakes and Gulf Coast regions102 may lead to broader adoption of LNG. An indirect safety 

impact of LNG-fueled MD/HDVs could also be an increase in the number of tanker trucks 

transporting LNG to many more refueling stations. This would increase the potential for road 

incidents involving LNG tanker trucks. However, if fueling stations were to perform their own 

liquefaction, the current natural gas pipeline distribution system could be used instead of tanker 

trucks. This would reduce the existing hazards from hauling diesel and gasoline fuel on roads 

and highways.103

 100 Foss, M. M. 2006. LNG Safety and Security 

www.beg.utexas.edu/energyecon/lng/documents/CEE_LNG_Safety_and_Security.pdf

101 Clark, K., Paul, N.. Clark, N. 2001. Waste Management's LNG Truck Fleet Final Results 

www.afdc.energy.gov/pdfs/waste_lng_final.pdf

102 See http://breakingenergy.com/2013/04/23/natural-gas-in-transport-tomorrow-s-fuel-today/ 103 National Petroleum Council. 2012. An Initial Qualitative Discussion on Safety Considerations for LNG Use in 

Transportation at www.npc.org/FTF_Topic_papers/21-LNG_in_Transportation.pdf
31

2.2.1.5 Findings and Conclusions 

This literature review did not produce any clear safety contraindications to the use of CNG or 

LNG fueled vehicles. It did however, identify some educational and operating procedure gaps 

and the potential regulatory opportunity to refine fuel system and maintenance facility standards. 

Robust training for people who handle natural gas, as well as the sound design and manufacture 

of natural gas storage tanks, may reduce the potential for leaks, tank ruptures, fires, and 

explosions.

Both CNG- and LNG-powered vehicles present hazards distinct from those of diesel vehicles, 

and call for distinct engineering and process controls to assure safe operability. However, based 

on the reported incident rates of NGVs and the experiences of adopting fleets, it appears that 

NGVs can be operated at least as safely as diesel MD/HDVs. Using natural gas instead of diesel 

fuel helps fleets comply with the MD/HD greenhouse gas rules that require up to 20 percent 

emissions reduction by 2018. 

There are currently no generally accepted codes and building standards for facilities that house 

CNG/LNG vehicles but which do not include a fueling facility; thus, facility safety is handled at 

the local level by fire marshals.104,105 Natural gas is lighter than air, so ventilation rates near the 

ceiling should be high enough to disperse the gas well before it reaches the lower flammability 

limit. Since LNG vapor initially stays low in case of a leak, it is recommended that enclosed 

LNG vehicle facilities install only classified (explosion-proof) electrical wiring and equipment in 

all maintenance facility work areas, both at low elevations and near the ceiling. It is possible to 

reduce such potential hazards by eliminating potential ignition sources within the facility.106 In 

addition, combustible gas detection and alarm systems could be installed.107

Essentially the same safety procedural requirements applicable to CNG fleets are relevant to 

LNG maintenance facilities, with one significant difference. Since LNG vehicles are expected to 

normally vent natural gas from LNG tank PRVs, all maintenance and storage locations should be 

equipped with a device to connect to PRV outlet and vent escaping gas at the building roof level. 

Alternatively, LNG vehicles should be de-fueled before entering the facility. This is not 

necessary for CNG vehicles, which are not expected to vent gas from their PRDs except in the 

event of vehicle fire or equipment failure. Unlike for CNG vehicles, there is limited data on 

modifying maintenance facilities to accommodate LNG vehicles, and further facility standards 

development and their enforcement are warranted. Although the NHTSA regulatory mission 

does not cover the safety of and vehicle fueling interfaces, they impact both vehicle highway 

safety, and the adoption of alternative-fuel vehicles. 

 104 Reich, S. L., & Kolpakov, A. 2011. Tracking Costs of Alternatively Fueled Buses in Florida 

www.nctr.usf.edu/2011/12/tracking-costs-of-alternatively-fueled-buses-in-florida-2/ 105 Westport HD. 2012. Servicing Heavy-Duty Natural Gas Trucks: Preparation And Shop Requirements 

https://s3.amazonaws.com/automotiveworld/presentations/12-19-12-Servicing+Heavy-Duty-Natural-Gas-Trucks.pdf 106 NHTSA. 2010a. Study of Heavy Truck S-Cam, Enhanced S-Cam, and Air Disc Brake Models Using NADS 

www.nhtsa.gov/DOT/NHTSA/NVS/Vehicle%20Research%20&%20Test%20Center%20(VRTC)/ca/capubs/811314

.pdf

107 Adams, R. 2010a. CNG Transit Experience Survey 

www1.eere.energy.gov/cleancities/pdfs/ngvtf10_trans_lessons_learned.pdf
32

The literature indicates that a number of preventive and safe handling procedures would ensure 

the safety of vehicles, personnel, and facilities. These include:

• Equipping LNG storage vessels with redundant pressure release valves to prevent an 

unsafe buildup of pressure when the LNG warms over time and changes to a gaseous 

state. 

• Emptying the LNG tanks or reclaiming the venting gas if a vehicle is parked for 

prolonged periods. 108 During defueling of the LNG vehicle, 30 percent of the fuel is 

typically lost in transfer; thus defueling could only occur outdoors or in a properly 

ventilated indoor facility.109 Chemical exposure of CNG cylinders could be avoided 

through proper containment or isolation, e.g., from road salt exposure or battery acid.110

• Proper insulation, orientation and ventilation of hot components are also critical to 

safety.111 

• Electric accessory systems are recommended on board NGVs to reduce the risk of a 

vehicle fire.112 CNG cylinders should be anchored at all times to prevent rocketing 

should a rupture occur.113 Fleets with vehicles that have rooftop CNG cylinders could 

develop and follow Standard Operating Procedures (SOP) to avoid collisions with low 

garage doors, overpasses, etc. 

• The literature indicates that best practices for proactive maintenance and inspection and 

for driver training would help prevent hose fires or brake fires that could lead to a 

CNG/LNG fuel tank venting. 

• Personnel safety training such as strict enforcement of "no smoking" policies, the use of 

non-sparking tools, and the use of personal protective equipment would also reduce

operational CNG/LNG hazards.114

Some revisions to fuel tank fire exposure testing and to tank thermal protection and inspection to 

prevent future incidents are recommended in the literature. One source specifically recommends 

expanding the cylinder certification test-to-test survivability against a fire localized away from 

the PRD, instead of against a bonfire that equally heats the cylinder and the PRD.115

 108 National Petroleum Council. 2012. An Initial Qualitative Discussion on Safety Considerations for LNG Use in 

Transportation www.npc.org/FTF_Topic_papers/21-LNG_in_Transportation.pdf 109 Westport HD. 2012. Servicing Heavy-Duty Natural Gas Trucks: Preparation And Shop Requirements 

https://s3.amazonaws.com/automotiveworld/presentations/12-19-12-Servicing+Heavy-Duty-Natural-Gas-Trucks.pdf 110 Lapetz, J. 2009. Safety Warning: Acid Exposure of Vehicular CNG Cylinders www.ngvglobal.com/safetywarning-acid-exposure-of-vehicular-cng-cylinders-0911 111 Adams, R. 2010a. CNG Transit Experience Survey 

www1.eere.energy.gov/cleancities/pdfs/ngvtf10_trans_lessons_learned.pdf

112 Ibid. 113 NHTSA. 2010a. Study of Heavy Truck S-Cam, Enhanced S-Cam, and Air Disc Brake Models Using NADS 

www.nhtsa.gov/DOT/NHTSA/NVS/Vehicle%20Research%20&%20Test%20Center%20(VRTC)/ca/capubs/811314

.pdf

114 Ibid. 115 Wong, J. 2009. CNG & Hydrogen Tank Safety, R&D, and Testing 

www1.eere.energy.gov/hydrogenandfuelcells/pdfs/cng_h2_workshop_8_wong.pdf
33

2.2.2 Biodiesel

Biodiesel is a domestically produced renewable fuel that can be manufactured from vegetable 

oils, animal fats, or recycled restaurant grease. Biodiesel consists of fatty acid alkyl esters, fatty 

acid methyl esters, or long-chain mono alkyl esters. Like petroleum diesel, biodiesel is used to 

fuel compression-ignition engines, which run on petroleum diesel, but it is cleaner burning. 

Biodiesel can be blended and used in different concentrations, including B100 (pure biodiesel), 

B20 (20 percent biodiesel, 80 percent petroleum diesel), B5 (5% biodiesel, 95% petroleum 

diesel) and B2 (2% biodiesel, 98% petroleum diesel). B20 is the most common biodiesel blend in 

the United States. 

The EPA highlights the benefits and disadvantages of using biofuel blends. Disadvantages 

include: concerns about B100's adverse impact on engine durability and unsuitability for use at 

low temperature due to increase in viscosity; lower fuel economy and power (10% lower for 

B100, 2% for B20); current higher costs; and a possible increase in nitrogen oxide emissions.

2.2.2.1 Penetration/Adoption 

Between 2001 and 2011, domestic biodiesel production has increased from virtually zero to 

approximately one billion gallons per year (Figure 2-6). With the recent addition of Mack and 

Volvo Trucks, more than 65 percent of diesel vehicle manufacturers in the U.S. market now 

support B20 or higher biodiesel blends. Volvo and Mack are among the first OEMs to extend 

B20 approval both to their new 2010 EPA emissions certified engines as well as to their older 

legacy models.116 Moreover, the first hybrid-electric trucks to support the use of B20 biodiesel 

blends were recently introduced, further expanding the potential for biodiesel adoption.117 

Blends up to B20 can be used in existing equipment without modification. In September 2011, 

Alliant began to use soybean-based fuel with B5 in 126 utility trucks. The vehicles using the 

blended fuel are International straight trucks, including trenchers, digger derricks, tractors, and 

bucket trucks.118

According to the latest APTA survey, at least 40 public transit agencies in North America 

operate buses running on biofuels.119 The San Francisco Municipal Transportation Agency 

(SFMTA) has the largest municipal biodiesel fleet in the United States with 512 vehicles using 

B20 – including both biodiesel and biodiesel-hybrid buses. In 2011, biodiesel fueled about 8 

percent of U.S. buses.120 In 2011, biodiesel fueled about 8 percent of urban transit buses. 

 116 Biodiesel Magazine. 2012. Mack and Volvo Trucks join growing list of B20 supporters 

www.biodieselmagazine.com/articles/8470/mack-and-volvo-trucks-join-growing-list-of-b20-supporters 117 Construction Business Owners. 2012. Hino Trucks Honored at Biodiesel Conference 

www.constructionbusinessowner.com/topics/hino-trucks-honored-biodiesel-conference 118 Bennett, S. 2008. Alliant Switches Fleet Vehicles to Biofuels www.worktruckonline.com/Channel/FuelManagement/Article/Story/2008/07/Alliant-Switches-Fleet-Vehicles-to-Biofuels.aspx?prestitial=1 119 See APTA 2012 “Transit on the cutting edge of clean technologies” at 

www.apta.com/resources/statistics/Documents/Transit-Clean-Technology.pdf 120 Gentry, W. 2012. Statement of William Gentry, President, Gentry Trailways and on behalf of American Bus 

Association and United Motorcoach Association Subcommittee on Highways and Transit, House Transportation and 

infrastructure Committee, U.S. House of Representatives www.gpo.gov/fdsys/pkg/CHRG112hhrg75852/html/CHRG-112hhrg75852.htm
34

Biodiesel vehicles accounted for about 7 percent of the new buses and more than 14 percent of 

the new demand.

2.2.2.2 Benefits

Although biodiesel contains about 8 percent less energy per gallon than petroleum diesel, 

corresponding to a 1- 2 percent decrease in energy density and a commensurate FE penalty, 

engines operating on B20 blends have similar fuel consumption, horsepower, and torque to 

engines running on petroleum diesel. Most B20 users reported no noticeable difference in 

performance or fuel economy, according to the DOE/AFDC sources cited. B20 has a higher 

cetane number (a measure of the ignition value of diesel fuel) and higher lubricity (the ability to 

lubricate fuel pumps and fuel injectors) than petroleum diesel.121 B20 and lower-level blends 

generally do not require engine modifications. 

Figure 2-6: U.S. Biodiesel Production and Consumption122

Compared with using petroleum diesel, using biodiesel reduces tailpipe emissions of unburned 

hydrocarbons (HC), carbon monoxide (CO), sulfates, polycyclic aromatic hydrocarbons, nitrated 

polycyclic aromatic hydrocarbons, and particulate matter (PM). The reductions increase as the 

amount of biodiesel blended into diesel fuel increases. B20 has been shown to reduce PM 

emissions 10 percent, CO 11 percent, unburned HC 21 percent, and carbon dioxide emissions by 

15 percent. 123 In addition, retrofit diesel oxidation catalysts (DOCs) and diesel particulate filters 

(DPFs) can operate effectively on vehicles using a biodiesel blend fuel up to B20 provided that 

 121 DOE Alternative Fuels Data Center. 2012. Biodiesel Blends www.afdc.energy.gov/fuels/biodiesel_blends.html 122 DOE EERE. 2012a. Alternative Fuels and Advanced Vehicles Data Center Natural Gas at 

www.afdc.energy.gov/fuels/natural_gas.html

123 DOE Alternative Fuels Data Center. 2012. Biodiesel Blends www.afdc.energy.gov/fuels/biodiesel_blends.html

-100

0

100

200

300

400

500

600

700

800

900

2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Million Gallons

 

Production Net Exports Consumption www.afdc.energy.gov/afdc/data/
35

this biodiesel blend conforms to appropriate biodiesel specifications, e.g., ASTM Standard 

D7467.124

2.2.2.3 Safety Considerations

In contrast with petroleum diesel, biodiesel is nontoxic and contains no hazardous materials.125 

It causes less damage than petroleum diesel if spilled or released in the environment and it is less 

combustible. The flashpoint for biodiesel is higher than 150°C, compared to 52°C for petroleum 

diesel. Biodiesel is also safe to handle, store, and transport.126 When conditions do not permit 

biodiesel-fueled vehicles to return to their base, they can be refueled with conventional diesel. 

Since biodiesel vehicles are essentially bi-fueled, there is no increased risk of vehicle 

stranding.127

Operational safety concerns include:

• Higher biodiesel blends have a solvent effect that can clean a vehicle's fuel system and 

release sludge deposits accumulated from previous petroleum diesel use. The release of 

these deposits may initially clog filters. 

• Biodiesel blends can gel at temperatures below 20 degrees Fahrenheit if the 

manufacturing process leaves too much glycerin in the fuel.128

• Since biodiesel is a rich food source, microbial filter-clogging may occur over time. 

To combat potential biofouling accumulation, use of a moisture dispersant and biocide may be 

necessary.129 The literature provides best practices for biodiesel storage, such as topping off 

tanks to prevent condensation and keeping the tank temperature at least 10° F above the cloud 

point of the blended fuel. 

There are a number of material compatibility issues associated with B100 that could affect 

vehicles with engines built before 1994: 

• B100 can swell or degrade hoses, gaskets, seals elastomers, glues, and plastics with 

prolonged exposure. 130 Natural or nitrile rubber compounds, polypropylene, polyvinyl, 

and Tygon materials are particularly vulnerable. Fuel pumps contain rubber valves that 

may fail. 

• Extensive contact with copper, brass, bronze, lead, tin or galvanized surfaces may also 

accelerate fuel oxidation. 

 124 Manufacturers of Emission Controls Association (MECA). 2012. What is Retrofit? at www.meca.org/dieselretrofit/what-is-retrofit 125 National Biodiesel Board. 2009b. Biodiesel sample material safety data sheet 

www.biodiesel.org/extpages/search-results?q=msds 126 DOE Alternative Fuels Data Center. 2012. Biodiesel Blends www.afdc.energy.gov/fuels/biodiesel_blends.html 127 Bennett, S. 2008. Alliant Switches Fleet Vehicles to Biofuels www.worktruckonline.com/Channel/FuelManagement/Article/Story/2008/07/Alliant-Switches-Fleet-Vehicles-to-Biofuels.aspx?prestitial=1 128 DOE Alternative Fuels Data Center. 2012. Biodiesel Blends www.afdc.energy.gov/fuels/biodiesel_blends.html 129 DOE Biodiesel Handling and Use Guide. 2009. www.nrel.gov/vehiclesandfuels/pdfs/43672.pdf 130 Ibid.
36

However, most engines made after 1994 have been constructed with gaskets and seals that are 

generally biodiesel resistant.131 The National Biodiesel Board recommends a regular 

maintenance and leak inspection schedule for the fuel lines and components of biodiesel 

MD/HDVs for newer vehicles; this is critical for older vehicles. 

2.2.2.4 Findings and Conclusions 

Biodiesel is a drop-in alternative fuel that can support MD/HDV progress toward GHG and 

criteria pollutant reductions without major capital investment or infrastructure barriers. There is a 

minor decrease in fuel efficiency due to the 8 percent lower energy density of biodiesel versus 

petroleum diesel, but the magnitude of the emissions decrease is substantially larger.

As for any bi-fueled vehicles, the potential for refueling with petroleum diesel reduces potential 

safety risks associated with vehicle stranding due to fuel supply shortage or refueling 

infrastructure limitations.

In conclusion, the literature reviewed and DOE/AFDC resources132indicate that biodiesel, as a 

viable drop-in alternative fuel, has both safety and environmental benefits. 

2.2.3 Liquefied Petroleum Gas (Propane)

Propane, also known as liquefied petroleum gas (LPG) or autogas, has been used worldwide as a 

vehicle fuel for decades. It is stored as a liquid, and has a propane fueling infrastructure that is 

widespread. Propane is a three-carbon alkane gas (C3H8). Stored under pressure inside a tank, 

propane turns into a colorless, odorless liquid. As pressure is released, the liquid propane 

vaporizes and turns into gas that is used for combustion. Propane is a non-toxic, noncarcinogenic, and non-corrosive fuel. It is insoluble in water and does not impact groundwater, 

surface water, or soil.133 An odorant, ethyl mercaptan, is added for leak detection.134

Propane has a high octane rating and excellent properties for spark-ignited internal combustion 

engines. There is interest in propane as an alternative transportation fuel because of its domestic 

availability, potentially cleaner-burning qualities, and relatively low cost. It is the world’s third 

most common engine fuel and is considered an alternative fuel under the Energy Policy Act of 

1992.135

When sold as vehicle fuel, propane can be a mixture of propane with smaller amounts of other 

gases. According to the Gas Processors Association's HD-5 specification for propane, it must 

consist of 90 percent propane, no more than 5 percent propylene, and 5 percent other gases, 

primarily butane and butylene. 136 To compare fuel efficiency on a gallon-to-gallon basis, the 

 131 National Biodiesel Board. 2009a. B100 Material Compatibility www.biodiesel.org/docs/ffsperformace_usage/materials-compatibility.pdf?sfvrsn=4

132 See www.afdc.energy.gov/fuels/biodiesel_benefits.html 133 Bertram, M. R, Burham,. A. Bertram. K. 2010. Propane Vehicles: Status, Challenges, and Opportunities 

www1.eere.energy.gov/cleancities/pdfs/propanepaper09_final.pdf

134 DOE Alternative Fuels Data Center. 2013b. Propane www.afdc.energy.gov/fuels/propane.html 135 Ibid. 136 Ibid.
37

energy content of propane is 66 percent that of diesel, thus requiring more propane fuel to travel 

an equivalent distance even in an optimized engine.

137 Larger storage tanks can increase range, 

but the additional weight displaces payload capacity and can slightly increase the consumption of 

fuel.

Propane is stored and handled as a liquid at the fuel dispenser. New nozzles and valves have 

been introduced that meet vapor control standards and prevent volatile organic compound (VOC) 

emissions from escaping from refueling stations. Onboard a vehicle, propane is stored at about 

150 pounds per square inch—about twice the pressure of an inflated truck tire. Under this

pressure, propane becomes a liquid with an energy density 270 times greater than the gaseous 

form.138 To operate a vehicle on propane as either a dedicated fuel or bi-fuel (i.e., switching 

between gasoline and propane) vehicle, only a few modifications must be made to a gasoline 

engine. Propane cannot be used in a diesel engine without major modifications since a spark or 

diesel-pilot ignition system would be required. The propane tank is typically located under the 

body of the vehicle.

2.2.3.1 Penetration/Adoption

According to the Propane Education and Research Council, there are more than 270,000 on-road 

propane vehicles in the United States. Many are used in fleet applications, such as police cars, 

shuttles, street sweepers, and school buses.139 As shown in Table 2-1, the Energy Information 

Administration estimates approximately 64,000 propane MD/HDVs operate in the United 

States140 The World LP Gas Association reports more than 13 million propane-fueled vehicles 

in operation worldwide.141 Propane vehicles can either be conversions from gasoline/diesel 

vehicles or purchased from OEMs. There are about 2,600 propane refuelers in the United States, 

mostly in California and the southern United States, particularly Texas.142

 137 Bertram, M. R. W. A. B. K. 2010. Propane Vehicles: Status, Challenges, and Opportunities 

www1.eere.energy.gov/cleancities/pdfs/propanepaper09_final.pdf 138 DOE Alternative Fuels Data Center. 2013b. Propane www.afdc.energy.gov/fuels/propane.html 139 Propane Education and Research Council. 2013. Propane Exceptional Energy http://autogasusa.org/ 140 Energy Information Administration. 2010. How many alternative fuel and hybrid vehicles are there in the U.S.? 

www.eia.gov/tools/faqs/faq.cfm?id=93&t=4

141 Bertram, M. R. W. A. B. K. 2010. Propane Vehicles: Status, Challenges, and Opportunities 

www1.eere.energy.gov/cleancities/pdfs/propanepaper09_final.pdf

142 Motavalli, J. 2012. Making the case for propane: from gas grills to cars and trucks 

www.cartalk.com/content/making-case-propane-gas-grills-cars-and-trucks
38

Table 2-1: Alternative Fuel Vehicles in Use in the United States by Fleet Operators: 2010143

Medium Duty Heavy Duty

Natural Gas — Total 22,575 28,976

 Compressed Natural Gas 22,521 26,000

 Liquefied Natural Gas 54 2,976

LPG — Propane 27,771 36,057

School buses fueled by propane are available from several school bus manufacturers, including 

Blue Bird Corporation and Collins Bus Corporation.144 In 2012, CleanFUEL U.S.A and Collins 

Bus Corporation announced the delivery of 134 propane school buses to First Student, Inc. for 

serving the Seattle Public Schools and Portland Public Schools.145 In 2009, there were 

approximately 2,000 LPG school buses out of a nationwide fleet of about 480,000. Blue Bird 

plans to sell another 14,200 buses by 2013. Propane has also found significant adoption in the 

paratransit bus market. In 2006, according to the American Public Transportation Association, 

3.7 million gallons of LPG were consumed by paratransit buses. 146

In addition to commercially available single and bi-fuel propane MD/HDVs, a propane-electric 

hybrid transit bus is also manufactured by E-bus.147

2.2.3.2 Benefits

Propane fuel costs significantly less than diesel fuel. MD/HDVs fueled by propane have the 

same horsepower, torque, and towing capacity of diesel-fueled vehicles.148 Lower maintenance 

costs are another reason for propane use in high-mileage vehicles. Its high octane and low carbon 

and oil contamination characteristics have resulted in greater engine life than conventional 

engines. Because the fuel mixture of propane and air is completely gaseous, cold start problems 

associated with liquid fuel are reduced.

Since propane requires relatively low compression, a pump of only a few horsepower can be 

sufficient. A home propane refueler can be purchased for approximately $1,500.149

 143 Energy Information Administration. 2010. How many alternative fuel and hybrid vehicles are there in the U.S.? 

www.eia.gov/tools/faqs/faq.cfm?id=93&t=4 144 Propane Education and Research Council. 2013. Propane Exceptional Energy http://autogasusa.org/ 145 Business Wire. 2012. CleanFUEL USA and Collins Bus Corporation Deliver More than 100 Propane AutogasPowered Buses www.businesswire.com/news/home/20121114005626/en/CleanFUEL-USA-Collins-BusCorporation-Deliver-100 146 Bertram, M. R. W. A. B. K. 2010. Propane Vehicles: Status, Challenges, and Opportunities 

www1.eere.energy.gov/cleancities/pdfs/propanepaper09_final.pdf

147 Ibid. 148 Propane Education and Research Council. 2013. Propane Exceptional Energy http://autogasusa.org/ 149 Motavalli, J. 2012. Making the case for propane: from gas grills to cars and trucks 

www.cartalk.com/content/making-case-propane-gas-grills-cars-and-trucks
39

Compared with vehicles fueled by conventional diesel and gasoline, propane vehicles can 

produce lower amounts of some harmful air pollutants and greenhouse gases, depending on 

vehicle type, drive cycle, and engine calibration.150

2.2.3.3 Safety Considerations

Similar to natural gas, there are two potential failure scenarios for propane: overpressure due to 

tank overfill, and unintended ignition (due to leaks or crash). 

In the first scenario, to prevent overpressure, most propane vehicles have a bleed, or splitter 

valve attached to the tank. During refueling the valve is opened, releasing vapor from the fuel 

tank and making room for the liquid propane to enter the tank.151 Once the tank is filled with 80 

percent liquid, additional propane will cause the valve to vent liquid fuel. This Overfill 

Prevention Device (OPD) supplements the automatic stop-fill system on the fueling station side. 

There are both safety and emissions implications of overfilled fuel tanks, so a pressure release 

device (PRD) is provided to release propane gas if pressure rises in the tank beyond safe 

levels.152

The DOE Clean Cities program conducted tests on 105 propane vehicles across seven fleets 

during fueling. Results showed that the OPDs on nearly 16 percent of the tanks failed to stop 

fueling at the necessary level.153 While the 16 percent OPD failure rate is potentially significant, 

tanks are also equipped with pressure relief devices specifically to ensure safe pressure levels. 

If a tank is overfilled during a cool time of day and sits without being used, warmer temperatures 

later in the day will expand the fuel, which could lead to a fuel release or leak through the 

pressure relief device.154 Indeed, overfill is one of three necessary conditions for a potentially 

hazardous ignition incident to occur: 1) overfilling leaves inadequate room in the tank for 

expansion; 2) rising ambient temperatures then cause fuel expansion and release; and 3) an 

ignition source is present. 

The physical properties of propane that are relevant to fire and explosion hazard in the event of a 

leak are different from the properties of natural gas. Propane, unlike natural gas, is heavier than 

air. Vapors therefore tend to fall to the ground level and can collect to a flammable level in low 

areas such as service pits. In contrast to natural gas, which dissipates as it rises, the dissipation of 

propane vapors is primarily based on air movement, and it will dissipate to non-hazardous 

concentrations faster in windy conditions than in still conditions.

 150 DOE Alternative Fuels Data Center. 2013b. Propane www.afdc.energy.gov/fuels/propane.html 151 Bertram, M. R. W. A. B. K. 2010. Propane Vehicles: Status, Challenges, and Opportunities 

www1.eere.energy.gov/cleancities/pdfs/propanepaper09_final.pdf 152 DOE Alternative Fuels Data Center. 2010. Propane Tank Overfill Safety Advisory 

www.afdc.energy.gov/technology_bulletin_1008.html 153 Ibid. 154 Bertram, M. R. W. A. B. K. 2010. Propane Vehicles: Status, Challenges, and Opportunities 

www1.eere.energy.gov/cleancities/pdfs/propanepaper09_final.pdf
40

Safety training manuals for propane-fueled vehicles include appropriate emergency responses to 

crash-related propane tank truck fire scenarios .155

2.2.3.4 Conclusions and Recommendations

LPG or propane fueled vehicles can be as operationally safe as the conventionally-fueled 

MD/HDVs. The pressurization of the tanks is relatively low, and there are overfill protection 

measures on both the fuel station and on board tank sides. Although tank OPDs have been found 

to have a failure rate of about 16 percent, excess pressure in the tank resulting from overfill is 

bled out through a PRD. Similarly, if the pressure in the tank increases due to ambient 

temperature after it is filled, the PRD is designed to protect against overpressurization. Thus, 

both the OPD and the PRD would have to fail for a tank to rupture due to overpressurization. No 

such documented incident could be found in the literature and on the Internet. However, it is 

critical to eliminate any ignition sources near a propane tank PRD. 

Best practices for fleets in the literature include ongoing inspections of the OPDs, training and 

educating propane vehicle users about preventing overfill, and having procedures to respond 

safely to a venting PRD. Safety standards for maintenance, inspection, and emergency response 

training have been promoted by industry groups, such as the National Propane Gas Association 

Technology Standards and Safety Committee, the Propane Education and Research Council 

(PERC), and the Underwriters Laboratory.156 The Propane Education and Research Council 

posts resources on codes, regulations and best practices for the safe operation of propane fueled 

fleets.157

There are also safety guidelines to be considered when developing propane-refueling 

infrastructure for MD/HDVs. This includes the National Fire Prevention Association’s NFPA 58 

Vehicular Liquefied Petroleum Gas Code, which applies to the design and installation 

requirements of propane refueling facilities.158 Local fire marshals typically ensure compliance 

with this code. In addition, propane suppliers can provide guidance to MD/HDVs owners and 

operators on the appropriate amount of propane to be stored on site to adequately meet vehicle 

fueling needs.159

 155 See “Tactical response guidelines for propane emergencies: Scenario #8

Propane Fueled Delivery Truck Fire” at 

www.propanesafety.com/uploadedFiles/Safety/Workforce_Training_programs/Propane_Emergencies_(PE)_Progra

m/Energencies_PDFs/Scenario%208.pdf

156 Bertram, M. R. W. A. B. K. 2010. Propane Vehicles: Status, Challenges, and Opportunities 

www1.eere.energy.gov/cleancities/pdfs/propanepaper09_final.pdf; and 2007 Propane Education & Research 

Council, Eight Step Incident Management Process©, Michael Hildebrand and Gregory Noll at 

www.propanesafety.com/uploadedFiles/Safety/Workforce_Training_programs/Propane_Emergencies_(PE)_Progra

m/REVISED_PE3_Text.pdf

157 See postings at www.propanesafety.com/ 158 National Fire Protection Association. 2011. NFPA 58: Liquiefied petroleum gas code 

www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=58&cookie_test=1 159 DOE Alternative Fuels Data Center. 2013b. Propane www.afdc.energy.gov/fuels/propane.html
41

2.3 Drivetrain Hybridization and Electrification

There are two main types of hybrid technologies that can be used in medium- and heavy-duty 

vehicles: 

• Hybrid electric vehicles (HEV) use an electric motor and generator, an energy storage 

system and power electronics, and an internal combustion engine. Hybrid electric 

vehicles are available across all weight classes in medium and heavy-duty vehicles.

• Hydraulic hybrid vehicles (HHV) use pressurized hydraulic fluid instead of electric 

charge as an additional energy storage system and as a power source, to complement the 

internal combustion engine. Due to its relatively high power density, but low energy 

capacity limited by accumulator size, the hydraulic system is eminently suitable for 

vehicles such as refuse trucks, transit buses, and delivery vehicles that operate in stopand-go traffic. 

HEV and EV adoption broken out by technology type includes:

• Series Hybrid

o Electric. 3,800 transit buses with the BAE series drivetrain are in service 

globally160, including 1,675 in New York City (28% of the bus fleet).161 

o Hydraulic. 11 Autocar E3 refuse trucks are in use by City of Miami with Parker 

RunWise drivetrain. UPS, Purolator, and FedEx are operating 48 Freightliner HH 

series trucks. Autocar E3 refuse trucks are also operating in NC, IN, TX, and CA.

• Parallel Hybrid

o Electric. 6,000 Eaton hybrid drivetrains are in service worldwide (2,500 electric 

buses in Asia, 300 electrics in Europe, and 3,200 in North America). Coca-Cola 

has operated 142 hybrid box trucks (33,000 lb.) in North American cities since 

2008, and larger parallel hybrid drive trucks (55,000 lb.) for its bulk-delivery 

applications since 2009.162

o Hydraulic. 100 Eaton “Hydraulic Launch Assist” drivetrains are in service on 

Peterbilt refuse trucks.

• Electric

o At least 20 fleets are using Smith or other electric MD box trucks. Frito-Lay 

Foods is the leading adopter (275 electric trucks), followed by AT&T, Staples, 

Coca-Cola, Pacific Gas & Electric, and Kansas City Power & Light. In addition, 

the Santa Barbara Metropolitan Transit District has operated 20 22-foot electric 

 160 BAE Systems. 2012. The Heavy-Duty Hybrid www.hybridrive.com/ 161 Gentry, W. 2012. Statement of William Gentry, President, Gentry Trailways and on behalf of American Bus 

Association and United Motorcoach Association Subcommittee on Highways and Transit, House Transportation and 

infrastructure Committee, U.S. House of Representatives www.gpo.gov/fdsys/pkg/CHRG112hhrg75852/html/CHRG-112hhrg75852.htm 162 DOE EERE. 2011. Project Startup: Evaluating Coca-Cola’s Class 8 Hybrid-Electric Delivery Trucks 

www.nrel.gov/docs/fy11osti/49621.pdf
42

buses for over two decades. In addition to Smith, at least 4 other manufacturers 

offer MD/HDV battery electric trucks for commercial sale in the United States:

 Electric Vehicles International,

 AMP Electric Vehicles,

 Boulder Electric Vehicles, and

 Motiv Power Systems.

The fuel consumption benefits of these technologies depend heavily on the vehicle applicationspecific duty cycle. Hybrid drivetrains provide less benefit when operation is mostly steady 

highway speed, because there is less opportunity for regenerative braking. Hence, they are not 

commonly incorporated into long-haul trucks. 

In addition, mild hybrid vehicles have electric or hydraulic power systems that are not part of the 

vehicle's power train but power ancillary equipment like refrigeration or an aerial lift, allowing 

the primary engine to be shut off during these non-propulsion activities. 

Electric trucks and buses exclusively rely on an electric motor for propulsion and on an electric 

storage device for all on-board systems, such as auxiliary power for hotel loads for heat, 

ventilation, lighting, and air conditioning. Typically, EVs draw power from an onboard battery 

or in some cases from ultracapacitors, overhead catenary, or inductive chargers.

Whether they are hydraulic or electric, hybrid MD/HDVs are also generally designed in either a 

series architecture or a parallel architecture. There is also a two-mode, or compound power split 

parallel electric hybrid, used in the GM-Allison hybrid electric buses, which can operate in either 

series, or a parallel configuration.163 Series hybrids supply power to drive wheels directly and 

exclusively from the electric motor or from a hydraulic accumulator tank. In the hydraulic case, 

the wheels may be powered by hydraulic pressure only for a brief period, such as launching from 

a stop, after which the internal combustion engine takes over. Electric series hybrids typically 

use a small internal combustion engine (ICE) to continually recharge a battery that powers an 

electric traction motor. Parallel hybrid vehicles split the power transmission to the drive wheels 

between the ICE and the second power source (Figure 2-7). 

While the range of a hybrid vehicle is usually comparable to an ICE, an electric-only MD/HDV 

will normally have a reduced range between charges. However, manufacturers of two MD 

electric trucks (E-trucks) claim to have a 100-mile-per-charge range: the Navistar eStar, with a 2-

ton payload, and the Smith Newton, with an 8-ton payload.164 Hino Trucks was honored in 2012 

with the National Biodiesel Board Impact Award for being the first manufacturer to support the 

use of B20 biodiesel blends in a hybrid-electric truck, as well as in its complete product line of 

class 6 and 7 conventional trucks.165

 163 See http://thisbluemarble.com/showthread.php?t=3155 and http://papers.sae.org/2007-01-0272/ 164 See 2011 DOE/EERE review at 

www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2011/2011_amr_01.pdf

165 Construction Business Owners 2012. Hino Trucks Honored at Biodiesel Conference 

www.constructionbusinessowner.com/topics/hino-trucks-honored-biodiesel-conference
43

Figure 2-7: Schematic of a Parallel Hybrid-Electric Utility Truck166

On the transit side, electric buses that wirelessly charge through induction when they are curbed 

at stops have been developed.167 Early adopters include Foothill Transit, with 12 Proterra buses, 

and the Utah Transit Authority.168

2.3.1 Penetration/Adoption

Hybrid and electric trucks still represent less than 1 percent of all commercial MD/HD trucks on 

the road in the United States A recent CALSTART survey of 82 fleets that represent 563,408 

vehicles included only 4,381 HV/EVs, including 413 EVs, or 0.78 percent of the total. This 

relatively low penetration may be due to the lack of national and/or State incentives to offset 

incremental costs of purchase, the limited amount of in-use performance data- particularly for 

electric trucks- and the lack of technology standardization. The Energy Information 

Administration (EIA) reported that there were 865 electric and 1,025 MD/HD hybrid MD/HDVs 

in 2010.169 APTA reported that in 2011 more than 35 percent of the public transit bus fleet used 

alternative fuels or hybrid technology, with 9 percent of buses either diesel-electric, or gasolineelectric hybrids, being operated by more than 60 public transit agencies.170 

 166 Mims, C. 2009. Hybrid Trucks Are Here for the Long (Medium and Short) Haul; An explosion in the number and 

kind of commercially available hybrid trucks means battery power isn't just for lightweight commuter vehicles 

anymore www.scientificamerican.com/slideshow.cfm?id=hybrid-trucks 167 Proterra. 2012. About the EcoRideTM BE35 www.proterra.com/index.php/products/productDetail/C22/ 168 Barry, K. 2012. Induction Charging Comes to Public Transit www.wired.com/autopia/2012/12/inductioncharging-bus/ 169 See Table at www.eia.gov/tools/faqs/faq.cfm?id=93&t=4 170“More than 35% of US public transit buses use alternative fuels or hybrid technology: Public transportation is 

leading the way in green vehicles: www.apta.com/mediacenter/pressreleases/2013/pages/130422_earth-day.aspx
44

Adoption of hybrid MD/HDVs is accelerating.

171 The 2012 industry estimates reported there are 

3,500-4,500 hybrid trucks currently operating in the United States 172 and 500-1,450 electric 

trucks in North America.173 More than 150 fleets use hybrid and/or electric trucks, of which 

there are about 40 models available.174 Market forecasts predict anywhere from 13,000 to 

39,000 hybrid and electric trucks sold annually in the United States by 2015-2017.175,176 

Early adoption of electric trucks has occurred in States such as CA and NY that have policies 

and programs to encourage these EVs. Hybridization shows the greatest current benefit in 

vocational vehicles.177 The most likely truck application areas for truck hybridization and 

electrification are municipal, delivery, and utility fleets, which tend to operate at lower speeds 

with frequent stop-and-go drive cycles, or which remain stopped for extended periods while 

operating onboard equipment, e.g., an aerial boom or a lift gate. 

Major truck manufacturers are now competing in the hybrid MD/HDV market niche: 

Daimler/Freightliner, International, Peterbilt, Paccar/Kenworth, Navistar, and Volvo/Mack. The 

electric truck OEMs include Smith, Navistar, Electric Vehicles International, VIA Motors, 

Quantum Technologies, and Boulder EV. 

In contrast to the truck market, there is already significant penetration of hybrid technology in 

U.S. transit bus fleets. Large public transit agencies in cities such as New York City, Chicago, 

Seattle and San Francisco have been at the forefront of adopting hybrid-electric buses, which 

now account for about 1 out of every 6 new buses that transit agencies have on order. A few 

agencies are pioneering plug-in electric hybrids and all-electric buses. By 2011, about 9 percent 

of buses were diesel-electric or gasoline-electric hybrids and more than 60 transit agencies had 

such buses in service.178

 171 Mims, C. 2009. Hybrid Trucks Are Here for the Long (Medium and Short) Haul; An explosion in the number and 

kind of commercially available hybrid trucks means battery power isn't just for lightweight commuter vehicles 

anymore www.scientificamerican.com/slideshow.cfm?id=hybrid-trucks 172 Mathers, J. 2011a. Hybrid Trucks: Creating Jobs, Reducing Emissions 

http://blogs.edf.org/innovation/2011/01/31/hybrid-trucks-creating-jobs-reducing-emissions/ 173 Walkowicz, K. 2012. Medium and Heavy Duty Vehicle Evaluations 

www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2012/veh_sys_sim/vss001_walkowicz_2012_o.pdf 174 Mathers, J. 2011b. Work Trucks and Fuel Consumption: 2011 http://blogs.edf.org/innovation/2011/03/10/worktrucks-and-fuel-consumption-2011/ 175 ABI Research. 2009. Commercial Hybrid Vehicles: A Global Analysis of Medium- and Heavy-Duty Hybrid 

Trucks and Buses www.abiresearch.com/mobile/research/1001997 176 CalStart. 2012a. Best Fleet Uses, Key Challenges, and the Early Business Case for E-Trucks 

www.calstart.org/Libraries/ETruck_Task_Force_Documents/Best_Fleet_Uses_Key_Challenges_and_the_Early_Business_Case_for_ETrucks_Findings_and_Recommendations_of_the_E-Truck_Task_Force.sflb.ashx 177 CalStart. 2012c. Market Demand Analysis For Hybrid and Electric Trucks 2012-2016 178 TRB/TCRP report 132 “Assessment of Hybrid Electric Transit Bus Technology,” 2009 at 

http://onlinepubs.trb.org/onlinepubs/tcrp/tcrp_rpt_132.pdf
45

2.3.2 Benefits

According to the Transit Cooperative Research Program, diesel-electric hybrid buses can have 

14 percent to 48 percent better fuel efficiency than conventional diesel buses.179 The breakeven 

period of hybrid-electric transit buses for most agencies is approximately 15 years (this will 

obviously vary depending on current diesel costs), not accounting for reduced brake maintenance 

costs.180 

With the exception of particulate matter from brake, tire, and road wear,181 an all-electric vehicle 

has no mobile emissions, or direct fuel consumption. Hybrid drivetrain technologies can 

significantly reduce fuel consumption and vehicle emissions, depending on the drive cycle and 

depending on the level of hybridization, from mild to full.182 Additionally, drivetrain 

hybridization and use of regenerative braking for energy recovery can reduce maintenance of 

some vehicle components such as brakes, based on NREL’s 13 month study of Coca Cola hybrid 

trucks.183 

Stored energy in an HEV also powers electric-only operation for electric Power Take-Off 

(ePTO) and auxiliary power generation.184

Wrightspeed’s “Route” retrofit plug-in hybrid powertrain for MD trucks claims to increase the 

fuel economy from 12 mpg to 25-40 mpg in a metro drive cycle. Because the factory truck’s 

engine and transmission are removed, the PHEV drivetrain adds no weight to the vehicle.185

 179 TRB/TCRP report 132 “Assessment of Hybrid Electric Transit Bus Technology,” 2009 at 

http://onlinepubs.trb.org/onlinepubs/tcrp/tcrp_rpt_132.pdf

180 U.S. transit buses average approximately 30,000 miles per year and diesel buses average approximately 3.5 

MPG. This means that diesel buses use on average about 8,600 gallons of fuel per year. At $4.00 per gallon each bus 

uses $34,400 worth of fuel per year. Most transit agencies will achieve no more than 30% reduction in fuel use, so 

they will save $10,000/year/bus in fuel costs. According to the American Public Transportation Association, the 

incremental purchase cost of a hybrid bus compared to a diesel bus is approximately $150,000.

181 www.fhwa.dot.gov/resourcecenter/teams/airquality/brochure/particulatebrochure.pdf 182 ils, H. 2009. Hybrid Solutions for MD Commercial Vehicles www.erc.wisc.edu/documents/symp09-Cornils.pdf 183 See “The case for Class 8 Hybrids” at http://ev.sae.org/article/11853 184 Freightliner. 2012. Freightliner hybrid trucks www.cleanandgreenvehicles.com/pdfs/M2eHybrid-Brochure.pdf 185 See page 10 of CalStart. 2012d. Wrightspeed brings a CNG plug-in at 

http://issuu.com/kfetzer/docs/showtimes_htuf2012_issuu?mode=window&viewMode=doublePage
46

Figure 2-8: Hydraulic Hybrid Refuse Truck186

The demonstrated FE gains for hydraulic hybrid systems range between 15 percent and 50 

percent, due primarily to the 71 percent brake energy recovery, which is significantly higher than 

that for electric hybrids. Assuming a high kinetic intensity drive cycle, such as the ones used for 

a refuse or delivery truck, parallel hybrids are at the lower end of this FE gain range, while series 

hybrids are at the upper end.187 A field test by NREL and the United Parcel Service (UPS) 

measured a 13-20 percent FE gain on 11 parallel hybrid step vans versus 11 conventional vans 

operated by UPS in Minneapolis.188 FedEx Ground, Purolator, and UPS report fuel savings of 

approximately 40 percent in tests of series hydraulic hybrids. 189 Miami-Dade’s Autocar E3 

refuse trucks with Parker RunWise burn 36 gallons per day versus 63 gallons for the 

conventional trucks.190 Additionally, up to an eight-fold increase in brake life is reported to 

reduce brake fade and other brake problems. Parker claims that a series hydraulic hybrid 

improves drivability, smoothes braking, and boosts acceleration versus an ICE equivalent. 191 

Hybrid systems potentially also allow a vehicle that runs out of fuel or that has an ICE 

breakdown to still pull off the road or drive to a safe location, which is not possible in an ICE 

vehicle failure.

 186 Cornils, H. 2009. Hybrid Solutions for MD Commercial Vehicles www.erc.wisc.edu/documents/symp09-

Cornils.pdf

187 DeCoster, T. 2012. Parker's hydraulic hybrid technologies 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26445

188 National Renewable Energy Laboratory. 2012. Eighteen-Month Final Evaluation of UPS Second Generation 

Diesel Hybrid-Electric Delivery Vans www.nrel.gov/docs/fy12osti/55658.pdf 189 CalStart. 2012a. Best Fleet Uses, Key Challenges, and the Early Business Case for E-Trucks 

www.calstart.org/Libraries/ETruck_Task_Force_Documents/Best_Fleet_Uses_Key_Challenges_and_the_Early_Business_Case_for_ETrucks_Findings_and_Recommendations_of_the_E-Truck_Task_Force.sflb.ashx 190 Hybrid Truck Users Forum. 2012. More Parker Hannifin Hydraulic Hybrids 

http://issuu.com/kfetzer/docs/showtimes_htuf2012_issuu?mode=window&viewMode=doublePage 191 DeCoster, T. 2012. Parker's hydraulic hybrid technologies 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26445
47

Since electric vehicles produce no tailpipe emissions, they also provide potential health benefits 

to operators through GHG reduction and complement fuel efficiency benefits. According to 

Smith Electric, the 20 fleets that are already using Smith trucks have prevented 15,000 hours of 

diesel inhalation for their drivers over five years (Figure 2-9).192 

Figure 2-9: Smith Electric Truck193

2.3.3 Safety Considerations

NHTSA recognized that potential hazards involving high voltage batteries and battery-related 

fires are different from those posed by conventionally-fueled vehicles. In January 2012, the 

agency issued Interim Guidance for Electric and Hybrid-Electric Vehicles Equipped With High 

Voltage Batteries, applicable to medium and heavy-duty hybrid vehicles.194 The literature has 

reported fires that were initiated by, or involved, the energy storage systems of hybrid-electric 

buses and trucks. For instance, in February 2012, a Zero Truck Co. hybrid electric truck with 24 

lithium ion batteries caught fire twice.195 Firefighters did not handle the high voltage 400-volt 

energy storage system (ESS) to avoid electrocution, but waited for the 24 lithium ion batteries to 

self-extinguish. Even though this battery-related fire led to the release of hydrogen gas, there 

were no adverse effects as the truck was located outside.196 

Introductions of new HEV/EVs to the market are rapidly increasing, with models from nearly 

every major OEM in production. One trend is that battery capacities are increasing with greater 

 192 Smith Electric Vehicles. 2012. Smith NTEA Green Truck Summit 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26498 193 Ibid.

194 See Interim Guidance for Electric and Hybrid-Electric Vehicles Equipped With High Voltage Batteries. 

www.nhtsa.gov/staticfiles/nvs/pdf/InterimGuide_TowTruck_012012_v3.pdf

195 See http://articles.mcall.com/2012-02-08/news/mc-allentown-electric-truck-fire-20120208_1_electric-trucklithium-ion-batteries-truck-box 196 Ibid.
48

vehicle electrification, owing to pressure to increase the vehicle range and to increase the range 

of applications for EVs.197

Due to their larger market penetration and longer (15 years) operational experience, hybrid and 

electric transit buses have more of a safety history than the more recently adopted hybrid electric 

trucks. Electric and hybrid-electric transit buses have been safely operated by many urban transit 

fleets. For example, the Santa Barbara’s Municipal Transit District has operated electric buses 

for over two decades without any safety incidents and with only minor service interruptions. This 

transit fleet experience shows that centrally maintained and properly operated electric battery 

buses have been operating and will continue to operate safely and reliably.

Transit fleet experiences also illustrate potential battery-related hazards. In March 2012, DC 

Metro recalled NewFlyer/BAE buses after fire problems, although there had been only 10 such 

incidents in North America.198 Fires resulted when debris in the rechargeable energy storage 

system on the bus roof shorted out the battery pack. The battery packs melted and smoked; 

however the smoke did not penetrate the passenger space.199 In 2011, NHTSA recalled over 

1,300 hybrid electric Orion VII Daimler 2007-2011 buses equipped with Lithium-ion batteries 

because of the potential for battery-related fire hazards.200 Problems of smoke and overheating 

and the potential for fire potential were due to debris accumulation in the roof-emplaced RESS, 

which compromised the electrical isolation and discharged or shorted out the battery.201 

As discussed recently in a May 2012 NHTSA workshop and in several SAE/NFPA annual EV 

Safety summits, the stranded energy in certain types and chemistries of lithium-ion batteries, 

which can enable thermal runaway given the right conditions, poses potential post-crash 

electrocution or fire hazards for HEVs/EVs operators and rescuers.202

Since the electrical systems of regular diesel vehicles are low voltage, a typical maintenance 

garage may not be equipped to safely service high voltage systems in hybrid-electric trucks or 

buses. A literature review identified several safety best practices for maintenance facilities of 

hybrid electric MD/HDVs: provide additional electrical safety equipment (e.g., fault ground 

detection and ground interrupters) and provide electrical safety training of maintenance and 

emergency response personnel. Specifically, high-voltage battery storage areas, and charging 

stations for charging several batteries at a time should be well ventilated and equipped with heat 

 197 Mackintosh, T. 2012. SAE J2990 Hybrid EV First and Second Responder Recommended Practice Overview 

www.sae.org/events/nevss/summit/2012_EV_Safety_Summit_mackintosh.pdf

198 Hedgpeth, D. 2012. Manufacturer needs to repair 47 hybrid buses bought by the Metro system 

http://articles.washingtonpost.com/2012-03-28/local/35449072_1_hybrid-buses-new-flyer-bae-systems 199 Ibid. 200 See www.dhses.ny.gov/ofpc/alerts-bulletins/safety/text/2011/20110065.cfm 201 NHTSA. 2011b. Orion VII Li-ion hybrid bus recall wwwodi.nhtsa.dot.gov/recalls/results.cfm?rcl_id=11V523000&searchtype=quicksearch&summary=true&refurl=rss

202 Harris, C. 2012. Overview of NHTSA EV Safety Program October 2012 

www.sae.org/events/nevss/summit/2012_EV_Safety_Summit_harris.pdf; and 

www.sae.org/calendar/techsess/203434.pdf
49

and smoke detectors to prevent overheating and the build-up of dangerous gases from overheated 

batteries during charging. 203

Since there are major differences between light-duty HEVs/EVs and their MD/HDV counterparts 

in terms of their power, size, weight and body ranges, appropriate consensus safety standards are 

being developed. These may mirror FMVSS 305 power level and battery sub-system crash 

integrity as assured by the NHTSA TP 3015-01 lab-testing procedure, which only applies to 

vehicles with a GVW less than 10,000 pounds.

204 For instance, OEMs have adopted voluntary 

safeguards and national and international specifications and standards for heavy-duty vehicle 

rechargeable storage system (RESS) safety testing. The SAE Truck and Bus Committee has 

issued Standard J2910 “Recommended Practice for the Design and Test of Hybrid Electric 

Trucks and Buses for Electrical Safety.”205

The SAE safety performance tests for high-voltage vehicle ESSs include206

1. Vibration: Pack Test

2. Thermal Shock: Pack Test

3. External Short Circuit Protection: Pack Test

4. Overcharge Protection: Vehicle Test

5. Over-Discharge Protection: Vehicle Test

6. Over Temperature Protection: Vehicle Test

7. Under Temperature Protection: Vehicle Test

8. Fire Resistance – Short Duration: Pack Test

9. Fire Resistance – Long Duration: Pack Level

10. Vehicle Crash Evaluations: Vehicle Test

11. Water Intrusion Test: Vehicle Test

Eaton, a major hybrid drivetrain manufacturer lists seven engineering and conspicuity measures 

to reduce the risk of stray energy and electrocution.207 These include orange coloring of all highvoltage lines, cable disconnect interlocks, ground fault detection, isolation from the vehicle 

chassis, and an ignition interlock that electrically isolates the ESS when the vehicle ignition is 

 203 Reich, S. L., & Kolpakov, A. 2011. Tracking Costs of Alternatively Fueled Buses in Florida 

www.nctr.usf.edu/2011/12/tracking-costs-of-alternatively-fueled-buses-in-florida-2/

204 See Sept.11, 2008 TP 305-01, Laboratory Test Procedure for FMVSS 305, Electric Powered Vehicles: 

Electrolyte spillage and electric shock protection” at www.nhtsa.gov/Vehicle+Safety/Test+Procedures 205 NHTSA. 2012. Electric Vehicle Safety Technical Symposium 

www.nhtsa.gov/staticfiles/nvs/pdf/EV_Symposium_agenda.pdf

206 Wilson, K. 2012. SAE Cooperative Research Project to Develop Repeatable Safety Performance Test Procedures 

for Rechargeable Energy Storage Systems (RESS) 

www.sae.org/events/nevss/summit/2012_EV_Safety_Summit_wilson.pdf

207 Eaton. 2011. Eaton Hybrid Electric Power Systems Emergency Response Guide TRDR1100 

www.roadranger.com/rr/CustomerSupport/Support/LiteratureCenter/index.htm?litlibtarget=1162919212387
50

off. Truck OEMs are backing the reliability of their HEVs. Freightliner, for example, provides a 

3-year/150,000 mile warranty on the hybrid system.208

The emergency response guide for Eaton HEV drivetrains outlines safety procedures for first 

responders that are similar to current methods used for ICE trucks.209 The guide notes that the 

lithium salt, which is dissolved in an organic solvent, will not normally spill or leak if the battery 

is damaged. If the 1.2 gallons of contained electrolyte does leak from the battery system, it can 

be wiped up by a towel. In a non-fire situation, no toxic gases are emitted, and there is no 

inhalation hazard. If there is a fire, inhaling gas released by the battery is dangerous and the 

guide advises moving immediately to fresh air and seeking medical attention.

Hydraulic hybrid vehicles entail a different set of potential safety concerns than electric hybrids.

The emergency response guide for Parker drivetrains states that two 20-gallon hydraulic 

accumulators are constantly pressurized to 2,450 psi with nitrogen gas. Nitrogen gas is not 

flammable and is chemically inert, but the pressure of the nitrogen gas could propel objects at 

deadly speed if the tank is pierced or broken. Hoses in the cradle contain 5400-psi hydraulic 

fluid. A hose puncture could lead to severe injection wounds by expelling high-pressure fluid, 

and a spray of leaking hydraulic fluid would be explosively flammable. If a hose in this system 

were to get loose, it could violently flail and cause death or injury by impact. If the containers of 

gases were pierced or opened, an explosion could also occur.

There are a variety of safety features incorporated in the hydraulic hybrid to prevent personal 

injury or component damage in the event of system pressure leaks or electrical failure. Safety 

features include a variety of fuses, discs, and valves activated by set points in velocity, 

temperature, or pressure. Additionally, orange coloring marks all high-pressure hoses.210

As one data point, following 15 months of testing HHVs, Miami-Dade County has not 

encountered any safety problems and has reported uptime of 99 percent in South Florida refuse 

collection service211

Parallel hybrid vehicles offer a potential safety advantage over series hybrid vehicles in the event 

that the hybrid drive unit fails, since the diesel powertrain remains fully functional.212 Series 

hybrid vehicles are not redundantly powered, so if there is a significant hydraulic leak or a 

traction motor or battery failure, the vehicle is disabled and must be towed. 

It is worth noting that a series HH drivetrain adds about 1,700 lbs. over a conventional drivetrain 

to the GVW of a truck. This is approximately 5 percent additional weight, which may affect the 

handling or stopping distance of the vehicle as well as its maximum payload.

 208 Freightliner. 2012. Freightliner hybrid trucks www.cleanandgreenvehicles.com/pdfs/M2eHybrid-Brochure.pdf 209 Eaton. 2011. Eaton Hybrid Electric Power Systems Emergency Response Guide TRDR1100 

www.roadranger.com/rr/CustomerSupport/Support/LiteratureCenter/index.htm?litlibtarget=1162919212387

210 Parker Hannifin Corporation. 2012. Parker RunWise Advanced Series Hybrid Drive AutoCar E3 First 

Responders Guide

211 Hybrid Truck Users Forum. 2012. More Parker Hannifin Hydraulic Hybrids 

http://issuu.com/kfetzer/docs/showtimes_htuf2012_issuu?mode=window&viewMode=doublePage

212 Freightliner. 2012. Freightliner hybrid trucks www.cleanandgreenvehicles.com/pdfs/M2eHybrid-Brochure.pdf
51

2.3.4 Findings and Conclusions 

Hybrid-electric transit buses have demonstrated a long and safe operational history, with vehicle 

handling and a breakdown rate that are comparable to conventional ICE buses.213 The recent 

NHTSA/ODI recall of lithium-ion RESS packages in Orion VII and Xcelsior hybrid buses214 for 

potential battery-related fire hazards indicate that unforeseen safety issues occurred. The roof 

placement of batteries to allow for passive cooling also allowed debris and moisture to 

accumulate that could cause battery short-outs with the potential for a fire. 

To date, no safety incidents have been reported in the shorter history of hybrid-hydraulic trucks. 

The underlying hydraulic technology is much more mature than that of HEVs. Multiple fail-safe 

design features prevent electrocution during operation of hybrid-electric vehicles, although postcrash hazards due to stranded energy may be present. In contrast to an ICE vehicle, hybrid 

vehicles (in particular parallel models) offer redundancy of propulsion: if one power source fails, 

the other can still propel the vehicle, even if it is for a short distance on electric or hydraulic 

system power alone.

Inputs from SMEs suggest that the full vehicle testing and modeling of EVs/HEVs is more 

limited for small volumes of production and downstream integration of novel RESS units by 

body-builders than it is for high-volume production of ICEs. Continued evaluation of best 

practices and lessons learned from hybridization and electrification across all adopting MD/HDV 

fleets would likely help to prevent failure of novel FE subsystems.

2.4 ITS and Telematics for Fuel Efficiency and Operational Safety

2.4.1 Driver and Vehicle Monitoring

Technologies that influence MD/HDV fuel efficiency can also affect driver behavior and 

operational safety, and vice versa. These telematics technologies include GPS-based route 

navigation, on-board computers that track driver and vehicle behavior, and associated mobile 

communications and displays that convey this information in real time to the vehicle driver and 

to fleet managers. Information that is typically gathered includes route choice, speed, braking,

and acceleration, measured using devices such as accelerometers, GPS, and on-board cameras. 

Telematics systems are intended to correct the behavior of MD/HDV drivers over time to 

increase their fuel efficiency, either by providing real-time feedback on their driving 

performance or by relaying performance information to supervisors for individual driver 

coaching. 

There are various implementations of telematics for route optimization, location, and navigation 

that simultaneously address the fuel efficiency and safety of vehicle operations. Camera-based 

 213 See series of NREL reports comparing new York City Transit hybrid, CNG and diesel buses at 

www.nrel.gov/vehiclesandfuels/fleettest/pdfs/42534.pdf and at 

www.nrel.gov/vehiclesandfuels/fleettest/research_hybrid_nyc.html

214 See Recall notice at wwwodi.nhtsa.dot.gov/owners/SearchResults?searchType=ID&targetCategory=R&searchCriteria.nhtsa_ids=11V523000

&refurl=rss
52

systems such as SmartDrive record comprehensive video-based data from the road. High 

acceleration or force thresholds are used to filter the events that are captured. The event data are 

reviewed for fuel consumption and safety performance by off-site analysts, thus informing the 

fleet managers for driver coaching and training.215 Non-camera based systems such as 

GreenRoad216 (Figure 2-10) use accelerometers and other sensors for real-time assessment and 

visual feedback for driving maneuvers, e.g., with a green/yellow/red performance meter. 

In addition to maintaining safe and fuel-efficient driver behavior, ITS technologies are also 

intended to prevent crashes, offering additional safety benefits. Although not classified as FE 

technologies, the Insurance Institute for Highway Safety (IIHS) has recommended four ITS crash 

avoidance technologies it claims have the potential to prevent or mitigate one-fourth of the 

384,000 annual large truck crashes, including one-third of injury crashes and about 20 percent of 

the 4,100 fatal crashes.217 

2.4.1.1 Penetration/Adoption

The penetration rate in the total population of non-privately owned commercial vehicles is 

estimated to increase from 7.9 percent in 2011 to 16.4 percent in 2016.218 The market leaders 

include a range of different companies. Qualcomm Enterprise Services is ranked as the largest, 

with an estimated total installed base of approximately 450,000 units in North and Latin 

America. Trimble which previously mainly focused on service fleets now has a total installed 

base of 360,000 units following the acquisition of PeopleNet. Other companies focusing on 

service fleets include FleetMatics, Networkfleet, NexTraq and Wireless Matrix. Several 

additional companies have a broader market scope, covering both light and heavy vehicles. 

Examples include Telogis, Teletrac, Zonar Systems and Webtech Wireless. GreenRoad states it 

is used by more than 85,000 drivers worldwide, in a wide range of vehicle types, including EV 

vans, and NG tractors. “Green telematics” that are claimed to save fuel, reduce emissions, and 

optimize routing have wide market penetration and have been adopted early by large MD/HDV 

fleets, such as Frito Lay.219 

 215 SmartDrive Systems. 2012. SmartDrive www.smartdrive.net/ 216 http://greenroad.com 217 See “Large trucks to benefit from technology designed to help prevent crashes,” IIHS Status Report, Vol. 45, No 

5, May 20, 2010.

218 www.prnewswire.com/news-releases/fleet-management---world-2012-166386546.html 219 Frito-Lay North America. 2011. Frito Lay Frito-Lay Electric Delivery Trucks hit the Green Streets of Orlando 

www.pepsico.com/PressRelease/Frito-Lay-Electric-Delivery-Trucks-Hit-the-Green-Streets-ofOrlando12022011.html
53

Figure 2-10: GreenRoad On-Board Driver Behavior and Telematics

2.4.1.2 Benefits

According to one telematics manufacturer, Telogis,220 the FE performance spread between the 

best- and worst-trained drivers is about 25 percent.221 The Telogis Fleet telemetrics program 

monitors when fleet vehicles violate defined speed parameters, including posted speed limits; 

when “hard” braking occurs; and engine off/on status, which can be critical when loading and 

offloading hazardous cargo.222

GreenRoad claims that in a case study its users’ fuel consumption decreased 6-15 percent,223

while Telogis claimed its telematics package demonstrated 11 percent fewer VMT through route 

optimization and monitoring driver behavior for subsequent coaching.224 A comparison of 

telematic deployments for real-time fleet management and improved fuel economy found that the 

benefits do include safer driving, with a 50 percent crash reduction and lower insurance and 

crash-related costs. Additionally, the systems have led to less aggressive acceleration and 

significantly decreased speeding. Altogether, these behavioral changes add up to 10 percent 

reduced fuel consumption, a longer vehicle life, and a positive ROI in 3-6 months. 225 

GreenRoad claims that its system decreases maintenance for vehicle wear by up to 10 percent.

2.4.1.3 Safety Considerations

The Federal Motor Carrier Safety Administration and the NHTSA jointly conducted the Large 

Truck Crash Causation Study (LTCCS) to better understand the critical events that increase the 

 220 See www.telogis.com/solutions/mobile/navigation/ 221 Koch, J. 2012. Technologies for increasing fuel efficiency and eliminated fuel waste 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26464

222 Telogis Fleet Safety. www.telogis.com/benefits/improve-driver-safety-and-asset-security 223 GreenRoad. 2012. http://greenroad.com/ 224 Koch, J. 2012. Technologies for increasing fuel efficiency and eliminated fuel waste 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26464

225 Renaud, P. S., & Coleman, D. 2012. Telematics: real-time fleet management and improved fuel economy 

www.ntea.com/worktruckshow/index.aspx?id=25469
54

probability of large truck crashes.226 Fatigue, alcohol, and speeding were the most common 

critical events for the 87 percent227 of large truck crashes attributed to driver error, as shown in

Table 1-3 above, that listed the numbers and percentage of trucks involved in crashes by safetycritical factors. Only 10 percent of crashes were attributed to vehicle issues, and only 3 percent to 

environmental reasons such as snow or ice. NCSA crash reports do not include FE technologies 

or alternative fuel tanks on state crash report forms. 

Telematic technologies claim to both reduce fuel consumption and to improve safety by 

monitoring driver performance, including either displaying immediate feedback to the driver or 

recording unsafe operations for later coaching. GreenRoad claims that its users’ crash-related 

costs decrease 50-70 percent (including lower insurance premiums) compared to baseline fleet 

operations. Telogis claimed that its telematics package leads to a 90 percent reduction in 

speeding events after one year of use; in one case study, Telogis demonstrated 11 percent fewer 

VMT through route optimization and monitoring driver behavior for deviations and to support 

coaching.228 Although no crash reduction figures are provided, GreenRoad and Telogis data 

indicated that lower VMT and reduced speeding generally correlated with fewer crashes.

At the same time, a potential safety concern related to telematics systems that provide real-time 

feedback to drivers is the potential for additional driver distraction from these systems.

2.4.1.4 Findings and Conclusions

Driver behavior was a critical causal safety factor in the LTCCS for 87 percent of large truck 

crashes. By all indications, telematics and crash avoidance systems offer potential improvements 

in both driver safety performance and fuel efficiency. Both camera and non-camera based 

telematics setups are currently available and appear to be well accepted by MD/HDV fleet 

drivers. 

2.5 FE Technologies Affecting Operations

2.5.1 Speed Limiters

Speed limiters (or speed governors) are on-board electronic control modules that limit the top 

speed of a vehicle to a pre-set value. They are intended to reduce the fuel consumption of 

MD/HDVs when traveling on highways. Since the devices also prevent excessive speeding, their 

safety effects have been a recurring issue in truck safety discussions. Heavy-duty trucks are 

delivered from the factory equipped with speed limiting devices that can be configured by the 

 226 FMCSA. 2006. Report to Congress on the Large Truck Crash Causation Study www.fmcsa.dot.gov/factsresearch/research-technology/report/ltccs-2006.htm#TABLE1 227 This figure refers to all drivers in a crash. 228 Koch, J. 2012. Technologies for increasing fuel efficiency and eliminated fuel waste 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26464
55

vehicle owner. However, these devices may sometimes be disabled by trucking companies in 

order to maximize operating speed and save travel time, as reported in Quebec, Canada.229

2.5.1.1 Penetration and Adoption

Electronic control modules that can be programmed to limit speed have been standard equipment 

in all trucks for a number of years. Typically, the top speed is programmed at the factory 

according to buyer specification; many U.S. trucking companies already are programming to 68 

mph or slower. Estimates on how many U.S. fleets already use speed limiters ranges from 60 to 

80 percent.230

To date, 33 countries (excluding the United States) including the world's leading developed 

nations, require speed limiters in heavy-duty vehicles.231 The EU has required trucks and buses 

to be limited to 56 mph since 1994; Australia to 62 mph since 1990; Japan to 56 mph since 2003; 

and Quebec and Ontario to 65 mph since 2009.232 A Canadian study of the environmental and 

safety implications of speed limiter requirements for trucks233 reported maximal safety gains at 

90 kph on uncongested roads, with safety benefits decreasing at higher speeds (105 kph) and 

greater traffic congestion levels. The UK and other countries have also imposed speed limiter 

requirements, resulting in road safety improvements. 234 The European Commission introduced 

Intelligent Speed Adaptation (ISA)235 combining ITS “speed alert” technologies to automatically 

adjust speed limits depending on traffic conditions. NHTSA has initiated the rulemaking process 

for speed limiter regulations in the United States.236

2.5.1.2 Benefits

The FMCSA noted the European Commission (EC) multiple benefits of speed limiters:

“...lower fuel consumption (from 3 percent to 11%), lower maintenance costs (tires, 

brakes, and engine), increased road safety (fewer casualties), more relaxed driving and 

lower insurance premiums as a consequence of less accidents.” The negative effects 

noted were: “...decreased road safety when performing an overtaking maneuver as 

overtaking another vehicle takes relatively longer. An indirect effect is that the long 

 229 Purchase, E. 2011. Trucking Speed Governors: PA Semi Accident Lawyers on NHTSA Study 

www.yourerieaccidentlawyers.com/blog/2011/09/trucking-speed-governors-pa-semi-accident-lawyers-on-nhtsastudy.shtml

230 Trucker News Services. 2012. Speed limiter report: Trucks with devices had 50% lower ‘speed-limiter relevant’ 

crash rate www.thetrucker.com/News/Stories/2012/3/27/SpeedlimiterreportTruckswithdeviceshad50lowerspeedlimiterrelevantcrashrate.aspx

231 Road Safe America. 2012. The Problem: Physics 

www.roadsafeamerica.org/docs/RSASpeedLimiterWhitePaper_0039.pdf

232 Patton, O. 2011. NHTSA clears path for speed-limiter proposal www.truckinginfo.com/news/newsdetail.asp?news_id=72581

233 Summary Report - Assessment of a Heavy Truck Speed Limiter Requirement in Canada, TP14808 

www.tc.gc.ca/eng/roadsafety/tp-tp14808-menu-370.htm 234 FMCSA. 2008. CTBSSP SYNTHESIS 16 Safety Impacts of Speed Limiter Device Installations on Commercial 

Trucks and Buses A Synthesis of Safety Practice http://onlinepubs.trb.org/onlinepubs/ctbssp/ctbssp_syn_16.pdf 235http://ec.europa.eu/transport/road_safety/specialist/knowledge/esave/esafety_measures_known_safety_effects/inte

lligent_speed_adaptation_isa.htm

236 See www.gpo.gov/fdsys/pkg/FR-2011-01-03/html/2010-33057.htm
56

overtaking maneuvers of vehicles fitted with speed limitation devices have the effect of 

reducing the average speed of other road users.” 

The EC report concluded overall that:

“It is clear that the known effects of speed limitation devices are generally very positive 

for drivers, for companies, for society and for the environment. The negative aspects are 

small and avoidable: if all the speed limitation devices were set accurately to the same 

speed, there would be less need for overtaking.”237

According to the EPA SmartWay Partnership and the Truck Maintenance Council, reducing 

highway speed by 5 miles per hour reduces fuel use and greenhouse gas emissions by about 7 

percent while extending the life of a truck’s engine, tire treads, and brakes. Decreasing MD/HDV 

speed by one mile per hour would increase the FE by about 0.1 miles per gallon.238

According to an American Transportation Research Institute survey of U.S. trucking fleets, the 

second most common intended goal of fleets that choose to implement speed limiters is reduction 

of crashes.239

2.5.1.3 Safety Considerations

Approximately 4,000 people die annually in multi-vehicle crashes involving large trucks, of 

which 98 percent are the occupants of passenger vehicles, motorcyclists, bicyclists, or 

pedestrians. This represents over 20 percent of all multi-vehicle crash deaths in the United States 

According to a recent FMCSA study,240 approximately 15 percent of all truck crashes were 

identified as related to excessive speed.241 The speed limiter-relevant crash rate242 for trucks 

without speed limiters was 5 crashes per 100 trucks/year, compared to much lower 1.4 per 100 

trucks/year crash rate for trucks equipped with speed limiters. The study showed that the overall 

crash rates for trucks without speed limiters were higher than for trucks with speed limiters, 

namely 16.4 versus 11 crashes per 100 trucks/year. “Results from multiple analyses indicated a 

profound safety benefit for trucks equipped with an active speed limiter,” the report concluded. 

These findings are consistent with reports from countries in which speed limiters have been 

mandated. Road Safe America reported that fatal commercial truck accident rates in those 

 237 FMCSA. 2008. CTBSSP SYNTHESIS 16 Safety Impacts of Speed Limiter Device Installations on Commercial 

Trucks and Buses A Synthesis of Safety Practice http://onlinepubs.trb.org/onlinepubs/ctbssp/ctbssp_syn_16.pdf 238 See NHTSA Oct 2010 study “Factors and Considerations for Establishing a Fuel Efficiency Regulatory Program 

for Commercial Medium- and Heavy-Duty Vehicles” at 

www.nhtsa.gov/staticfiles/rulemaking/pdf/cafe/NHTSA_Study_Trucks.pdf

239 American Transportation Research Institute. 2011. Predicting Truck Crash Involvement: A 2011 Update 

www.atri-online.org/research/results/ATRI_Crash_Predictor_One_Pg_Summary_Apr_2011.pdf 240 See FMCSA, march 21012, “Research on the Safety Impacts of Speed limiter Devices on Commercial Motor 

Vehicles-Phase II” at www.fmcsa.dot.gov/facts-research/research-technology/report/Speed-Limiters.pdf 241 Trucker News Services. 2012. Speed limiter report: Trucks with devices had 50% lower ‘speed-limiter relevant’ 

crash rate www.thetrucker.com/News/Stories/2012/3/27/SpeedlimiterreportTruckswithdeviceshad50lowerspeedlimiterrelevantcrashrate.aspx 242 Assessment of whether a crash was speed limiter-relevant was based on four types of information in the dataset 

used for the study: speed limit where the crash occurred, crash type (e.g., rear-end truck striking), contributing 

factor(s) in the crash (used to exclude crashes; e.g., weather-related), and crash narrative. 
57

countries are lower than those in the United States.243 In the UK, all articulated heavy-goods 

vehicles were speed limited after 1993 and the accident involvement rate for that vehicle class 

fell from 40 per hundred million vehicle kms in 1993 to 30 in 2005—a 26 percent decrease. 244

Moreover, the heavy goods vehicles involvement in fatal crashes per 100,000,000 km has 

dropped to 1.1 in 2005 from 3.1 in 1990, a reduction of 65 percent.245 A large truck traveling 75 

mph requires approximately one-third more distance to stop compared with a truck traveling 65 

mph.246 Speed also exacerbates the size and weight differences between large trucks and

passenger vehicles, leading to more severe crashes. ABF Freight System Inc. credits slower 

speeds with strengthening safety performance, although the exact benefit is impossible to 

calculate.247 JB Hunt Transport stated that speed limiters will create a speed differential on the 

highway, but that the risk from that differential is outweighed by the risk of speeding. Schneider 

National reported that before there were universal speed limiters in its fleet, trucks without speed 

limiters accounted for 40 percent of collisions while traveling only 17 percent of the route 

miles.248

An analysis of nearly 19,000 truck crashes in Kansas found that 73 percent had contributory 

causes related to the truck driver. The top five contributors to truck crashes were: failing to give 

enough attention to the task, speeding, failing to yield the right of way, improper lane changes 

and following another vehicle too closely. Additionally, researchers found that more truck 

crashes happened in locations with a high speed limit.249 Similarly, in the FMCSA LTCCS 

“traveling too fast for conditions” was the most frequently cited factor in crashes where trucks 

were assigned the critical reason for the crash.250 Note that this may be due to lack of driver 

training rather than the presence or use of speed limiters.

Data from the FARS for 2005 analyzed by the IIHS showed that an estimated 328 fatal crashes 

involved drivers of large trucks who were speeding, resulting in 374 deaths. About 24 percent of 

drivers of large trucks involved in fatal crashes in 2005 had at least one speeding conviction 

 243 Road Safe America, 2009. RSA Speed Limiter White Paper 

www.roadsafeamerica.org/docs/RSASpeedLimiterWhitePaper_0039.pdf 244 FMCSA. 2008. CTBSSP SYNTHESIS 16 Safety Impacts of Speed Limiter Device Installations on Commercial 

Trucks and Buses A Synthesis of Safety Practice http://onlinepubs.trb.org/onlinepubs/ctbssp/ctbssp_syn_16.pdf 245 Road Safe America. 2012. The Problem: Physics 

www.roadsafeamerica.org/docs/RSASpeedLimiterWhitePaper_0039.pdf

246 McCartt, A. T. 2007. IIHS Letter re: Motor Vehicle and Carrier Safety Standards; Devices to Limit the Speed of 

Certain Trucks Request for Comments; Docket No. NHTSA-2007-26851 

www.iihs.org/laws/comments/pdf/nhtsa_fmcsa_ds_atm_032707.pdf

247 Insurance Institute for Highway Safety. 2010. Speed Limiters in Trucks Would Serve 2 Purposes 

www.iihs.org/externaldata/srdata/docs/sr4508.pdf 248 Patton, O. 2011. NHTSA clears path for speed-limiter proposal www.truckinginfo.com/news/newsdetail.asp?news_id=72581

249 Whistler, D. 2012. Researchers sift through crash statistics http://fleetowner.com/safety/researchers-sift-throughcrash-statistics? 250 FMCSA. 2007a. Large Truck Crash Causation Study http://ai.fmcsa.dot.gov/ltccs/default.asp
58

within the past 3 years; this compared with 19 percent of passenger vehicle drivers involved in 

fatal crashes due to speeding.251

A NHTSA 1991 report stated that the potential crash reductions from truck speed limiters were 

not sufficient to justify mandating them, since they would have little effect on speeds or crash 

likelihood below the set maximum speed.252 The Owner Operator Independent Drivers 

Association (OOIDA) cited this document in a 2007 opposing position statement.253 OOIDA 

also has expressed the concern that when there is a speed differential between trucks and 

passenger cars, this increases the rate of lane changes and sudden braking events, and in turn 

increases potential for car–truck crashes. However, a number of reports have found that speed 

limiters actually reduce speed variability and reduce lane change and deceleration maneuvers.254 

Europe’s experience with speed limiters that cap truck speeds at a speed substantially lower than 

the average speed of passenger vehicles also suggests no degradation in safety. 

However, since the 1995 repeal of the national maximum speed limit, which was in effect at the 

time of the previous NHTSA conclusion, there have been widespread increases in speed limits 

and in average travel speeds.255 In 2009, the American Trucking Association Safety Task Force 

endorsed electronically speed limiting all Class 7 and 8 trucks and setting a national 65 mph 

speed limit for all motor vehicles.256 

On January 3, 2011, NHTSA published a notice granting petitions for rulemaking from 

American Trucking Association (ATA), Road Safe America, and nine major motor carriers. The 

notice announced that the agency would initiate the rulemaking process with a notice of 

proposed rulemaking (NPRM) on speed limiters for certain heavy trucks. 

FMCSA sponsored a Transportation Research Board (TRB) study on “Commercial Truck and 

Bus Safety Synthesis Program (CTBSSP),” which assessed the safety impacts of speed limiter 

 251 McCartt, A. T. 2007. IIHS Letter re: Motor Vehicle and Carrier Safety Standards; Devices to Limit the Speed of 

Certain Trucks Request for Comments; Docket No. NHTSA-2007-

26851www.iihs.org/laws/comments/pdf/nhtsa_fmcsa_ds_atm_032707.pdf 252 See “Commercial Motor Vehicle Speed Control Devices, 1991, Report No. DOT HS 807 725. It was supportive 

of fleet applications of speed monitoring and speed limiting devices, but concluded that, because of the small target 

size, there was not sufficient justification to require the application of speed limiting devices at that time.

253 Johnston, J. 2007. Comments of the owner-operator independent drivers association, inc in response to notice and 

request for comments Motor Vehicle and Carrier Safety Standards [Docket No. NHTSA-2007-26851] 

www.ooida.com/Issues&Actions/Regulatory/commentdocs/speed%20limiter%20comments%20final%203-27-

07.pdf

254 Varhelyi, A., & Makinen, T. 2000. The effects of in-car speed limiters: field studies 

http://partnet.vtt.fi/evaserve/evaserve_tool/aineisto/limiter.pdf, Toledo, H., Albert, G., and Hakkert, S., 2007. A 

Simulation-Based Evaluation of the Impact of Active Speed Limiters on Traffic Flow and Safety 

http://pubsindex.trb.org/view.aspx?id=801530

255 McCartt, A. T. 2007. IIHS Letter re: Motor Vehicle and Carrier Safety Standards; Devices to Limit the Speed of 

Certain Trucks Request for Comments; Docket No. NHTSA-2007-26851 

www.iihs.org/laws/comments/pdf/nhtsa_fmcsa_ds_atm_032707.pdf 256 American Trucking Association. 2009. Expanding ATA’s Safety Agenda Executive Summary 

www.trucking.org/ATA%20Docs/What%20We%20Do/Image%20and%20Outreach%20Programs/ART/Safety%20

Task%20Force%20Report.pdf 
59

device installations on commercial trucks and buses through a fleet survey.257 The majority of 

U.S. fleet managers (although based on a low survey response rate) indicated that speed limiters 

were either “successful” or “very successful” in reducing crashes. In operational terms, speed 

limiter users believed that limiters were either “successful” or “very successful” in reducing tire 

wear (44%) and increasing fuel economy (76%). Almost 96 percent of respondents indicated 

speed limiters did not negatively affect safety or productivity. 

A recent FMCSA study on the efficacy of truck speed limiters in improving road safety258 used 

truck crash data collected directly from truck fleets representing 138,000 trucks and over 15,000 

crashes. The findings showed strong positive safety benefits for speed limiters. Trucks equipped 

with speed limiters had a significantly lower overall crash rate (approximately 50%) compared to 

those without, and a more than three-fold decrease in speed-limiter-relevant crash rate. A less 

comprehensive 2009 Transport Canada study of 400 power units across two fleets concurred that 

lower speeds have resulted in an improved safety record. Based on the collision data reviewed, 

there was no evidence that speed limiters were contributing to the occurrence of collisions. In 

fact, there were no rear end collisions attributed to operating with a speed limiter.259 

2.5.1.4 Findings and Conclusions 

Speed limiters on MD/HDVs have demonstrated significant fuel efficiency and safety benefits, 

while also reducing vehicle wear and tear and maintenance costs.260 Case studies cited above 

from multiple adopting countries and from U.S. truck fleets indicate net positive safety benefits 

from broad adoption of this FE technology. Speed limiters appear to provide a consistent and 

effective means to maintain lower truck speeds, save fuel, and improve road safety.

2.5.2 Idle Reduction

Idle reduction technologies allow vehicle operators to refrain from long-duration idling of the 

main propulsion engine by using one of several alternative technology options.261 An idle 

reduction device may be either installed on board a vehicle or at a wayside station to provide 

auxiliary power services (e.g., heat, air conditioning, and/or electricity) instead of the main drive 

engine, while the vehicle is parked or stationary. On-board idle reduction technologies include: 

• automatic start-stop, which shuts down the motor when the vehicle is stationary and 

starts it again once power is needed; 

• direct-fired heaters, which are small, lightweight heaters that burn fuel from the main 

engine fuel supply; and 

 257 FMCSA. 2008. CTBSSP SYNTHESIS 16 Safety Impacts of Speed Limiter Device Installations on Commercial 

Trucks and Buses A Synthesis of Safety Practice http://onlinepubs.trb.org/onlinepubs/ctbssp/ctbssp_syn_16.pdf 258 FMCSA. 2012b. Research on the Safety Impacts of Speed Limiter Device Installations on Commercial Motor 

Vehicles: Phase II www.fmcsa.dot.gov/facts-research/research-technology/report/Speed-Limiters.pdf 259 Transport Canada. 2009b. Summary Report - Assessment of a Heavy Truck Speed Limiter Requirement in 

Canada www.tc.gc.ca/eng/roadsafety/tp-tp14808-menu-370.htm 260 See IIHS Status Report, Aug.21, 2010 “Speed limiters in trucks would serve two purposes” at 

www.iihs.org/externaldata/srdata/docs/sr4508.pdf 261 See EPA SmartWay verified idle Reduction options at www.epa.gov/smartway/technology/idling.htm
60

• Auxiliary Power Units (APUs), which are small diesel engines that are installed on the 

truck to provide air conditioning, heat, and electrical power to run accessories like lights, 

on-board equipment, and appliances. 

Electrification can also provide the operator with climate control and auxiliary power without 

idling. This additional power may be delivered by on-board equipment (e.g., generator sets-GS, 

power inverters, solar panels, thermal storage systems-TSS), and/or off-board equipment (e.g., 

shore power at truck stops).

2.5.2.1 Penetration/Adoption

The North American Council for Freight Efficiency (NACFE) 2012 benchmarking study of antiidling technology penetration262 found only a modest (less than 10%) adoption rate for antiidling technologies by the truck fleets surveyed. Penetration of anti-idling devices has been 

driven both by economic payoff from cumulative fuel savings, as well as by compliance with 

anti-idling pollution laws263 in 46 jurisdictions in 31 States. These laws carry fines ranging from 

$100 - $25,000 per violation, and in some States, repeat offenders may be jailed.264 A wide 

range of truck OEMs, drivetrain suppliers, and integrators are offering trucks with built-in idle 

reduction systems, including Eaton, Altec, and Freightliner.265

Anti-idling systems offer a good opportunity for petroleum and emissions reductions in heavyduty vehicles—particularly vocational vehicles, as well as for long-haul routes. Common idling 

situations occur at worksites where power from the main engine of a vocational truck is needed 

to operate equipment such as an aerial lift, and when delivery vehicles stop for loading and 

unloading. Long-haul trucks idle for cab comfort during mandatory driver rest periods. Work 

truck fleets, such as Verizon, are implementing idle reduction solutions that operate the aerial 

using electric power, turning on the engine only when the energy storage system runs low.266

2.5.2.2 Benefits

Class 7 and 8 trucks alone consume over a billion gallons of diesel fuel per year when idling. 

Adoption of anti-idling options helped the eight study fleets surveyed by NACFE to lower their 

annual per truck fuel expense by an average of $4,500.267, 268 An Argonne National Laboratory 

analysis found that an idling model year 2001 truck uses 0.77 gal/hour for heating and 0.98 

 262 NACFE 2012 Anti-Idling Study http://nacfe.org/wp-content/uploads/2012/07/NACFE-2012-Anti-Idling-Study062812.pdf

263 See CARB HDV idle reduction requirements at www.arb.ca.gov/msprog/truck-idling/truck-idling.htm 264 Cascade Sierra Solutions. 2011. Shorepower Truck Electrification Project www.the-step-project.org/greenanswers/fleet-managers/ 265 TruckingInfo. 2012. Eaton enhances hybrid drive system for faster payback www.truckinginfo.com/news/newsdetail.asp?news_id=77134, Freightliner. 2012. Freightliner hybrid trucks 

www.cleanandgreenvehicles.com/pdfs/M2eHybrid-Brochure.pdf, Altec Industries. 2012. Altec Green Fleet 

www.altec.com/green-fleet.php 266 Beegle, B. 2012. Worksite idle reduction 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26496

267 North American Council for Freight Efficiency. 2012b. 2012 Anti-Idling Study http://nacfe.org/wpcontent/uploads/2012/07/NACFE-2012-Anti-Idling-Study-062812.pdf 268 Ibid.
61

gal/hour for cooling. In comparison, a direct-fired heater only uses 0.06 gal; a storage air 

conditioner uses 0.2 gal; an APU uses 0.23 gal; and an electrified parking space uses between 1.7 

and 2.4 kWh (between $1.00 and $2.45 per hour, well below the cost of diesel fuel consumed by 

idling).269 Any truck or bus that idles 20 hours or more per week is projected to realize payback 

and return on investment within five years from implementing any of the idle reduction 

technologies.270 

2.5.2.3 Safety Considerations

There were no safety incidents relating to the use of automatic start-stop, APUs, electrified 

parking spaces, or other idle reduction technologies identified in the literature. In comparison, 

potential safety benefits associated with idle reduction equipment adoption appear to be indirect, 

relating to driver in-cab air quality health impacts, and to better sleep quality. Using either 

electrified parking space power or an auxiliary power unit for air conditioning improves in-cab 

air quality for nitrogen oxide (NOx), carbon monoxide (CO), particulate matter (PM) and total 

hydrocarbons compared to an idling vehicle. Idling emissions can contribute to premature 

mortality, bronchitis (chronic and acute), hospital admissions, respiratory symptoms (upper and 

lower), cancer, asthma attacks, work loss days, and minor restricted activity days.271 Given that 

up to one-third of drivers suffer from sleep apnea and many require continuous positive airway 

pressure machines in their cabs, improving conditions for sleep could reduce operator fatigue and 

thus reduce the potential for crashes due to sleeplessness.272

2.5.2.4 Findings and Conclusions 

Idle reduction technologies can provide significant FE benefits to MD/HDV fleets, particularly 

for high-idling applications such as vocational and delivery vehicles, and long-haul sleeper 

tractor cabs. When used in work trucks and delivery vehicles, these FE technologies also lead to 

a significant reduction in point source emissions of criteria pollutants in urban locations, where 

human exposure is high. 

Further research is needed to quantify the safety benefits of idle reduction relating to improved 

in-cab air quality and potentially improved operator sleep, and any related effect on maintaining 

hours of service safety. This literature review did not identify any safety disbenefits of anti-idling 

technologies.

2.6 Longer Combination Vehicles

Longer combination vehicles (LCV) allow trucks to improve efficiency and save fuel by hauling 

more freight in a single load by pulling multiple trailers per tractor, thereby making fewer trips 

and reducing the vehicle miles travelled. There are three common types: Rocky Mountain 

 269 Gaines, L. 2008. Which Idling Reduction Technologies Are the Best? 

www.transportation.anl.gov/pdfs/EE/533.pdf

270 Ibid. 271 DOE EERE. 2012b. Idle Reduction Research and Development 

www.afdc.energy.gov/afdc/vehicles/idle_reduction_research.html 272 Cascade Sierra Solutions. 2011. Shorepower Truck Electrification Project www.the-step-project.org/greenanswers/fleet-managers/
62

Doubles, Turnpike Doubles, and Triple Trailers.273 (See Figure 5-4 for the corresponding FHWA 

classification graphic.274) 

2.6.1 Penetration/Adoption

LCVs have operated in 19 American States for a number of decades, and Canada for several 

decades. LCVs are suited to light, bulky freight as their maximum allowable weight is no greater 

than other tractor-trailers currently in operation. The Surface Transportation Assistance Act 

(STAA) of 1982 provided for the unrestricted use of two-trailer combinations with two 28-foot 

to 28.5-foot trailers on the National Network (NN).275 The National Truck Network (NTN) 

comprises 209,000 miles of highways that can accommodate large trucks, including the 47,000 

mile Interstate Highway System. 

Some States allow LCVs with greater lengths or different tractor-semitrailer combinations, but 

most do not. LCVs are primarily allowed in Western States but are also permitted on some 

turnpikes, for example in New York and Massachusetts.276 For example, LCVs currently operate 

in 16 States west of the Mississippi River and on turnpikes in 5 States east of the Mississippi 

River. Among the States that do allow LCVs, 11 allow operation of triples, 8 allow triples with 

permits, and 8 allow Rocky Mountain Doubles. Only 3 States allow operation of LCVs without 

restrictions.277 

Of the eight major fleets surveyed by NACFE regarding LCV adoption, only one has begun to 

use double or triple trailers between 2003 and 2011.278

2.6.2 Benefits 

Each LCV uses about one-third less fuel and emits one-third fewer greenhouse gas emissions 

than two tractor-trailers that would carry the same amount of freight.279 LCVs introduce an 

increase of cargo-carrying capacity of 30 percent to 100 percent per driver. This results in fewer 

truck trips and fewer miles driven.280 Fewer trucks on the road would also reduce congestion 

and improve safety. If freight that is currently moved by railroad is shifted to LCVs due to lower 

cost, the above benefits could decrease. However, the existence or extent of mode shifting and 

cross price elasticity has not been well studied.

 273 DOE EERE. 2006. Fact #411: February 13, 2006 States that Allow Longer Combination Vehicles 

www1.eere.energy.gov/vehiclesandfuels/facts/2006_fcvt_fotw411.html

274 See original FHWA graphic at www.fhwa.dot.gov/policy/otps/truck/wusr/fig02_2.gif 275 The National Network consists of the Interstate System and routes designated by the FHWA in consultation with 

the States.

276 Ibid. 277 EPA. 2011a. EPA Smartway Longer Combination Vehicle Factsheet 

www.epa.gov/smartway/documents/partnership/trucks/partnership/techsheets-truck/EPA420F10-053.pdf 278 North American Council for Freight Efficiency. 2012a. 2011 Fleet Fuel Efficiency Benchmark Study 

http://nacfe.org/

279 British Columbia Ministry of Transportation and Infrastructure. 2011. Long Combination Vehicles Program 

www.th.gov.bc.ca/cvse/lcv/faqs.htm

280 California Department of Transportation. 2009. Longer Combination Vehicles 

www.dot.ca.gov/hq/traffops/trucks/exemptions/lcvs.htm
63

2.6.3 Safety Considerations

The safety of freight moving on highways and roadways is a combination of many factors, 

including exposure due to vehicle miles traveled, vehicle performance characteristics, driver 

fatigue and capability, enforcement, roadway design, road conditions, motor carrier 

management, and vehicle condition and maintenance. It is difficult to isolate the impact of truck 

size and weight (TS&W) among these factors. 

LCVs are allowed to operate on certain interstate and State highways in only 19 U.S. States 

primarily due to concerns over safety and potential infrastructure damage and maintenance 

costs.281 An advocacy organization, OOIDA,282 has raised safety concerns about LCVs, 

including the off-tracking of rear wheels when turning due to rearward amplification,283 the 

incompatibility of LCVs with the turning radii of existing interchanges and intersections, their 

ability to maintain speed on inclines, and their potentially slower acceleration and braking in 

traffic. A related LCV hazard scenario and the references it is based on is discussed in detail in 

Chapter 5.2.5. 

Statistics on LCV crashes are difficult to obtain because of the low number of vehicles and 

limited areas allowing them (See Section 6.6 and Figure 6-1). It is known that triple trailers tend 

to experience rearward amplification and can leave the lane they are traveling in, although this 

can be lessened by advanced connector types or with advanced active steer correction 

technology. Triples also require more passing length, spray more rain and snow, and have a 

history of being underpowered while climbing steep grades. 284

Heavier truck traffic can deteriorate pavement structures at an accelerated rate, and serious 

roadway defects can subsequently lead to crashes. One combination vehicle pass may cause wear 

equivalent to 2,000 to 3,000 cars. Excess pavement damage from LCVs is dependent on whether 

the increase in vehicle weight is distributed across a sufficiently increased number of axles. 285

In Canada, LCVs are no heavier than current tractor-trailers, yet they generally have more axles 

and tires to grip the road and are equipped with enhanced braking systems. Their stopping ability 

is generally superior to other tractor-trailers and their road damage should be no greater than 

current tractor-trailers. A number of Canadian reports suggest that collision rates for LCVs in 

Canada are relatively low, given the numerous restrictions of operation.286, 287 A 2005 study by 

the Canada Safety Council found that overall there is little difference in crash rates between 

LCVs and other trucks when operated under similar conditions of weather, road, and driver 

 281 Adams, T., Kleinmaier, D., Marach, A., Helfrich, G., Levine, J., & Bittner, J. 2012. Longer Combination 

Vehicles: An Estimation of their Benefits and Public Perception of Their Use www.wistrans.org/cfire/.../CFIRE_05-

01_Final_Report.pdf

282 Owner-Operator Independent Drivers Association. 2005. Longer Combination Vehicles (LCVs) 

www.ooida.com/OOIDA%20Foundation/Issues/LCVs.asp

283 Paul S. Fancher and Kenneth L. Campbell, Truck Size and Weight Study Phase I: Working Papers 1 and 2 

combined: Vehicle Characteristics Affecting Safety. www.fhwa.dot.gov/reports/tswstudy/vehiclsaf.htm 284 California Commercial Driver handbook 2010-2011 at www.dmv.ca.gov/pubs/comlhdbk/comlhdbk.pdf 285 Ibid. 286 British Columbia Ministry of Transportation and Infrastructure. 2011. Long Combination Vehicles Program 

www.th.gov.bc.ca/cvse/lcv/faqs.htm

287 Ibid.
64

experience levels. However, LCVs reduce the vehicle miles traveled to move the same volume of 

freight, since each LCV typically carries as much freight as two single trailer trucks. Factoring in 

the reduced exposure and freight demand, the Safety Council concluded that LCVs have a safety 

record equivalent to standard trucks.288 This may be because of the strict controls in terms of 

routing, weather restrictions, and driving placed on them by the carriers and by the road 

authorities, and because the drivers must meet higher standards of skill and experience. Given 

the confounding factors and smaller sample size of LCVs, it is difficult to assess their relative 

safety performance.

In 2010, the National Research Council reviewed and summarized a compendium of LCV 

studies from Canada, the United States, and Australia. A study published by the Canadian 

Journal of Civil Engineering reviewed accident rates in Alberta and concluded that from a 

collision rate perspective, LCVs as a group have better safety performance than other articulated 

trucks. Turnpike doubles, it reported, have the lowest collision rate of all articulated truck types, 

followed by Rocky Mountain doubles. Yet the collision rate for triple trailer combinations is 

higher than the collision rates for tractor semitrailers.289 

Previous studies of potential safety impacts of TS&W policy changes have relied primarily on 

studies that compared crash rates of single- and multi-trailer combinations. However, most multitrailer combinations are STAA290 short (maximum 28’ 6”) doubles that are comparable in length 

and weight to single-trailer combinations. While these doubles are less stable than standard 

single-trailer tractor-semitrailers, they perform better than tractor-semitrailers in terms of their 

static rollover threshold and offtracking. The various LCV configurations analyzed typically fall 

between the tractor-semitrailer and the STAA double in terms of stability and control properties. 

Yet they are longer and heavier than either of those standard vehicles and they have greater 

offtracking. The opposite safety differences along these two dimensions are occluded in 

aggregate crash rate studies, making it difficult to compare the relative safety performance of 

different LCV configurations.291

The literature review offers no clear consensus on the safety of LCV operations to date and 

efficiency implications of expanding them nationwide. Some studies found LCVs to be less safe 

in terms of fatal crashes, or when cars pass LCVs. Others concluded that LCVs are safer because 

of lower collision rates, or lower crash costs versus tractor semi-trailers. The lack of sufficient 

data and controls in these studies, as well as inconsistent study methodologies, may prevent a 

definitive safety evaluation of LCVs at this time.292 The Federal Highway Administration Office 

of Freight Management issued in December 2012 a notice soliciting bids for a new study on 

specific areas of Federal truck size and weight TS&W limits, their operation, and their impacts, 

 288 Ibid. 289 Regehr, J,. Montufar, J., Rempel, G. 2008. Safety performance of longer combination vehicles relative to other 

articulated trucks www.nrcresearchpress.com/doi/abs/10.1139/L08-109 290 Surface Transportation Assistance Act 291 FHWA. 2004. Western Uniformity Scenario Analysis: A Regional Truck Size and Weight Scenario Requested by 

the Western Governors' Association www.fhwa.dot.gov/policy/otps/truck/wusr/chap07.htm 292 Adams, T., Kleinmaier, D., Marach, A., Helfrich, G., Levine, J., & Bittner, J. 2012. Longer Combination 

Vehicles: An Estimation of their Benefits and Public Perception of Their Use www.wistrans.org/cfire/.../CFIRE_05-

01_Final_Report.pdf
65

responding to a mandate in the Moving Ahead for Progress in the 21st Century (MAP-21)

legislation.293 This requirement to further study the issue was a compromise between proponents 

of allowing six-axle truck/trailer configurations to gross 97,000 pounds and opponents of any 

size or weight increases.

2.6.4 Literature Findings 

Based on references discussed above LCVs can result in significant fuel efficiency improvement 

of 30 percent over conventional tractor trailers, with proportional reductions in pollutant and 

greenhouse emissions. Due to a decrease in VMT per unit of cargo, reduced crashes would also 

be expected. Whether these expected safety benefits are realized, could be influenced by 

whether, and to what extent, any rail cargo is diverted to LCVs. If this occurs, the benefits could 

be reduced. 294

Even assuming there is no significant freight rail diversion to the highway mode, it is still 

difficult to extrapolate the positive safety findings from studies conducted outside the United 

States to U.S.-wide LCV operations because the operating environments differ. Regulatory 

differences could also be expected to have a significant impact on the safety of LCV 

operations. 295 Potential LCV hazards (see Section 5.2.5), such as offtracking and rearward 

amplification of LCVs require further evaluation. Advanced rearward amplification-reducing 

bogies or active steering correction systems may offer some safety mitigation. Operational 

restrictions, including strict route designations and strict driver training standards for LCVs, may 

also be required. Oversized and overweight LCVs may be good candidates for the use of 

telematics systems that monitor and help ensure the safety performance of drivers. 

2.7 Tire Technologies

2.7.1 Tire Pressure Monitoring Systems and Automatic Tire Inflation Systems 

Based on a representative study of hundreds of trucks, FMCSA determined that tire pressure is 

within 5 psi of proper pressure in only half of all truck tires.296 In addition to programs of 

routine manual pressure checking and inflation procedures, a number of tire pressure monitoring 

systems (TPMS) and automatic tire inflation systems (ATIS) can be installed onboard 

MD/HDVs. These systems report the tire pressures to the driver or issue an alert if tire pressure 

is below a set point. Furthermore, in-pavement tire pressure pads have become available for fleet 

terminals that measure the tractor and trailer’s tire pressure as it drives over the device. 297

 293 Se FHWA Talking Freight at 

www.fhwa.dot.gov/planning/freight_planning/talking_freight/nov282012transcript.cfm

294 FHWA. 2004. Western Uniformity Scenario Analysis: A Regional Truck Size and Weight Scenario Requested by 

the Western Governors' Association www.fhwa.dot.gov/policy/otps/truck/wusr/chap07.htm 295 Ibid. 296 Lockridge, D. 2010. FMCSA talks safety technology www.truckinginfo.com/news/newsdetail.asp?news_id=70422

297 North American Council for Freight Efficiency. 2012a. 2011 Fleet Fuel Efficiency Benchmark Study 

http://nacfe.org/
66

2.7.1.1 Penetration/Adoption

According to NACFE, only automatic tire pressure inflation systems on trailers had significant 

adoption rates, with nearly 30 percent of the trailers in the eight surveyed fleets incorporating 

ATIS by 2011.298 There does not seem to be a predominant tire FE technology or practice 

emerging, so NACFE is continuing annual tire pressure benchmarking surveys299 of fleet 

adoption and FE gains. Most fleets still choose to manage correct tire pressure through routine 

manual maintenance rather than procuring systems that will monitor and inflate tires on their 

own. 300 Approximately only 5 percent of national truck fleets employ TPMS technology. The 

low penetration rate appears to be based on concerns of system reliability, maintenance costs, 

and initial system costs.301 

2.7.1.2 Benefits

Correct tire inflation is important for achieving optimal fuel economy in commercial motor 

vehicles (CMVs). Preliminary results from a FMCSA field operational test with one truck fleet 

indicated a 1.8 percent increase in FE due to TPMS and found that the tires on non-equipped 

control trucks showed faster wear than those on TPMS trucks. The extra wear corresponded to 

one extra road call per year.302 Another FMCSA study of three different TPMS models recorded 

a fuel economy increase of 1.4 percent and positive ROI in less than one year. Tread wear 

improvements were measurable, especially on the drive tires. According to the study, some 

drivers who were initially wary of the systems eventually asked management to equip entire 

fleets with TPMS and ATIS.303

TPMS systems, whether on-board or terminal-based, greatly assist in a fleet’s ability to monitor, 

take action and continue to educate drivers and maintenance technicians on the importance of 

correct tire inflation for both FE gains and for safe stopping distance when braking. 

In combination with a telematic system, TPMS can also report tire pressures to a web portal and 

warn the fleet manager or dispatcher of underinflated tires. One TPMS manufacturer claims that 

this extends the life of tires up to 20 percent.304

NACFE studies reported that fleets adopting tire pressure inflation and monitoring systems for 

use on vehicles, using drive-over tire pressure measurement mats or strictly requiring drivers to 

routinely check pressure saved $4,500 per year.305

 298 Ibid. 299 See http://nacfe.org/wp-content/uploads/2012/07/NACFE-2012-Tire-Pressure-Study-062812.pdf 300 North American Council for Freight Efficiency. 2012a. 2011 Fleet Fuel Efficiency Benchmark Study 

http://nacfe.org/

301 Lockridge, D. 2010. FMCSA talks safety technology www.truckinginfo.com/news/newsdetail.asp?news_id=70422

302 Ibid. 303 Park, J. 2012. DOT study proves ROI on tire inflation and pressure monitoring systems 

www.truckinginfo.com/news/news-detail.asp?news_id=76201 304 WABCO. Integrated Vehicle Tire Pressure Monitoring (IVTM) www.wabco-auto.com/ivtm 305 North American Council for Freight Efficiency. 2012c. 2012 Tire Pressure Study http://nacfe.org/wpcontent/uploads/2012/07/NACFE-2012-Tire-Pressure-Study-062812.pdf
67

2.7.1.3 Safety Considerations

Tire under-inflation results in irregular tire wear, which can reduce the ability of tread to grip the 

road in adverse conditions.306

Currently, NHTSA requires TPMS for proper tire inflation for light duty cars and trucks in view 

of safety benefits.307 A concern with some automatic tire inflation systems noted in the NACFE 

reference cited is that a driver may be over reliant on TPMS/ATIS and continue to drive on a flat 

tire, expecting it to self-inflate, only to have a blowout later if the ATIS fails.308 As with any 

new technology, driver education is required to understand the limitations of the technology.

2.7.1.4 Literature Findings 

A tire can lose up to half of its air pressure and still not appear to be flat, so either regular checks 

by the driver, or automatic systems such as TPMS/ATIS, are critical for both vehicle safety and 

for optimal fuel efficiency. It is not clear from the available literature whether such automatic 

systems for MD/HDVs lead to safer vehicle operations, versus diligent visual checking 

procedures. However, there is also no evidence to indicate that TPMS/ATIS adoption leads to 

lower safety performance. Given that a wide range of tire checking diligence may be expected 

across fleets and drivers, automatic systems appear likely to raise the lowest common 

denominator of diligence, and therefore operational safety. 

2.7.2 Single Wide Tires

Although heavy and medium trucks still most commonly use double tires at each wheel site, over 

the past decade major tire suppliers have begun to offer single wide base – or “super single” –

tires for MD/HDVs. With single wide tires, only one tire is installed per wheel site instead of two 

(Figure 2-11); meaning an 18-wheeler becomes a 10-wheeler. This design change is intended to 

improve fuel efficiency through a combination of decreased rolling resistance, lower 

aerodynamic drag, and reduced weight.

Figure 2-11: Single Wide Base Tires Replacing Duals at Each Wheel Site309

 306 HCI Corporation. 2011. Tire-SafeGuard www.tiresafeguard.com/ 307 See Safercar.gov TPMS postings at www.safercar.gov/Vehicle+Shoppers/Tires/Tires+Rating/TPMS 308 North American Council for Freight Efficiency. 2012a. 2011 Fleet Fuel Efficiency Benchmark Study 

http://nacfe.org/

309 North American Council for Freight Efficiency. 2010. Executive Report – Wide Base Tires 
68

2.7.2.1 Penetration/Adoption

More than two decades ago, single wide tires were seen as an alternative to the conventional dual 

truck tire assembly that had been the industry standard since its introduction in the early 20th

century. Numerous fleets and owner operators have adopted this technology.310 

Two of the eight major truck fleets surveyed by NACFE have exclusively adopted single wide 

tires, driven by weight reduction and fuel economy goals.311 One fleet surveyed by NACFE had 

no plans to adopt over concerns about the initial cost and the break even period. All surveyed 

fleets believed that tire and wheel availability and other issues that prevented widespread single 

wide tire adoption no longer exist. These fleets indicated that pressure from heavier 

tractorsdue to new EPA emissions equipmentand from denser loadsled them to consider 

weight-saving measures, including wide-base tires. 

2.7.2.2 Benefits

Studies indicate that single wide tires may yield fuel savings of 2-6 percent, as a result of 

reduced aerodynamic drag, rolling resistance, and weight reduction.312,313 Single wide tires were 

also reported to improve vehicle stability and handling, especially for tanker trailers by allowing 

the tank to be mounted lower.314 A lower center of gravity should reduce the risk of vehicle 

rollover. 

The total weight savings for a typical combination truck using single wide base tires on its drive 

and trailer axle ranges from 800 to 1,000 pounds. The weight savings reduce fuel consumption or 

increase cargo capacity for trucks that are weight-limited. Single wide tires may offer other 

benefits in combination with improved truck stability.315

Using single wide tires, there are also fewer tires and no hard-to-reach inner tires. Most uneven 

tire wear occurs because one dual has less air in it than another. That problem is eliminated with 

singles.316 Increased brake life is also a possible benefit of using single wide tires. Depending on 

axle width and wheel offset, more of the brake drum is exposed to open air, allowing the drum 

and shoe to stay cooler, with claimed benefits of up to an extra 20,000 miles of brake life per 

tractor. 

 http://nacfe.org/wp-content/uploads/2010/12/NACFE-ER-1002-Wide-Base-Tires-Dec-2010.pdf 310 Ibid. 311 North American Council for Freight Efficiency. 2012a. 2011 Fleet Fuel Efficiency Benchmark Study at 

www.greenfreightandlogistics.org/news/2011-fleet-fuel-efficiency-benchmark-study// 312 Salari, K. 2012. DOE’s Effort to Reduce Truck Aerodynamic Drag through Joint Experiments and Computations 

www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2012/veh_sys_sim/vss006_salari_2012_o.pdf

313North American Council for Freight Efficiency. 2010. Executive Report – Wide Base Tires 

http://nacfe.org/wp-content/uploads/2010/12/NACFE-ER-1002-Wide-Base-Tires-Dec-2010.pdf 314 EPA. Smartway: a glance at clean freight strategies: single wide based tires at 

www.epa.gov/smartway/forpartners/documents/trucks/techsheets-truck/EPA-420-F10-041.pdf 315 EPA. 2012. Low Rolling Resistance Tires A Glance at Clean Freight Strategies 

www.epa.gov/smartway/documents/partnership/trucks/partnership/techsheets-truck/EPA-420-F10-041.pdf 316 Overdrive. 2012. Single-Minded www.overdriveonline.com/single-minded/
69

2.7.2.3 Safety Considerations

A potential safety concern is that drivers may lose control when a single wide tire blows out on 

the highway, since there is no remaining dual to shoulder the load on that wheel site. This 

appears unlikely to happen because most combination trucks have tandem axles, so they can still 

pull over to the shoulder if a wide base tire fails. The single wide tires also lower the vehicle’s 

center of gravity, so the vehicle may be less likely to lose control than one with dual wheels.317 

When a single wide tire fails, the vehicle is certainly down and cannot be “limped in” to the 

nearest point of service, as can be done with dual wheels. However, with the implementation of 

new CSA rules “limping in” may no longer be accepted practice.318 

Single tires eliminate the potential for the two tires on a dual wheel to have different pressures. 

Unequally inflated dual wheel tires shift the load to the tire with higher pressure. This may not be 

visually apparent and therefore is likely to occur without the driver’s knowledge, increasing the 

probability of a blowout.319 Fleets that have adopted single wide tires report that breakdowns 

with wide base tires occur less frequently because the driver can visually detect when they have a 

low tire and address the issue sooner than on a vehicle with dual wheels.320

Testing by tire manufacturers and fleets suggests that traction of wide base tires is comparable to 

that of dual tires.321

2.7.2.4 Findings and Conclusions 

Given advances in single wide tire design over the past decade, and in light of positive fuel 

efficiency gains and safety experience to date reported by NACFE and major fleet adopters, wide 

base tires were found to offer a straightforward and low-risk way to reduce weight and to 

improve the FE of MD/HDVs fleets by a significant 2-6 percent. 

2.7.3 Low Rolling Resistance Tires

Overcoming tire rolling resistance accounts for nearly 13 percent of the total energy required to 

keep a combination truck in motion.322 Tire design plays a key role in determining the level of 

rolling resistance, and innovations in tire tread design have made it possible to decrease 

resistance. While there have been indications that for some rugged duty cycles these lower 

rolling resistance tread designs may not provide sufficient traction, most long-haul highwaybased trucks have already adopted lower rolling resistance tires.323 

 317 Ibid. 318 NACFE 2010 Wide base tires report at http://nacfe.org/wp-content/uploads/2010/12/NACFE-ER-1002-WideBase-Tires-Dec-2010.pdf 319 Overdrive. 2012. Single-Minded www.overdriveonline.com/single-minded/ 320 See ref 282 NACFE 2010 report 321 North American Council for Freight Efficiency. 2010. Executive Report – Wide Base Tires http://nacfe.org/wpcontent/uploads/2010/12/NACFE-ER-1002-Wide-Base-Tires-Dec-2010.pdf 322 EPA. 2011c. SmartWay Technology Program: Verified Low Rolling Resistance Tires 

www.epa.gov/smartway/technology/tires.htm 323 Carbon War Room Research & Intelligence Group. 2012. Unlocking fuel-saving technologies in trucking and 

fleets 
70

2.7.3.1 Penetration/Adoption

All eight fleets surveyed by NACFE, representing 75,000 tractors and 130,000 trailers, have 

adopted low rolling resistance tires.324 The EPA references a test of LRR tires on 15 fleets that 

logged 57 million miles.325 More extensively, all non-short-haul heavy-duty tractors that pull 

53-foot or longer box-type trailers in California have been required to install LRR tires since 

2010 following the adoption of an Air Resources Board greenhouse gas reduction regulation.326

2.7.3.2 Benefits

The EPA’s SmartWay Partnership states that the FE improvement due to LRR tires is 2-5 

percent. SmartWay acknowledges that LRR tires may have slightly decreased tread life, but cites 

a manufacturer report that found the tires to wear at a rate comparable to conventional tires. 327 

These improvements are achieved when verified low rolling resistance tires are installed on all of 

the axle positions of the tractor and trailer and when all tires are properly inflated according to 

the manufacturer's specifications.328

2.7.3.3 Safety Considerations

In a 2009 NHTSA study of low and standard rolling resistance tires for light-duty vehicles, the 

tire models tested were found to show a strong and significant relationship between lower rolling 

resistance and lower wet slide number (i.e., locked-tire wet traction coefficient). This may be 

significant to light-duty vehicles that are not equipped with anti-lock braking system (ABS), 

since the wet slide number relates most closely to locked-wheel emergency stops on wet road 

surfaces. For newer vehicles with ABS or electronic stability control systems, the tradeoff was 

predicted to be less significant.329 However, it is important to note that ABS have been required 

by NHTSA on all trucks and buses since the 1997-1999 implementation timeframe.330 A recent 

 http://carbonwarroom.com/sites/default/files/reports/Unlocking%20Fuel%20Saving%20Technologies%20in%20Tru

cking%20and%20Fleets%20(Carbon%20War%20Room).pdf 324 North American Council for Freight Efficiency. 2012a. 2011 Fleet Fuel Efficiency Benchmark Study 

http://nacfe.org/

325 EPA. 2012. Low Rolling Resistance Tires A Glance at Clean Freight Strategies 

www.epa.gov/smartway/forpartners/documents/verified/420f12024.pdf

326 California Air Resources Board. 2012. Heavy-Duty (Tractor-Trailer) Greenhouse Gas Regulation 

www.arb.ca.gov/cc/hdghg/hdghg.htm

327 EPA. 2012. Low Rolling Resistance Tires A Glance at Clean Freight Strategies 

www.epa.gov/smartway/documents/partnership/trucks/partnership/techsheets-truck/EPA-420-F10-041.pdf 328 EPA. 2011c. SmartWay Technology Program: Verified Low Rolling Resistance Tires 

www.epa.gov/smartway/technology/tires.htm 329 Evans, L. R., MacIsaac, J. D., Harris, J. R., Yates, K., Dudek, W., Holmes, J., Popio, J., Rice, D., & Salaani, M. 

K. 2009. NHTSA Tire Fuel Efficiency Consumer Information Program Development: Phase 2 – Effects of Tire 

Rolling Resistance Levels on Traction, Treadwear, and Vehicle Fuel Economy 

www.nhtsa.gov/DOT/NHTSA/NVS/Vehicle%20Research%20&%20Test%20Center%20%28VRTC%29/ca/Tires/8

11154.pdf 330Jermakian, J.S. 2012. Crash avoidance potential of four large truck technologies. Accident Analysis and

Prevention 49:338-46, at www.iihs.org/iihs/topics/t/crash-avoidance-technologies/qanda#antilock-brakes
71

Transport Canada study331 of Class 8 long-haul trucks equipped with SmartWay-certified LRRs 

demonstrated safe traction performance in winter conditions, while increasing fuel economy and 

reducing emissions.

2.7.3.4 Findings and Conclusions 

Although safety trade-offs from lower traction are theoretically possible, the literature search did 

not identify data on safety impacts of LRR tires in MD/HDVs, or reports of safety hazards from 

lower LRR road grip in snow, ice or wet conditions that could result in potential degraded 

handling or longer braking distance. The average age of Class 8 tractors, which stand to benefit 

the most from LRR tires, is approximately 9 years.332 This means that LRR replacement tires 

would have to be installed to continue the FE benefits over the life of the vehicle. 

2.8 Aerodynamic Components

Aerodynamic technologies minimize drag and improve the airflow around a moving MD/HDV. 

Examples of aerodynamic technologies include gap fairings that reduce turbulence between the 

tractor and trailer, side skirts that minimize wind under the truck body, and rear fairings that 

reduce turbulence and pressure drop at the rear of a trailer. Additionally, streamlined side 

mirrors, roof deflectors, cabin extenders, and other systems are available.

The EPA SmartWay program has verified four categories of aerodynamic components through 

its fuel efficiency technology certification:333

• Trailer Gap Reducer—fairings attached to front of trailer 

• Trailer Boat Tail—fairings attached to rear of trailer

• Trailer End Fairing—attached to underbody of trailer

• Trailer Skirts—fairings attached to sides of trailer underbody

2.8.1 Penetration/Adoption

Eight major truck fleets, representing 75,000 tractors and 130,000 trailers, participated in a 

benchmarking technology adoption study led by NACFE from 2003 to 2010. The tractors studied 

drove over 3 billion miles in 2010, by which time all eight fleets had adopted most, if not all of 

the available tractor aerodynamic features, including bumpers, mirrors, roof fairings, chassis 

skirts, and minimized fifth wheel height. OEMs are now typically filling the gaps between 

bumpers and fenders.334 Figure 2-12 illustrates the relative adoption rate of aero FE 

technologies, relative to other FE technologies discussed here.

 331 www.tc.gc.ca/eng/mediaroom/releases-2013-h007e-7042.htm and Packed snow performance of low rolling 

resistance class 8 heavy truck tires - Feb. 2013 Executive Summary at www.tc.gc.ca/eng/programs/environmentetv-packed-snow-performance-eng-2913.htm

332 Heavy-Duty Truck Fleet Ages as Carriers Seek Cost Savings. www.trustheart.com/news-bulletins/ltl-and-tlbulletins/91-heavy-duty-truck-fleet-ages-as-carriers-seek-cost-savings 333 EPA. 2011b. SmartWay Technology Program: Verified Aerodynamic Technologies at 

www.epa.gov/smartway/forpartners/technology.htm

334 North American Council for Freight Efficiency. 2012a. 2011 Fleet Fuel Efficiency Benchmark Study /

http://nacfe.org
72

While aerodynamic improvements for tractors are already being widely adopted, aerodynamic 

devices for trailers are not yet as well used. In part, this is because most trailer aerodynamic 

devices were not widely available or adopted until recently. For instance, boat tails on the backs 

of trailers have only been available since 2007. Trailer skirts have been quickly adopted in the 

past 3 years, accelerated both by a California Air Resources Board (CARB) requirement and by 

the increased price of fuel—given that trailer skirts offer greater FE benefit than other 

component types. In December 2008, CARB adopted a regulation to reduce greenhouse gas 

emissions from 53-foot or longer box-type tractor-trailers. A combination of tractor and trailer 

aerodynamics and the use of low rolling resistance tires are mandated by the CARB 

regulation.335 Although the NHTSA/EPA MY2014-18 regulation does not cover trailers, the 

adoption rate trends shown in Figure 2-12 suggest that the gap between tractor and trailer 

aerodynamics may narrow in the next few years. 

Figure 2-12: FE Technology Adoption Across Eight Major Fleets336

A number of new concepts have recently emerged for trailer aerodynamic upgrades. For 

example, a device known as an undertray fairing may be substituted for the more prevalent truck 

skirts and boat tails. Competition of aero designs may accelerate aerodynamic devices adoption 

by long-haul truck fleets.337

 335 California Air Resources Board. 2012. Heavy-Duty (Tractor-Trailer) Greenhouse Gas Regulation 

www.arb.ca.gov/cc/hdghg/hdghg.htm 336 North American Council for Freight Efficiency. 2012a. 2011 Fleet Fuel Efficiency Benchmark Study 

http://nacfe.org/

337 Ibid.
73

For package delivery and utility trucks that operate in stop-start mode, or short ranges, aero 

devices add weight and may reduce fuel efficiency, although medium-duty trucks operating on 

highway-intensive duty cycles offer benefits and growth potential for aerodynamic upgrades.338

2.8.2 Benefits

According to a DOE modeling study and testing, aerodynamic devices offer significant potential 

to improve the fuel efficiency of medium and heavy-duty trucks. Specifically, for a box-type 

tractor-trailer, the combination of a gap reducer, a trailer skirt, and a boat tail can yield 26 

percent drag coefficient reduction. This corresponds to FE gains respectively of a:

• gap reducer: 1-2 percent,

• underbody skirt: 5-7 percent, and

• boat tail: 4-7 percent.

DOE indicates that a 12 percent reduction in fuel use from decreased aerodynamic drag would 

translate to about 3.2 billion gallons of diesel fuel saved per year and 28 million tons of CO2 

emission reduction for a Class 8 tractor-trailer. Also that that additional FE reductions could 

result from combining aerodynamic upgrades with single wide tires.339 

The 2010 National Research Council (NRC) report estimated that the fuel efficiency 

improvement available today for trailer aerodynamics is 5.5 percent but concurs with the DOE 

analysis that the benefit can increase to 11.5 percent in just a few years.340 EPA stated that using 

fairings can increase fuel efficiency by as much as a 15 percent when applied to an early model 

combination tractor-trailer truck and 3-11 percent when applied to a newer Class 8 truck in longhaul highway type operation.341 Manufacturers claim that using only one aerodynamic surface, 

such as a rounded air deflector, can increase fuel efficiency by up to 5 percent, reducing fuel use 

by about 80 gallons, thus saving over $100. The range of boat tails tested in another study, 

ranging from a 2 feet to 5 feet extension, were reported to reduce tractor-trailer fuel consumption 

between 4.7 percent and 7.3 percent, similar to the DOE figures.342

According to the NACFE technology adoption study, large truck fleets have reported varying 

degrees of real world fuel economy performance gains from 2 percent to about 5 percent for 

aerodynamic devices.343

 338 Carbon War Room Research & Intelligence Group. 2012. Unlocking fuel-saving technologies in trucking and 

fleets 

http://carbonwarroom.com/sites/default/files/reports/Unlocking%20Fuel%20Saving%20Technologies%20in%20Tru

cking%20and%20Fleets%20(Carbon%20War%20Room).pdf

339 Salari, K. 2012. DOE’s Effort to Reduce Truck Aerodynamic Drag through Joint Experiments and Computations 

www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2012/veh_sys_sim/vss006_salari_2012_o.pdf 340 National Research Council. 2012. Review of the 21st Century Truck Partnership, Second Report 

www.nap.edu/openbook.php?record_id=13288&page=R1

341 EPA. 2011b. SmartWay Technology Program: Verified Aerodynamic Technologies 

www.epa.gov/smartway/technology/aerodynamics.htm

342 Patten, J., Poole, G., Mayda, W., & Wall, A. 2010. Trailer Boat Tail Aerodynamic and Collision Study 

www.tc.gc.ca/eng/roadsafety/safevehicles-motorcarriers-environment-boat-tails-1307.htm 343 California Air Resources Board. 2012. Heavy-Duty (Tractor-Trailer) Greenhouse Gas Regulation 

www.arb.ca.gov/cc/hdghg/hdghg.htm
74

2.8.3 Safety Considerations

Many boat tail configurations that provide high drag reduction possibilities include boat tail 

material that is as low as 44 inches from the ground, which could cause a vehicle to strike the 

boat tail before or instead of the rear impact guard. For many configurations, this material is 

sufficiently short that the collisions are mostly grille or hood strikes.344 This Transport Canada 

aerodynamics air flow analysis and collision estimation of boat tails was based on scale models 

and wind tunnel testing. It concluded that in truck-truck collisions, class 4-6 straight trucks with 

cab-over engine designs have the greatest potential for occupant injury. If a cab-over engine 

driver rear-ends a truck equipped with boat tails, the horizontal boat tail structural member could 

protrude through the windshield.345 The data required to quantify these strikes were not included 

in the report, however, and the report recommended further study to expose cab-over engine 

vehicle populations in the analysis. No specific windshield strike cases were found in literature. 

The Transport Canada study identified tradeoffs between optimal FE gain and minimizing the 

risk of other vehicles' collision with the boat tail.346

Even though the bottom panel of boat tail configurations provides up to 20 percent of the overall 

aerodynamic benefit, the study concluded that it could result in safety hazard in colder climates. 

Ice and snow can potentially accumulate on the bottom panel and later become dislodged, 

creating dangerous shedding conditions for cars behind the truck. 

Damage from road debris, or detachment of aerodynamic skirts and tails in normal vehicle 

operation are a potential safety issue for aerodynamic features, particularly for underbody 

devices. However, no reported incidents of aerodynamic component detachment or structural 

failure while en route could be found in the literature. Trailer skirt developers have prioritized 

the design of components that are resilient against road damage. Various aerodynamic skirts 

have been in continuous use on MD/HDVs in European and certain Asian countries for over two 

decades, where regulatory and economic factors promote both fuel efficiency and the provision 

of vulnerable road user (VRU) underride protection.347 Newer skirts are typically constructed of 

fiber-reinforced plastic and appear to resist damage when repeatedly impacted by curbs, loading 

dock ramps, railroad crossings, and snow (Figure 2-13).348 Some designs have full-flex struts 

that allow them to bend up to 90 degrees rather than snap, as in older monolithic designs, and 

they weight as little as 190 pounds. 349,350 There is a relatively low impact on the vehicle 

weight—approximately 0.25 percent of an 80,000 GVWR.

 344 Patten, J., Poole, G., Mayda, W., & Wall, A. 2010. Trailer Boat Tail Aerodynamic and Collision Study 

www.tc.gc.ca/eng/roadsafety/safevehicles-motorcarriers-environment-boat-tails-1307.htm

www.tc.gc.ca/eng/roadsafety/safevehicles-motorcarriers-environment-boat-tails-1307.htm 345 Ibid. 346 Ibid. 347 National Research Council. 2010a. Technologies and Approaches to Reducing the Fuel Consumption of MD/HD 

Vehicles www.nap.edu/openbook.php?record_id=12845&page=R1 348 www.youtube.com/watch?v=PzGhXtJyIBU 349 Freight Wing Incorporated. 2011. Aeroflex www.freightwing.com/aeroflex.php 350 Green Big Truck. 2010. Laydon Composits Gets CARB Certified for Trailer Skirt Products 

http://greenbigtruck.com/2010/02/laydon-composits-gets-carb-certified-for-trailer-skirt-products/
75

Figure 2-13: Aerodynamic Trailer Skirt Survivability to Impacts351

Aerodynamic features can provide both truck occupant and non-occupant safety benefits 

unrelated to reducing fuel consumption and greenhouse gas emissions. The safety benefit of 

skirts that also act as VRU sideguards in certain operating environments are discussed separately 

below.

2.8.3.1 Side Skirts

There are various reports that side skirts and boat tails improve trailer-tracking stability, 

especially in crosswinds. Additionally, skirts appear to significantly reduce road spray from 

trailer tires, improving visibility for nearby vehicles on the highway. One trailer skirt 

manufacturer, Laydon Composite, states that its devices are both damage resistant and comply 

with the European heavy truck sideguard regulation.352 European Council Directive 89/297/EEC 

(ECD, 1989) mandates side under-ride protection on trucks over 3.5 tons to prevent pedestrians, 

bicycle riders, and motorcyclists from falling and being crushed under the wheels of a moving 

vehicle.353 Based on data from the European Union, the number of deaths and serious injuries 

for VRUs involved side-impact crashes with heavy vehicles has been reduced since the universal 

implementation of sideguards in 1986 in the United Kingdom354 and on the Continent in 1989.355

Side impacts comprise a large subset of VRU-truck fatalities.356 In the United States, 55 percent

of bicyclist fatalities and 29 percent of pedestrian fatalities involving tractor trailers in 2005-

2009 followed initial point of impact with the right or left sides of the truck.357 The 

 351 Freight Wing Incorporated. 2011. Aeroflex www.freightwing.com/aeroflex.php 352 National Research Council. 2010b. Technologies and Approaches to Reducing the Fuel Consumption of 

Medium- and Heavy-Duty Vehicles www.nap.edu/openbook.php?record_id=12845&page=R1 353 European Union. 1989. European Council Directive 89/297/EEC http://eurlex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31989L0297:EN:HTML

354 UK Road Vehicles (Construction and Use) Regulations 1986. www.legislation.gov.uk/uksi/1986/1078/made 355 Patten, J. D., & Tabra, C. V. 2010. Side Guards for Trucks and Trailers Phase 1: Background Investigation 

www.safetrucks.ca/resources/National_Research_Council_Truck_Side_Guard_Study_2010-03-01.pdf 356 Ibid. 357 NTSB, Safety Recommendations H-14-001 through -007, 2014. www.ntsb.gov/safety/safety-recs/RecLetters/H14-001-007.pdf
76

corresponding side-impact fractions of bicyclist and pedestrian fatalities involving single-unit 

trucks during 2005-2009 were 44 percent and 25 percent.

358 Sideguard devices that comply with 

the EU-directive have been correlated with a 61 percent bike fatality reduction in side collisions 

with heavy trucks in the United Kingdom following implementation on most of its truck fleet. 

Two examples of safety-driven truck side-guard adoption in the United States are the municipal 

truck fleets of Portland, Oregon, which performed the retrofits in 2008359, and Boston, MA, 

which fitted its municipal trucks in 2013.360 An ordinance requiring the Washington, D.C. 

municipal fleet to be fitted with sideguards was enacted in 2008.361 None of these safety-driven 

efforts has simultaneously considered impacts on vehicle fuel efficiency.

It is unclear whether all U.S. models of aerodynamic side skirts would also provide comparable 

safety benefit for VRU under-ride protection. This question, as well as a comprehensive costbenefit analysis that jointly considers VRU safety and fuel efficiency benefits across different 

MD/HDV classes and operating environments, could benefit from further study. Such a study 

could explore the effects on VRU safety and fuel consumption of a design that attempts to 

address both issues.

If VRU under-ride protection were considered as a core function of the side skirt, the skirt’s 

impact resistance, geometry, and visibility would need to be considered in tandem with its fuel 

efficiency effects. This could mean minimizing gaps in coverage, increasing conspicuity, and 

ensuring a minimum lateral resistance to force, commensurate with the impact of a cyclist or 

pedestrian, or possibly of a motorcyclist, lower ground clearance, making the sideguard flush 

with the side of the truck body, and by extending the sideguard longitudinally.362 

2.8.4 Findings and Conclusions 

Aerodynamic technologies offer significant fuel savings in the 5-15 percent range for 

MD/HDVs, depending on how full a package is installed, and on the drive cycle of the vehicle. 

Maximum benefit will be realized for long-haul Class 8 trucks, as recognized in the EPA 

SmartWay program. While the literature indicates that some users have concerns about damage 

to underbody fairings or skirts, there were no documented incidents of any safety-critical 

damage.

 358 NTSB, Crashes Involving Single-Unit Trucks that Resulted in Injuries and Deaths, 2013. 

www.ntsb.gov/safety/safety-studies/Documents/SS1301.pdf 359 City of Portland. 2007. Improving Bicycle Safety in Portland 

www.portlandonline.com/shared/cfm/image.cfm?id=191337

360 City of Boston Mayor’s Office of New Urban Mechanics http://newurbanmechanics.org/project/vehicle-sideguards/

361 Patten, J. D., & Tabra, C. V. 2010. Side Guards for Trucks and Trailers Phase 1: Background Investigation 

www.safetrucks.ca/resources/National_Research_Council_Truck_Side_Guard_Study_2010-03-01.pdf 362 Riley, B. S., Penoyre, S., & Bates, H. J. 1985. Protecting Car Occupants, Pedestrians, and Cyclists in Accidents 

Involving Heavy Goods Vehicles by Using Front Underrun Bumpers and Sideguards 

http://trid.trb.org/view.aspx?id=237955
77

2.9 Lightweight Materials

Use of lighter materials substitution for steel in body and chassis components also reduces 

vehicle fuel consumption, although material strength and the role of vehicle mass and design for 

crash safety must be considered. According to the DOE Materials Technology Program, reducing 

the vehicle weight by 10 percent can improve the fuel economy by 6 to 8 percent.363 With 75 

percent of vehicle fuel consumption directly related to factors associated with vehicle weight, the 

potential benefits of weight reduction enable downsizing the engine and energy storage systems, 

with corresponding cost and/or performance benefits.364 Alternatively, lightweighting can 

maximize the vehicle payload to increase fuel-efficiency (see Sec. 2.9.4). NHTSA-sponsored 

Volpe Center studies of the crash safety performance of future plastics and composite intensive 

vehicles (PCIV) evaluated the potential safety benefits and remaining research needs of plastics 

and composites applications in emerging lighter weight, more fuel efficient, and environmentally 

friendly vehicles.365

Aluminum is a mature material that can provide 40 percent weight reduction compared to steel. 

Advanced high-strength steel (AHSS) can provide weight savings of up to 25 percent over 

conventional steel. First generation AHSS is already finding applications throughout the heavyduty vehicle industries. One downside is that AHSS still has some of the corrosion issues of 

conventional steel.366

A variety of even more lightweight and corrosion-resistant composite materials is in MD/HDV 

production. Fiberglass has a useful lifetime of 20-plus years. Plastic composites also provide 

significant weight reduction, while carbon fiber achieves the highest weight savings: 40- to 50-

percent lighter than fiber-composite. Carbon fiber reinforced composite (CFRP) is extremely 

strong, durable, and has no corrosion concerns, but comes at a high cost of production and long 

processing time, which could slow adoption rate.367 Typically, a core material such as foam core 

is encased in a fiberglass shell and other reinforced CFRP configurations to ensure structural 

strength in a crash. 

 363CalStart. 2012b. Leading Fleets Take Delivery of World’s First Commercially Available Hybrid Hydraulic Parcel 

Delivery Trucks – “Hybrids Without Batteries” www.calstart.org/news_and_publications/CALSTART-in-thenews/CALSTART-Press-Releases/worlds-first-hybrid-hydraulic-delivery-trucks.aspx DOE VTP materials 

Technologies: Goals, Strategies and Top Accomplishments at 

www1.eere.energy.gov/vehiclesandfuels/pdfs/materials_tech_goals.pdf

364 Oak Ridge National Laboratory. 2012. Oak Ridge National Lab Lightweight Materials Program 

web.ornl.gov/sci/lightmat/Lightweight.html

365 “Characterizing And Enhancing The Safety Of Future Plastic And Composite Intensive Vehicles (PCIVs),” 2009, 

A. Brecher, J. Brewer, S. Summers, and S. Patel at www-nrd.nhtsa.dot.gov/pdf/esv/esv21/09-0316.pdf 366 DOE. 2012. Energy Department Investments to Develop Lighter, Stronger Materials for Greater Vehicle Fuel 

Economy www.doe.gov/articles/energy-department-investments-develop-lighter-stronger-materials-greater-vehiclefuel, Lyden, S. 2012. It's a Lightweight Material World www.worktruckonline.com/channel/vehicleresearch/article/story/2012/01/it-s-a-lightweight-material-world.aspx?prestitial=1 367 Lyden, S. 2012. It's a Lightweight Material World www.worktruckonline.com/channel/vehicleresearch/article/story/2012/01/it-s-a-lightweight-material-world.aspx?prestitial=1
78

2.9.1 Penetration/Adoption

Consumer preferences have limited the downsizing options available to vehicle manufacturers, 

and safety and performance standards have resulted in a very limited ability to reduce weight 

further with conventional materials.368 However, there are a growing number of lightweighting 

cases in MD/HDV fleets, especially in transit buses.

In 2003, North American Bus Industries began manufacturing a 45-foot transit bus with a lightweight all-composite body, called the Compo Bus. Approximately 400 were manufactured 

between 2004 and 2008, and the largest fleet is in Los Angeles. Some of them have now been in 

service for more than 10 years and continue to operate. Mobile Energy Solutions LLC introduced 

a lightweighted transit bus in 2007 with an all-composite body featuring a fuel cell/hybrid 

electric propulsion system.369 In 2009, Proterra debuted a lightweighted fast-charge battery 

electric composite bus, using a light yet durable fiberglass-balsa wood composite. The EcoRide 

BE35 buses, now operating in several U.S. cities, have a reduced composite body weight of 20-

40 percent over a conventional steel or aluminum bus, is climate resistant, and could provide up 

to a 600 percent improvement in fuel economy based on FTA testing at Altoona due to their 

combination of lightweighting, regenerative braking, and electric power source.370 

UPS tested prototype diesel-fueled delivery trucks with lightweight bodied constructed of a 

plastic resin material instead of steel. The lightweighted trucks achieved a 40 percent increase in 

fuel efficiency largely due to the truck’s 10 percent weight reduction, the advanced powertrain 

technology, and a more aerodynamic design. The body uses high-impact plastic for the lower 

cladding and one-piece plastic molded roof. With a payload capacity that is only 10 percent (700 

lb.) less than the company’s standard delivery truck, the 150-horsepower trucks ran on an Isuzu 

four-cylinder diesel engine. The lightweighted trucks were successfully field tested in extreme 

weather conditions over the course of one year, after which UPS ordered 150 of these trucks to 

run on high-mileage routes.371

To date, carbon fiber has been an enabling technology for the composite body of the Proterra 

bus, and the fuel cells in advanced Van Hool fuel cell buses,

372 but could also become a broadly 

adopted material in the future if its fabrication costs and volume issues were resolved. 

 368 Oak Ridge National Laboratory. 2012. Oak Ridge National Lab Lightweight Materials Program 

web.ornl.gov/sci/lightmat/Lightweight.html

369 Composites World. 2007. New hybrid electric bus takes advantage of composites 

www.compositesworld.com/articles/new-hybrid-electric-bus-takes-advantage-of-composites 370 Proterra Inc. 2009. Proterra Brings Future of Green Commercial Transit to California Cities With Zero Emission 

Clean Bus Tour www.prweb.com/releases/2009/02/prweb1968224.htm; see also 

www.proterra.com/index.php/products/productDetail/C29/ and www.proterra.com/product-tech/durabilityreliability/composite-body/ 371 Kaye, L. 2012. UPS Rolls Out Lightweight Composite Trucks In a Move Towards Energy Efficiency 

www.triplepundit.com/2012/06/ups-composite-trucks-energy-efficiency/ 372 See Regional Buses Boos CFRP in Alternative Energy at www.compositesworld.com/articles/regional-busesboost-cfrp-in-alternative-energy; and Proterra at www.proterra.com/index.php/products/productDetail/C32/
79

2.9.2 Benefits

The Department of Energy launched new initiatives in 2012 for further reducing vehicle weight 

to improve fuel efficiency.373 Lightweighting can also mean a reduction in maintenance costs 

because a lighter vehicle body requires less brake power to stop and may cause less wear on tires 

and brakes.374

Reduction in MD/HDV mass will reduce crash forces to other vehicles, although this effect is 

likely to be small for light-duty vehicles since truck lightweighting will have only a small effect 

on the mass differential. This benefit may be canceled out further by the fact that light-duty 

vehicles are also undergoing lightweighting. 

Replacing cast iron and traditional steel components with lightweight materialsincluding 

advanced high-strength steel, magnesium, aluminum, and carbon fiber composites  can allow 

vehicle manufacturers to include additional safety devices, integrated electronic systems, and 

emissions control equipment on vehicles without increasing their weight. In this way, advanced 

lightweight materials promise to enhance vehicle safety and performance of vehicles, while 

boosting fuel economy.375

As with most FE technologies, individual payback and economic benefits will depend on the 

vehicle class and drive cycle. Vehicles that travel long distances and those with high kinetic 

intensity (e.g., transit buses) stand to benefit the most from lightweighting.376

2.9.3 Safety Considerations

In all cases, the safety and crashworthiness of lighter weight vehicles is a significant 

consideration, as well as other environmental health and safety issues associated with new 

materials and process technology.377

The predominant grade aluminum for body manufacturing loses 80-percent of its strength when 

welded, so it may not be a suitable material for work trucks with a long service cycle. For 

fiberglass composite materials, it is important to correctly mount accessories. Concentrated loads 

at the point of attachment may result in damage of a fiberglass panel. The downside of some 

plastic composites may include their susceptibility to heat, degradation, and brittleness and over 

 373 See www.doe.gov/articles/energy-department-investments-develop-lighter-stronger-materials-greater-vehiclefuel and http://energy.gov/articles/obama-administration-announces-142-million-new-funding-develop-lightweightmaterials

374 Lyden, S. 2012. It's a Lightweight Material World www.worktruckonline.com/channel/vehicleresearch/article/story/2012/01/it-s-a-lightweight-material-world.aspx?prestitial=1 375 DOE. 2012. Energy Department Investments to Develop Lighter, Stronger Materials for Greater Vehicle Fuel 

Economy www.doe.gov/articles/energy-department-investments-develop-lighter-stronger-materials-greater-vehiclefuel

376 Lyden, S. 2012. It's a Lightweight Material World www.worktruckonline.com/channel/vehicleresearch/article/story/2012/01/it-s-a-lightweight-material-world.aspx?prestitial=1 377 Quintiere, J. G., Walters, R. N., & Crowley, S. 2007. Flammability Properties of Aircraft Carbon-Fiber Structural 

Composite www.fire.tc.faa.gov/pdf/07-57.pdf
80

time when exposed to UV radiation from sunlight.378 However, thermoplastics resistant to UV 

and weathering have been developed,379 and no examples of such lightweight material failures 

on MD/HDVs were identified in the literature. The use of composites, aluminum, and fiberglass 

may result in less protection to truck drivers in crashes because of lower crush strength, but no 

studies or work with significant results were identified. The FAA has sponsored and evaluated

fire-safe polymers and polymer composites380 and examined the flammability properties of 

carbon fiber composites for structural applications.

381

2.9.4 Findings and Conclusions 

No safety incidents involving or attributable to lightweighting of MD/HDVs were identified in 

the literature. The fuel economy benefits of reducing vehicle weight have been clearly 

demonstrated over the past few years in operational fleet pilots and all-composite buses in transit 

fleets. Lightweighting vehicles may make it possible to install additional safety or environmental 

equipment on MD/HDVs to further improve their performance. Moreover, rightsizing vehicle 

payloads, i.e., increasing payload per vehicle as a result of lightweight body materials, could 

enable reductions in the total truck fleet size, reduced VMT, and a proportional reduction in 

crashes.382 NHTSA sponsored research on the crash safety of Plastic and Composite Intensive 

Vehicles (PCIVs) identified progress in and research needs for characterizing and enhancing the 

safety of automotive lightweighting composites in structural applications.

383

 378 Lyden, S. 2012. It's a Lightweight Material World www.worktruckonline.com/channel/vehicleresearch/article/story/2012/01/it-s-a-lightweight-material-world.aspx?prestitial=1 379 See www.dotmar.com.au/uv-resistance.html 380 See “Fire safe polymers and polymer composites,” 2004, FAA report at www.fire.tc.faa.gov/pdf/04-11.pdf 381 See ”Flammability properties of aircraft carbon fiber structural composites,” 2007 at www.fire.tc.faa.gov/pdf/07-

57.pdf

382 Lyden, S. 2012. It's a Lightweight Material World www.worktruckonline.com/channel/vehicleresearch/article/story/2012/01/it-s-a-lightweight-material-world.aspx?prestitial=1 383 A. Brecher et al, “Characterizing and enhancing the safety of future PCIVs,” ESV-21 paper 09-0316, at wwwnrd.nhtsa.dot.gov/pdf/esv/esv21/Track%2025%20Oral.pdf
81

3 SUMMARY OF SUBJECT MATTER EXPERT INPUTS ON MD/HD SAFETY 

IMPACTS OF FE TECHNOLOGIES

3.1 Introduction: Respondents and Fleet Profiles

To obtain primary experience information on safety benefits or disbenefits, interviews384 were 

conducted with managers of several large “green” and diverse corporate truck fleets and with 

managers of public transit fleets with significant fuel efficiency technology adoption. These 

fleets were selected based on published reports on green technology fleet adoption385 and are the 

same large “green” fleets that were analyzed for safety performance relative to conventional 

fleets in our queries of CSA and FARS databases. In addition, technical experts from 

medium/heavy-duty vehicle FE-related OEMs, suppliers, and integrators, as well as FE experts 

from Government agencies were interviewed. The results represent the viewpoints of each 

interviewee and do not represent the views of their organization or the U.S. Department of 

Transportation.

Table 3-1 lists the organizations of the subject matter experts who were interviewed.

Table 3-1: Broad Cross-Section of Subject Matter Experts Interviewed

Corporate fleets Public transit Industry Government

Coca Cola Enterprises Houston METRO EATON Corporation Argonne & National 

Renewable Energy 

Laboratories

Frito Lay Long Beach Transit Vanner, Inc. FMCSA

Verizon Communications Santa Barbara 

Metropolitan Transit 

District (SBMTD)

Parker Hannifin 

Corporation

EPA SmartWay

ARAMARK Washington DC 

Metropolitan Area Transit 

Authority (WMATA), or 

Metrobus system

BAE Systems CALSTART

DHL/Exel Daimler Trucks North 

America (DTNA) 

DOE Clean Cities 

Program

The private and public green fleet managers who were interviewed spanned a wide range of 

MD/HD vehicle types, FE technologies, geographic locations, types of services, and typical drive 

cycles. These major fleets are among those that have joined the DOE National Clean Fleets 

Partnership.386 

 384 The oral communications with fleet managers were non-standardized, and varied on subject matter depending on 

the fleet type (such as public transit or government). 

385 See www1.eere.energy.gov/cleancities/national_partnership.html for fleet list See 

www1.eere.energy.gov/cleancities/national_partnership.html for fleet list 386 Ibid.
82

The Coca Cola Enterprises fleet includes 15,000 MD/HDVs total; 10,000 are Class 7 or 8. 

Most of the fleet operates short distance trips, with fewer than 500 tractors driving long 

distances. Since 2007, the fleet has included 760 hybrid-electric trucks (representing a small 

vehicle fraction of about 5%), 2 LNG and 5 CNG trucks acquired 1.5 years ago, 8 fully electric 

Smith Newtons acquired two years ago (GVW 19,000 lbs.), and six Navistar E-star electric 

trucks (GVW 14,000 lbs.) acquired one year ago. Coca Cola Enterprises has also adopted idle 

reduction in all of its vehicles, speed governors, and telematics packages, along with a drivertraining program to ensure eco-driving and fuel savings.

Frito Lay has about 22,000 fleet vehicles, excluding trailers. The fleet includes 1,200 Class 8 

over-the-road tractors, 4,000 trailers, and a delivery fleet of 4,000 straight box trucks. The fleet 

has reduced its fuel consumption by 14 percent in the past four years despite a concurrent 3 

percent growth in goods volume. Frito Lay has set a corporate sustainability goal of 50 percent 

fuel burn reduction by 2020. It will deploy 275 electric trucks by the end of 2012 and become the 

largest domestic commercial fleet of all-electric trucks. The entire fleet has broadly implemented 

idle reduction since the late 1990s, aerodynamic technologies program for its trailers since 2007-

08, as well as speed limiters and required driver training. About 70 CNG tractors are also being 

deployed in 2012.

The Verizon Communications fleet, consisting of the Telecom, Wireless, and Enterprise 

divisions, has 6,000 MD/HDVs. The fleet-wide fuel efficiency average is 11 mpg (c.f. typical 

HD truck fleet average of 5.7 mpg). A large fraction of the MD/HDV diesels are equipped with 

5-minute idle reduction. Though there are no alternative-fueled MD/HDVs, the fleet includes 15 

hybrid and 10 CNG light aerials (up to 30 ft. reach booms). The earliest hybrids are from 2008-

09, with most hybrids and the CNGs online since 2010. 

Aramark’s Food and Facilities division fleet is piloting five MD hybrid gasoline-electric Azure 

Dynamics conversions of 14,000 lbs. GVW E-450s. Eight propane-powered vehicles are in 

service, though not currently fueled on site, and the fleet plans to deploy new Smith electric 

trucks soon. A multi-stop urban duty cycle is typical of Aramark vehicle fleet operation. Several 

different telematics pilot program are underway. 

DHL Supply Chain Unit’s fleet provides point-to-point delivery of heavy loads, with 1,100 Class 

7-8 tractors in the United States, but no alternative fuels are currently in use. “Teardrop” 

aerodynamic trailers have been use in the U.K. fleet, but not in the United States The fleet duty 

cycle includes a mix of highway and urban driving. Speed limiters have been universally adopted 

for several years. DHL has also been conducting a telematics and driver coaching studies on 150 

box trucks (under 26,000 lbs.). 

Houston Metro (Metropolitan Transit Authority of Harris County) has operated 350 hybridelectric buses, with 122 parallel hybrid-electric buses first deployed in 2007. Commuter buses 

travel mostly on the expressway, with little or no urban driving, while other city routes have 

more stops. More recently, 130 series hybrid-electric buses have been deployed, although the 

majority of the fleet remains diesel. The Houston transit bus fleet has gained experience with 

both lithium ion and nickel metal hydride batteries. 
83

Long Beach Transit (LBT) operates 87 gasoline electric series hybrid buses out of a fleet of 247 

buses, with hybrids in service since 2007. LBT maintenance and operations staff has had 5 years 

and more than 11 million hours of service with the hybrid electric buses, which use either 

ultracapacitors or batteries. 64 Gillig Corp. CNG buses were also rolled out in 2012, fitted with 

CNG tanks in rooftop compartments.

The Santa Barbara Municipal Transit District (SBMTD) has operated and maintained for 20 

years a fleet of electric 22-foot shuttle buses from Ebus, with a daily range of 60-75 mi between 

charges. Ten other electric buses from different manufacturers were tested, but withdrawn from 

service. Currently, the fleet of 108 buses consists of the 20 electrics; 18 diesel electric parallel 

hybrids (Gillig, with Allison drivetrains); and the rest diesel. SBMTD has experience with all 

battery types for energy storage: lead acid, lithium ion, and nickel cadmium batteries. To save on 

battery energy and extend range, the SBMTD electric buses have no hotel loads or auxiliary 

power drains and all HVAC needs are met by opening or closing the bus windows.

The Washington Metropolitan Area Transit Authority (WMATA) has operated New Flyer 

hybrid electric buses with Allison parallel drivetrains since 2006, including standard 40 foot and 

longer articulated models. The number of hybrid electric buses is approximately 400 out of a 

total bus fleet of 1,500. WMATA has also been operating over 300 CNG buses since 2002.

Although not interviewed, the Los Angeles Metro has safely operated CNG buses for over two 

decades: it completely converted its transit bus fleet to CNG in 2011, and currently operating 

over 2,200 CNG buses.

387 Similarly, the New York City Metro Transit Authority (NYMTA) has 

safely operated a large fleet of New Flyer CNG buses, ordering in 2010 475 new buses in 

addition to 190 older CNG buses.

388

3.2 Fuel Efficiency Technology Clusters and Safety Concerns Reported

A theme that emerged from interviews with the SMEs is that fuel efficiency technologies were 

often introduced into fleets as synergistic “clusters.” For example, intelligent transportation 

systems (ITS), GPS vehicle tracking, and route optimization were frequently implemented 

together. Automatic stop-start and other idle reduction technologies were typically bundled with 

driver training. Hybridized or electric drivetrains typically incorporate regenerative brake 

technology. As pointed out by a number of SMEs, combining complementary technologies and 

driver training is important to obtain maximum FE, environmental, and economic benefits. 

3.2.1 Alternative Clean Fuels

One corporate fleet manager reported that the only problem with their CNG tractors was a 

cylinder liner issue that was not specific to the CNG propulsion. It was not a significant safety 

hazard. 

 387 See “Diesel era ends for MTA buses” 2011 at http://articles.latimes.com/2011/jan/13/local/la-me-buses20110112

388 See http://energy-vision.org/new-york-citys-mta-upgrades-its-cng-bus-fleet/
84

Another corporate fleet with CNG trucks in service since 2010 reported no safety incidents 

related directly to FE technologies. This fleet has experienced the same accident rates across its 

FE and conventional vehicles. Even though employees usually blamed any accidents on the truck 

equipment, they did not blame deployed FE technologies, such as hybrid, CNG, and idle 

reduction. While he did not report any first-hand safety incidents, the fleet manager was very 

skeptical of CNG vehicles due to concerns about the safety of high pressure (4000 psi) CNG 

cylinders potentially exploding in a crash scenario or if ruptured. He also pointed out that shared 

gaseous/liquid fueling garages are spark- protected at ground level, but not necessarily at ceiling 

level where natural gas would rise and create the potential for ignition. Particularly in diesel 

garages that only serve a few CNG vehicles, and especially in cold climates, he stated that best 

practices and engineering controls for safe ventilation are important. He related anecdotal reports 

that more people were hurt by opening high pressure CNG lines than by electric shock, e.g., from 

batteries and high voltage wiring in electric hybrids.

The fleet manager of a third corporate fleet believed that propane is as safe as or safer than 

gasoline, based on personal experience filling propane vehicles. This manager reported no safety 

issues with the company’s propane or hybrid gasoline-electric trucks. Though he did not perceive 

any safety issues with alternative clean fuels, he recommended “better public education about 

fuels like CNG” so that potential adopters “would not be scared.”

Engineers at a major truck OEM alluded to the general safety risk arising from multiple small 

suppliers that implement components downstream without performing extensive tests, owing to 

the low volumes of retrofitting or producing alternative fuel vehicles. They related an incident in 

which a supplier began fitting natural gas engines onto one of this company’s truck models, but 

incompatibilities led to overheating and brake fade. This was only discovered after several 

vehicles had been sold. The engineers said that the risk associated with the high number of 

configurations and low volumes of alternative fuel vehicle production also applies to the OEMs. 

HD vehicles are a mix of components from many different suppliers, and unlike commoditized 

light-duty vehicles there are not generally the engineering resources to system-test, integrate, and 

model all potential configurations. The engineers noted that there have not been any significant 

safety problems to date, other than the NG engine retrofit problem.

A U.S. Government official stated that as the first generation of CNG tanks nears the end of their 

15-year service life, there is concern within the MD/HDV OEM and engine industry that some 

expired tanks will not be properly disposed. He reported the case of a salvage yard operator who 

mishandled the CNG tanks on a vehicle and caused them to rocket into a nearby building. He 

added that conventional automotive technicians may have the skills or knowledge to deal with 

natural gas fuel systems. Often NGVs have been unsafely returned to service after a collision 

without proper inspection for fuel system damage. These defective or weakened NG tanks have 

resulted in fatal or injurious explosions when they were filled. The official also stated that 

enforcement of the Federal Motor Vehicle Safety Standard (FMVSS) 304389 end-of-service 

replacement provisions for CNG tanks would be critical to make sure that expired tanks are not 

 389 NHTSA. 2009. FMVSS 304 CNG fuel container integrity 

www.nhtsa.gov/DOT/NHTSA/NVS/Crashworthiness/Alternative%20Energy%20Vehicle%20Systems%20Safety%2

0Research/811150.pdf 
85

unscrupulously re-sold, which could result in similar safety incidents.390 Concerning the indoor 

servicing of CNG vehicles, he was aware of one case in which an NGV was operated in a shop in 

the presence of an active flame heater that led to an explosion.

3.2.2 Drivetrain Hybridization and Electrification Safety

One corporate fleet manager reported that heavy-duty hybrid electric tractors achieved an actual 

13-22 percent fuel efficiency FE gain compared to ICE tractors. The fleet manager at this 

company was very satisfied with their alternative fuel medium- and heavy-duty vehicles’ 

operation and maintenance safety. The manager’s main concern was about the too-short (5-year) 

battery life on the hybrids, as in his experience batteries are dying faster than expected and are 

expensive to replace. This fleet manager found that some drivers refused to drive the non-airconditioned E-Star electric trucks in the summer. However, there were no safety issues for EVs 

related to hot weather. Indeed, both the E-Stars and the Smith electric trucks were reported as 

“fairly reliable” in high summer heat. Technician safety training was identified as a need for the 

potential high voltage hazards involved in servicing hybrid and electric vehicles, but he stressed 

that there have been neither electrocution incidents nor close calls.

Although electric motors have more torque at low speeds than ICEs, a corporate fleet operating 

Smith Newton electric trucks (E-trucks) found that they do not accelerate faster than ICE trucks, 

due to programming that prevents breaking of traction. The reliability and availability of the 

Smiths was also measured as about 300 days meantime between failures (MTBF), which is 50 

percent better than the MTBF of ICE trucks. The only potential concern for E-trucks is their 

limited all-electric range of 50-65 miles, which could become a safety issue if it were stranded in 

traffic. In addition, while the maximum vehicle speed is 55 mph; going uphill could be limited to 

lower speeds. Therefore, each E-truck route must be “certified” for safe operation to prevent 

stranding. With this best practice and driver training, the company has never experienced a 

stranded E- truck, except for one case in very cold weather. 

In one private fleet, no safety issues were reported with regard to gasoline-electric hybrid truck 

conversions. The manager said that they have experienced 35 percent FE savings and that the 

hybrid conversions have been “beneficial overall.” 

A major manufacturer of parallel hybrid drivetrains pointed to the extensive incident-free history 

of its hybrid electric MD/HDVs (6,000 vehicles over 300 million miles) as evidence of HEV 

safety for the past five years. The company engineers stated that the rechargeable energy storage 

system undergoes multiple safety tests and is designed with fail-safes for various misuse and 

abuse scenarios. Rogue acceleration is prevented through error checking in the hybrid control 

unit, and less wear and tear on brakes in the hybrid vehicles may lead to improved brake safety 

by reducing brake fade. 

 390 American Trucking Association. 2009. Expanding ATA’s Safety Agenda Executive Summary 

www.truckline.com/Newsroom/Policy%20Papers/Safety%20Task%20Force%20Report.pdf; Natural Gas Vehicle 

Owner Community. 2011. CNG tanks never expire? http://cngchat.com/forum/archive/index.php/t-7264.html

Review of Limited Calendar Service Life for ANSI NGV2/FMVSS 304 CNG Fuel Containers, Aug. 14, 2012 at 

www.cleanvehicle.org/technology/CVEFLimitedLifeRationaleR3.pdf
86

The weights of parallel hybrid vehicles are similar to those of conventional counterparts, so their 

handling is similar, according to these engineers. What may be a safety risk is the highly variable 

integration of hybrid components downstream by bodybuilders: the case was recalled of an 

operator being stranded in a bucket without power. Better standardization, knowledge sharing, or 

new best practices may be needed in the future to ensure operational safety. Another potential 

safety concern is the uncertain battery lifetime due to variability of climate and duty-cycles. 

Without state-of-charge indicators, this could conceivably leave vehicles underpowered or 

stranded if the battery degrades, and is not serviced or replaced in a timely manner. The 

engineers noted that HEVs and EVs share the same potential battery-related safety issues and 

required safeguards and best-practices in prevention and maintenance. 

Another hybrid-electric drivetrain company expert concurred that its 3,800 series hybrid electric 

buses (HEB) in operation around the world had a long and mostly incident-free history. This 

chief engineer pointed out that minimal vehicle chassis changes are needed, and that the vehicle 

handling is deliberately similar to a conventional bus. Multiple fail-safes are designed against 

electrocution, and the engineer stated that the bus’s quieter operation is not a detectability hazard 

to other road users. The engineer stated that the breakdown rate of hybrid electric buses is not 

any higher than that of ICE buses. Moreover, a hybrid bus is still able to pull off the road when 

there is a failure with the ICE or its electric motor, and this feature has been used in service. At 

the same time, though, a battery cooling system failure had to be addressed through engineering 

redesign, suggesting the need to evaluate best practices for preventing damage to new FE 

subsystems that may not be compatible with existing protocols—in this case, pressure washing 

of the vehicle that short circuited the ESS.

The operations manager of a transit agency that has used hybrid diesel-electric buses with two 

different drivetrains since 2007 reported occasional early safety issues with over-speeding of the 

traction motor; when this occurred, the bus entered a creep mode and had to be taken out of 

service. There was an instance of an operator who tried to keep the traction motor working in 

creep mode, shorted out the plate, and caused hydraulic fluid to leak down the bus. This failure 

mode only happened once on one of the hybrid drivetrain types operated by the transit agency. A 

recommended countermeasure was better operator training to deal with emergencies on hybrid 

buses and to understand their failure modes.

Another transit agency operations manager reported that their HEBs did not fully deliver the 

promised FE gains, but that they had brought a 90 percent decrease in brake maintenance costs 

versus diesel buses. The ultracapacitors used as the energy storage system on some of the buses 

developed an acetonitrile electrolyte leak that had to be fixed under warranty. The mean time 

between road calls for hybrid buses at this agency was comparable to diesel: 9,000 versus 11,000 

hours. 

The electric buses (EBs) operated by an urban transit agency have had no safety issues, but they 

have had service interruptions, according to the fleet manager, which mainly involved older lead 

acid batteries. Newer lithium ion batteries used by the fleet have been far more reliable. When 

the hybrid electric buses occasionally break down while in service, they have to be towed to the 

central depot. However, experience with HEB/EBs shows that their brakes last longer than the 

brakes of diesel buses. The fleet manager said that the agency’s experience with electric and 

hybrid buses has been positive overall and that they are very popular with users.
87

Regarding hybrid hydraulic vehicles (HHVs), an engineer with a major drivetrain manufacturer 

stated that the basic technology used in these drivetrains is proven technology, with many 

decades of engineering knowledge. He stated that the company’s HHV drivetrains are designed 

for the 10-year lifetime of a heavy-duty truck. Also, there are no safety issues associated with the 

mass transfer of hydraulic fluid between accumulators, and multiple redundancies exist against 

rogue vehicle movement commands. His chief recommendation was for continued improvement 

of vehicle part integration to prevent any possibility of conflicting system signals. 

Hydraulic hybrid drivetrains have similar failure modes to existing hydraulic systems on 

MD/HDVs , i.e., leaks. Due to the high fluid pressure (5,800 psi), a leak can lead to high-velocity 

fluid ejection that could cause potential injury to bystanders. There is always a risk of vehicle 

stranding due to leaks and loss of propulsion. Several of these types of strandings were reported 

by another HHV drivetrain manufacturer. At the same time, the anti-lock brake system (ABS) 

friction brakes on HHVs provide a backup in case the hydraulic system fails, ensuring safe 

stopping capability. The fact that the friction brakes are cool may also improve their stopping 

performance, a safety benefit. The engineer stated that acceleration of the HHV is not unsafely 

quiet, and that drivers report they like operating the HHVs. Although the engineer did not have 

data, he suggested that it is possible that the extra 1,700 pounds of weight added by a series HHV 

drivetrain may affect the braking distance and handling of the truck. Maintenance best practices 

implementation in an HHV fleet is considered safety-critical to and includes regular inspection 

of hydraulic hoses, oil, and accumulators to detect and address leaks.

3.2.3 Intelligent Transportation Systems and Telematics

The manager of one corporate fleet reported that since installing both auto start-stop and 

telematics for monitoring of vehicle operation on all their vehicles two years ago, the fraction of 

unproductive engine operating time has decreased from 30 percent to 9 percent. He added that no 

safety incidents were documented in the fleet for this cluster of FE technologies.

Another company that had outfitted 80 percent of its fleet with GPS and telematics that monitor 

FE reported that out-of-route miles, acceleration, and congregation events (when drivers use their 

trucks to meet and socialize during work hours), decreased 10 percent under engineered route 

lengths, compared to 20-30 percent over engineered route lengths prior to having training and 

installing telematics. Hence, the use of telematics on vehicles has led to a reduction in vehicles 

miles traveled VMT, which likely led to fewer crashes. The fleet manager of this company also 

believed that efficient drivers are safer drivers, though the company did not yet have data to 

quantify this.

3.2.4 Speed Limiters

Two corporate fleet managers both stated that 65-mph speed limiters have been universally in 

place on all their respective vehicles since the late 1990s, with no resulting safety issues 

reported. One fleet manager believed that speed-limiter-related safety concerns could only be 

encountered when trucks pass other vehicles on the highway instead of staying in the right-hand 

lane behind other vehicles as suggested. He recommended combining speed limiters and driver 

training programs to improve truck safety.
88

3.2.5 Idle Reduction (Auto Stop-Start)391

Idle reducing auto start-stop was implemented on nearly the entire fleet of a major company 

about 15 years ago. Although the resulting FE gain is difficult to measure, there have been no 

safety-related concerns. 

Although FE gain data is operationally difficult to measure at one company with broad idle 

reduction adoption, there have been no safety issues with the engine shut-off feature. No 

employees have blamed any mishaps on the idle reduction feature.

A truck auxiliary power supplier and integrator stated that idle reduction technology and electric 

auxiliary power systems in aerial work trucks have shown no known safety issues. According to 

the engineer, the proof that the system is working seamlessly is that drivers don’t realize the 

system is ever turning the engine on and off to recharge the vehicle batteries. He cited a test of 

the system on 22 aerial trucks in Montreal that demonstrated 16-30 percent FE gains. The utility 

company testing the system enforced idle reduction by cutting off power to work systems 

whenever the engine was on, so drivers were unable to cheat. 

One U.S. Government idle reduction program leader/coordinator stated that in 10 years she has 

seen no safety incidents or issues due to idle reduction equipment. If there had been any serious 

safety concern, this researcher believed it would have shown up. In an extreme temperature 

scenario when a vehicle engine auto-stops and then has to restart, the auto-start would usually be 

run by an APU not linked to the rest of the motor system; the researcher did not believe that auto 

stop-start could cause a breakdown on the highway or at a stoplight, which are the failure 

scenarios that could potentially cause a crash or congestion.

3.2.6 Driver Training: Human Factors and Operation Issues

One of the corporate green fleets partnered with two outside companies to implement a 

comprehensive driver-education program focused on FE techniques: anticipation, coasting, 

eliminating unnecessary idling, keeping momentum, and keeping distance. Eco-driving “rodeos” 

are also held to train drivers.

Another corporate fleet manager stated that he is confident that “fuel efficient driving is safe 

driving” and vice versa. His company has committed to train all of its drivers and reports that all 

trained over-the-road drivers have achieved a 6-20 percent FE improvement. As a general 

observation, the fleet manager stated that when deciding on fuel efficiency technology 

implementation, “Fleets don’t necessarily consider the safety impact unless there is a negative 

risk when picking a fuel efficiency technology, or unless there is good data on the positive safety 

impact of the FE solutions.” 

LCVs were supported by one of the corporate fleets as a fuel efficient way to transport low 

weight cargo. The corporate fleet SME interviewed stated that he “could not think of a single 

road safety incident involving LCVs in the western United States.” This anecdotal view was 

 391 The North American Council for Freight Efficiency (NACFE) 2012 study on idle reduction early adoption costs 

and benefits at http://nacfe.org/wp-content/uploads/2012/07/NACFE-2012-Anti-Idling-Study-062812.pdf
89

supported by engineers at a truck OEM, although they added that the longer stopping distance of 

heavier LCVs is a potential safety disbenefit.

One large corporate green fleet has deployed a telematics driver-monitoring system called 

SmartDrive392 on 150 box trucks and combined this technology (which has internal cameras to 

monitor texting, distractions, and about 70 other “driving defects”) with daily driver coaching. 

Monitoring and recording of driver behavior, combined with coaching, has reduced distracted 

and aggressive driving and provided significant FE and safety benefits. Since implementation, 

the mean distance between driver defects such as texting and hard braking has doubled to 600 

miles from 300 miles. This fleet manager believes that technological solutions and driver 

coaching should occur in tandem for maximum FE and safety benefit, and he believes that driver 

training is the more critical element of the two. He agreed that there is evidence of telematics 

alone decreasing unsafe behavior. His company in the UK-implemented GreenRoad393 for onboard behavioral feedback found it reduced “driver defects” by 75 percent on 15 test vehicles. 

3.2.7 Tire Systems: Wide Base,394 LRR,395 and TPMS396 Safety Issues

Wide base tires were considered for use by one corporate green fleet manager, who recognized 

some benefits, but was concerned that if a tire blowout occurs there is no redundancy. This lack 

of redundancy was considered a potential safety hazard of wide base tires. However, a mechanic 

at the same company stated that he had never experienced any problems with the wide base tires 

installed on the two tractors and 14 trailers within the fleet. Other concerns articulated were not 

safety-related, e.g., the tires are “very expensive” and so far have poor availability. 

Another corporate fleet manager reported that he installed a TPMS without real payoff, because 

the TPMS flaw detection was not early enough to prevent tire damage. This fleet manager 

believed that TPMS is unlikely to prevent blowouts, when compared to monthly yard checks of 

tires. At the same time, his parent company has been running wide base tires for some years now, 

but it is not yet clear if these tires have produced notable FE gains.

Engineers at one truck OEM stated that they were unsure why super-single tires are not catching 

on despite the technology’s proven FE benefit. They believed that the slow adoption of single 

tires—and of other FE technologies, for that matteris probably not due to degraded safety 

performance, but rather due to fears of increased downtime, specifically for tires. The perception 

is that the downtime for single tires is twice as high as for duals. This would therefore be a 

reliability issue and not a safety issue.

 392 SmartDrive Systems. 212. SmartDrive www.smartdrive.net/ 393 California Natural Gas Vehicle Coalition. 2012. Cummins Plans 15-Liter HD Engine 

www.cngvc.org/pdf/newsletters/CalNGVNews_041612.pdf 394 See EPA SmartWay Verified Technologies for fuel efficiency improving tires at 

www.epa.gov/smartway/forpartners/technology.htm

395 EPA. 2012. Low Rolling Resistance Tires A Glance at Clean Freight Strategies 

www.epa.gov/smartway/documents/partnership/trucks/partnership/techsheets-truck/EPA-420-F10-041.pdf 396 North American Council for Freight Efficiency. 2012c. 2012 Tire Pressure Study http://nacfe.org/wpcontent/uploads/2012/07/NACFE-2012-Tire-Pressure-Study-062812.pdf
90

Without citing specific experience, these engineers suggested that LRR tires could negatively 

affect truck stopping distance and stability control, a potential safety disbenefit. They stated that 

LRR tires can make passing NHTSA stability control tests more difficult for vehicles, and they 

were hesitant to recommend them. 

3.2.8 Aerodynamic Components and Safety

An aerodynamic retrofit program for trailers of a green corporate fleet started in 2007. Although 

the associated FE gain has been difficult to measure, the safety performance of aero components 

was tested extensively in wet and other driving conditions, including pressure envelope 

interactions with other vehicles, and crosswinds. Aerodynamic system safety was deemed solid, 

with no instances of detaching. 

A major truck manufacturer has deployed fuel tank fairings, side extenders around sleepers, and 

over-the-sleeper fairings as part of its high-efficiency line of products for over-the-road trucks. 

While the engineers had heard concerns about railroad crossings and dock approaches damaging 

under-trailer skirts, they had never actually observed such problems. They also had no 

knowledge of any aerodynamic components that have detached from vehicles. 

This same manufacturer is currently studying the use of cameras to replace rear view mirrors on 

over-the-road trucks, which would offer approximately 1 percent FE improvement. The 

engineers stated that rear-view cameras have the potential to fail and that there are more ways for 

cameras and displays to fail than for mirrors. According to these engineers, the failure of a rearview camera is a safety risk comparable to losing a tire, since the vehicle could then no longer 

legally (or safely) be operated. 

These engineers said that an important consideration for aerodynamic components is that drivers 

are required to regularly inspect their vehicles. Covering underside or other components with 

aerodynamic fairings can make them harder to inspect; for example, CNG relief valve shrouds, 

wheel covers, and certain fairings. Drivers and inspectors need to be able to see through wheel 

covers and to be able to access lug nuts through them. These covers must also be durable to 

withstand frequent abuse. 

While this manufacturer did not offer an active fifth wheel, intended to reduce the cab-trailer 

gap, the engineers stated that its safety could be ensured through a variety of fail-safe 

mechanisms, such as ratcheting. 

3.2.9 Lightweighting Materials

As discussed in Section 2.9, there are multiple lightweighting materials options for weight 

reduction of MD/HDVs to improve their fuel efficiency without loss of crashworthiness. These 

materials include aluminum, fiberglass composites, advanced high-strength steel, plastic, and 

carbon fiber composites in either structural or internal padding applications. Although lightweighted vehicles offer improved maneuverability and shorter stopping distance, for specific 

lightweighting materials, they may also result in decreased survivability in vehicle fires. 

Engineers at one heavy-duty truck OEM suggested that new lightweight materials (such as 

carbon fiber composites and resin epoxies) are making it harder to fight vehicle fires. They stated 

that it may take less time for a vehicle to become fully engulfed in fire with more such materials 
91

incorporated, although they did not cite any actual experience. These engineers recalled a 

conference speaker who claimed that when “a lightweight vehicle fire begins at highway speed, 

it could be too late by the time the vehicle stops.” 

More potential crashworthiness safety issues related to lightweighting vehicles with plastics and 

composites were discussed in a 2007 NHTSA/Volpe report and research roadmap.397

3.3 Conclusions Regarding Key Safety Issues

Based on the collective input from SMEs and the concerns raised, the perceived potential 

hazards associated with the adoption of FE technologies by MD/HDVs were summarized. 

Available countermeasures were also identified, largely based on the SME inputs. The top five 

FE MD/HDV safety issues by perceived potential hazard were identified as follows, in no order 

of precedence.

• Potential ignition or explosion scenario in shared natural gas and liquid fuel indoor 

fueling facilities. 

Since gaseous fuels rise whereas liquid fuel vapors sink, the fire protection requirements 

for servicing vehicles indoors that use each type of fuel are different. Ignition protection 

in diesel or gasoline facilities is typically at ground level; natural gas facilities must 

eliminate ignition sources near the ceiling. Due to the high cost of a dedicated NG 

fueling facility, some fleets may use shared fueling facilities that service both kinds of 

vehicles. Robust countermeasures include strictly fueling NGVs in outdoor facilities, if 

the climate permits, or enforcing strict compliance with NFPA 52 and/or NFPA 57 codes 

for any facility that serves CNG or LNG vehicles, at any point in time. Engineering 

controls such as methane detectors, visual/audible alarms, induction-motor HVAC 

equipment, and other safeguards to prevent ignition in NGV depots are likely to be more 

effective than relying on process controls, such as manually opening doors or vents 

whenever an NGV is brought into a shared indoor facility.

• Degraded or expired CNG tanks that remain in service are more likely to rupture. 

The first generation of CNG tanks is reaching the end of its service life. There is no 

national uniform oversight of their proper decommissioning and destruction. There are 

reports of expired or damaged CNG tanks being unscrupulously resold through shops 

and online, e.g., on eBay.398 FMVSS 304 requires a statement to be placed on the tank 

by the manufacturer: “This container should be visually inspected after a motor vehicle 

accident or fire and at least every 36 months or 36,000 miles, whichever comes first, for 

damage and deterioration.” Although FMVSS 304 does not require destruction of the 

aging CNG tank after 15 years, it does require a statement: “Do Not Use After 

(manufacturer must insert the month and year that mark the end of the manufacturer’s 

recommended service life for the container).” In addition, NGV training would

 397 Report No. DOT HS 810 863 “A Safety Roadmap for Future Plastics and Composites Intensive Vehicles,” Nov. 

2007, at http://volpedb.volpe.dot.gov/outside/owa/vntsc_outside.product_repository.display_item?p_doc_id=30707 398What happens when your CNG tanks expire? At www.government-fleet.com/article/print/story/2010/05/whathappens-when-your-cng-tanks-expire.aspx
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minimize any safety risk for automotive mechanics who service NGVs to ensure that 

damaged CNG tanks are not placed back in service without the necessary inspection 

and/or replacement at end-of-service life. 

• Flammability of lightweighting materials and toxicity of burning byproducts. 

The SMEs stated that fiber reinforced composites, such as the carbon reinforced 

polymers, used in high end and NASCAR racing cars, are more flammable than steel. 

The fire safety and health hazards of burning composites include the potential release of 

volatile organic compounds, toxic microscopic fibers, and combustion products. For 

instance, carbon fiber composites and resin epoxies used in aircraft structural 

applications are more flammable and can burn completely, releasing toxic airborne 

fibers and other byproducts.399 The SMEs suggested that although the probability of 

such an event might be low, the consequence could be of medium to high severity and 

the fire safety and environmental health performance would benefit from further study. 

Passive fire protection coatings, such as Vermitex, can be integrated to address potential 

flammability hazard of these materials. Fire safety of light-weighted MD/HDVs could 

benefit from further study, since both fire performance and burn byproducts toxicity 

depend on the choice of lightweighting materials and their structural or interior 

application.

• Low-rolling resistance tires may increase stopping distance and degrade stability 

control. 

An SME suggested LRR tires could introduce a significant safety concern if they 

increase stopping distance and degrade stability control. Potential countermeasures to 

offset a degradation in stopping distance or in stability control include vehicle 

lightweighting, improving driver training, and reducing vehicle speed limits, e.g., by 

installing speed limiters. 

• Incompatible FE components and subsystems. 

The integration of FE vehicle subsystems has been low production volume, with 

relatively sparse engineering and validation resources allocated for each vehicle 

configuration. Although SMEs only cited two concrete examples where this led to safety 

issues, they suggested that the probability of integrating incompatible components might 

be medium to high (consistent with the Risk Matrix shown in Figure 5-1), and that the 

consequences warrant further study. Possible countermeasures include standardizing 

designs and equipment across manufacturers, and utilizing whole vehicle crash tests and 

safety certification that account for the entire suite of on-board subsystems. Increased 

standardization, knowledge sharing, and development of best practices could help to 

ensure operational safety of FE MD/HDVs.

In addition to the above top five perceived potential safety issues, the SME input revealed 

several other findings about the impacts of FE technology on safety performance, as well as on 

 399 See FAA 2007 report “Flammability properties of aircraft carbon fiber structural composite” at 

www.fire.tc.faa.gov/pdf/07-57.pdf; and Gandhi FAA article “Post-crash health hazards from burning aircraft 

composites” at www.aviationfirejournal.com/aviation/library/VOL6-Hazards.pdf
93

non-safety considerations. For example, multiple SMEs concurred that FE technologies offer 

benefits beyond fuel cost reduction and environmental compatibility. 

These SMEs reported that FE technologies reduced their fleet maintenance costs. This could 

result from less frequent brake replacements when regenerative braking was incorporated, fewer 

moving parts in the case of electric drivetrains, or less engine wear in hybrid hydraulics that 

launch the vehicle from a stop.

SMEs accounts indicated that, although electric and hybrid electric vehicles contain onboard 

electric propulsion and energy storage systems with high voltages, there have been no significant 

safety incidents. The probability of electrocution by electric vehicle energy storage systems is 

low, as long as personnel and first responders are properly trained. Furthermore, additional layers 

of engineering safeguards to prevent electric shock are continually being added by the OEMs. 

In terms of hybrid vehicle operational safety, the requirements for dealing with emergencies such 

as a disabled hybrid drivetrain MD/HDV can be quite different from those of a diesel truck or 

bus. Better operator training was recommended by the SMEs to safely and effectively deal with 

the distinct failure modes of FE technology-equipped vehicles, including proper use of 

emergency propulsion modes designed to steer a disabled vehicle to the roadside.

The general potential safety concern articulated for HHVs is the high pressure of the fluid 

system. However, the low number of in-service vehicles means that more time would be 

necessary to more thoroughly assess their safety performance. In the meantime, robust hydraulic 

line and accumulator tank inspection and maintenance schedules would minimize any safety risk 

for the limited existing fleets, as these appear to be the main potential points of failure. 

According to the SMEs queried, speed limiters in MD/HDVs have not led to safety issues so far. 

One SME specifically recommended their adoption in tandem with driver training to improve 

fleet safety. The only potential safety concern they identified was unsafe passing of other 

vehicles. Although a suggested countermeasure would be for speed limiter-equipped MD/HDVs 

to stay in lane and not pass, there are situations where passing at higher speed may be necessary.

Idle reduction technologies such as auto start-stop and auxiliary power units have demonstrated 

FE benefits of varying levels without any reported safety hazards.

SMEs disagreed on whether there is in fact any effect on safety performance, or only a potential 

effect on reliability. Further study with a larger sample size of users would provide more 

information on any potential risks. 

Aerodynamic components did not introduce any reported safety issues, although there is 

potential to interfere with preventative safety if the aerodynamic system makes vehicle 

components inaccessible for inspection and hazards are not detected in a timely manner. 

Removable aerodynamic components that can be temporarily detached, folded, or tilted to allow 

inspection could address these potential secondary safety hazards. For components whose 

inspection is visual, transparent aerodynamic components (e.g., polycarbonate) is another 

countermeasure.
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4 SCENARIO HAZARD ANALYSIS AND PREVENTION/MITIGATION 

STRATEGIES

4.1 Technical Approach to Hazard Analysis of FE Technologies and Fuels in MD/HDVs

Technically, the event Risk (R) is defined as the product of a hazardous event probability of 

occurrence (p) by its consequence severity (C). However, for lack of empirical safety 

performance data, the classic Risk Assessment Matrix (RAC) shown in Figure 4-1 below is not 

yet feasible for a safety analysis of specific FE technologies safety impacts. Ideally, a hazard 

characterization and classic probabilistic hazard analysis of MD/HDV crashes attributable to or 

involving FE technologies and alternative fuels should be based on ample statistics derived from 

real world data. However, though several types of FE technologies have been introduced 

individually or in clusters (e.g., speed limiters, idle reduction, route selection using GPS and 

ITS) over the past decade, most have been adopted only recently, and have very slowly 

penetrated truck and bus fleets. 

Figure 4-1: A Summary Risk Assessment Matrix Ranking and Prioritizing Hazards

400

A best practice for cost-effective risk management is to prioritize the hazard levels by Risk 

(which is the product of probability and consequence) .

401 This enables appropriate allocation of 

resources for risk reduction measures to address those high consequence events that result in loss 

of life, injuries and/or greatest property damage. The Hazard Matrix schematic in Figure 4-2

 400 Source: DOD MIL-STD-882E. See latest update of DOD-MIL-STD-882_E, System Safety Standard Practice, 

May 2012,at www.system-safety.org/Documents/MIL-STD-882E.pdf 401 See for instance the ANSI-endorsed standards ISO 27001-Risk Assessment and ISO 31000 (2009) Risk 

Management- Principles and Guidelines
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indicates potential risk mitigation responses (from low to high), based on combinations of event 

probability and consequence severity. Some risk levels related to vehicle operation can be

transferred via insurance coverage; while negligible risks are routinely accepted as the cost of 

doing business. While there are currently numerous Federal and State laws and incentives for 

manufacturing and buying fuel-efficient vehicles,402 there are as yet no auto insurance credits for 

better safety in operating vehicles with FE technologies that we could identify. 

The classic Hazard level ranking matrix in Figure 4-2 shows what types of hazards must be 

addressed versus those considered too improbable, unaffordable to control, or of negligible 

impact: most risk categories can be prevented or managed cost effectively through engineered 

controls, or with design or operational changes. The vertical axis corresponds to the 

Consequence severity, and the horizontal axis shows the frequency of event occurrence.

Figure 4-2: A Hazard Level Matrix Showing Ranked Event Probability and Consequence 

Severity. 

A preliminary hazard analysis (PHA), a probabilistic fault-tree analysis (FTA), or failure mode 

and effect analysis (FMEA) all require statistically meaningful information on single-point 

failures, or failure chain frequency of occurrence, and their respective severity of consequences. 

At present, the lack of statistical data on crashes attributable to a specific category of FE 

technology, or a cluster thereof, or to alternative fuel use precludes either quantitative or 

qualitative hazard characterization and ranking. 

The literature review (Chapter 2) and focused interviews conducted with SMEs familiar with

fuel efficiency technologies and MD/HDV fleet operations (Chapter 3) have also indicated that 

 402 See the DOE Alternative Fuels Data Center laws and Incentives at www.afdc.energy.gov/laws and 

www.afdc.energy.gov/laws/matrix/tech
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there is no statistically meaningful information causally linking crashes to any specific FE 

technology. Further, the introduction of new vehicle FE technologies and the integration of new 

or upgraded automotive subsystems (both FE related and unrelated), alternative fuels, and 

associated refueling infrastructure may lead to as yet unforeseen failure modes and failure chains 

due to greater complexity and to the “learning curve” period. 

The NACFE 2011 Freight Efficiency Technologies Benchmark report403 has evaluated the rate 

of adoption over time for six major FE technology clusters by 18 representative truck fleets, as 

shown in Fig. 2-12. NACFE has also conducted in-depth benchmarking studies of the adoption 

benefits for specific FE technologies, such as tire pressure monitoring, anti-idling technologies, 

and 6x4 packages. However, MD/HDV fleet-wide penetration and operational experience with 

vehicles equipped with FE technologies to date with are still too modest to quantitatively modify 

the crash safety performance of the national MD/HDV fleet. 

Only a few FE equipment failures may be safety-critical, and their identification is only possible 

through realistic scenario analysis and modeling, and/or through longer-term in-service fleet tests 

and evaluations, such as those performed by NREL404 for hybrid electric buses, trucks and 

delivery vans. Some safety-critical failures may show up after considerable time, after a warranty 

period or design life ends. Typically, new products may be overdesigned with extra redundancies 

and safeguards since bad publicity after a major a failure with severe public safety impacts can 

lead to massive recalls, close a new production line, and potentially bankrupt affected operators 

and OEMs. The OEM engineers interviewed indicated that they design, build and integrate new 

FE technology and fuel subsystems into MD/HDVs for safe operability, and that CMV drivers 

are trained and certified to operate them in a safe manner. Moreover, new fuel efficiency 

subsystems (e.g. hybrid drive, TPMS, aero-skirts) integrated into commercial vehicles are 

typically overdesigned to ensure their safe operation for the duration of the design life, and/or 

warranty coverage.

Furthermore, the NHTSA/FARS and FMCSA/CSA databases do not include detailed 

information on vehicle FE technology involvement in crash reports. The 2007 FMCSA LTCCS 

report to Congress405 showed that the large truck vehicle related relative contributions to crashes 

are smaller than driver-related crash causes, of which vehicle-related causes represent 10 percent.

At present, no quantitative risk ranking of safety hazards related to FEs and alt-fuels, or even a 

structured qualitative PHA is possible at present due to:

• The lack of statistical safety data of MD/HDV vehicle crashes linked to FE technologies.

• Insufficient and only short duration experience of SMEs who provided inputs with actual 

crash or other safety mishaps for “clean fleets.”

 403 See NACFE 2011 Benchmarking report and update at http://nacfe.org/wp-content/uploads/2012/09/NACFE2012-Benchmark-Study-Update-062812.pdf and related in-depth FE technology benchmarking studies and barriers 

to adoption at http://nacfe.org/ 404 See www.nrel.gov/news/press/2012/1987.html and 

www.nrel.gov/vehiclesandfuels/fleettest/publications_hybrid.html

405 See LTCCS FMCSA report to Congress at www.fmcsa.dot.gov/safety/research-and-analysis/report-congresslarge-truck-crash-causation-study 
97

• The lack of statistical confidence to enable ranking potential hazards by probability, 

consequence and overall risk level;

• Limited and sparse data in cited literature on hazardous incidents due to alternative-fuels

operations, and/or fires due to rechargeable Lithium Ion batteries.

• The SMEs consulted offered in Section 3 only limited available information and 

opinions on potential hazards and their top five safety concerns, based on their own 

experience with clean commercial MD/HDV fleets, or transit bus fleets. 

Credible hazard scenarios can be proposed and analyzed to provide insights into the worst-case 

consequence. A deterministic scenario-based risk assessment (RA) methodology was adopted, in 

which a “worst case” scenario and its consequence are assumed, and the potential prevention and 

mitigation options to reduce impacts (through compliance with safety regulations, applicable 

voluntary technical standards, or adoption of industry best practice) are explored. Some risk 

analysts object to this type of deterministic risk assessment because it focuses on worst case 

scenarios with severe consequences, which are considered to be a rare occurrence. Prevention or 

mitigation of such “worst case” events is often costly and considered unduly “conservative.”406 

Nevertheless, both “best case” and “worst case” scenarios can be used as a tool to bound or 

scope possible risk levels, and can offer insights into engineered or operational procedures that 

could prevent or respond to hazards. Given the lack of hard data on highway safety impacts, the 

hazard scenarios rely on the literature findings and on the SMEs inputs (Chapter 3) based on 

their limited experience or qualified opinions. Consulting experts for their informed opinions 

regarding hazard identification and ranking is called the Delphi Approach. News and literature 

accounts of real crashes and accidents involving CNG trucks and buses, or hybrid and electric 

vehicles identified through Internet searches and related trade safety information were also used 

in the detailed scenarios below. 

Scenarios involving FE technologies, alternative fuels, and operator factors leading to a vehicle 

crash, fire, or explosion are illustrated. Each scenario is discussed below in detail in context, and

can be supported by at least one reported actual incident, cited literature, or by SMEs inputs. 

This scenario analysis allows for identification of strategies to prevent and/or mitigate such 

hazardous occurrences. 

4.2 Scenario Analysis of Safety Hazards identified by SMEs

4.2.1 Hazard Analysis of CNG-Fueled Trucks and Buses

Cleaner CNG fueled MD/HDVs have been safely operated in the United States and worldwide

since the early 1980s with over 90,000 in use today. Public transit authorities have accumulated 

considerable experience with the safe operation of large CNG bus fleets over the past two 

decades, according to APTA407, such as Los Angeles Metro, which operates the largest (over

2200) urban fleet of CNG transit buses in the United States.

 406 See postings at www.palisade.com/risk/risk_analysis.asp 407 See “Transit on the cutting edge of clean technology,” Sept 2012 at 

www.apta.com/resources/statistics/Documents/Transit-Clean-Technology.pdf
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A recent FMCSA safety study of the commercial NGV fleet408 cited the DOE/EIA 2010 

inventory of 43,088 MD/HDV vehicles, including 21,510 urban transit buses, 7,466 heavy-duty

trucks, 5,925 medium duty trucks and 8,187 vans. As CNG MD/HDVs are now being rapidly 

deployed, attention to safe operability and refueling is paramount. 

NGV America safety statistics409 cite the safe and reliable in service operation of over 10,000 

CNG fueled transit and school buses and shuttles, stating that 1 in 5 new transit buses are CNGfueled. An NGV America survey of over 8,331 NGVs showed they have a better crash safety 

record than conventional counterparts, with a 31 percent lower crash rate, no fatalities and 37

percent fewer injuries. Of only 7 fire incidents noted, only one was due to the CNG fuel system 

failure. The strong cylinder tank survived intact in crashes, with no leakage or fire due to the 

automatic fuel release valve activated in case of excessive heat or pressure build-up.

Powertech410 noted that only 26 CNG cylinder failures occurred from 2000 to 2008, and that the 

probability of a CNG vehicle cylinder failure is quite low, and that catastrophic ruptures are very 

rare. The events cited through 2008 (see Figure 2-4 and general discussion above) included: 5 

failures due to mechanical damage, 12 due to environmental damage (exposure to temperatures 

too hot or too cold), 8 leaks due to metal liner and 6 due to plastic liner failures; only 16 resulted 

in vehicle fires. Aftermarket conversions to CNG using steel (Type 1) cylinders were involved in 

50 percent of these failures. The Powertech failure analysis also illustrated how CNG buses with 

Type 4 composite tanks mounted on the bus roof were damaged in collisions with low overhead 

barriers or tunnels. In these cases, the punctured tanks released the CNG, but did not burst, 

explode, nor ignite. Compliance testing with FMVSS 304 has reduced the number of deaths and 

serious injuries due to CNG leakage from tanks ruptured in crashes. 

Given the very few actual incidents either reported by fleet managers (in Chapter 3) or in the 

literature (Chapter 2), safety hazards due to CNG tank failures in MD/HDVs could not be ranked 

by probability, consequence, severity, or overall risk level. Only tank failures for CNG-fueled 

trucks are discussed in this scenario; crashes of CNG tanker trucks delivering fuel to refueling 

compressor stations or industrial sites are omitted, since they are already subject to compliance 

with PHMSA hazmat regulations. Greater highway safety risk could potentially result from 

highways crashes of loaded CNG tanker trucks delivering fuel to refueling stations and 

compressor mishaps, but no such incidents were found documented in the literature. 

4.2.1.1 Incidents of Vehicle Fires or Explosion Due to CNG Tank Rupture 

Several SMEs identified the potential for ignition or explosion scenarios due to pressurized 

(3,500-psi) CNG tank rupture, or to a leak or malfunction of the automated vent on the fuel tank 

as a risk concern in purchasing and operating CNG fueled trucks. The SMEs conjecture, rather 

than experience, was that the probability of such an event would be medium, but the 

 408 FMCSA Natural Gas Systems Safety Study, 2012 at http://ntl.bts.gov/lib/51000/51300/51334/Natural-GasSystems-Tech-Brief-508.pdf 409 See “How Safe are Natural Gas Vehicles?” at www.ngvc.org/tech_data/techbulletin2.html 410 See “CNG and Hydrogen Tank Safety, R&D and Testing,” Joe Wong, Powertech Labs, Inc., 12.10.2009, at 

www1.eere.energy.gov/hydrogenandfuelcells/pdfs/cng_h2_workshop_8_wong.pdf
99

consequence could be high. Tank failure either could occur due to a road crash, or due to 

accidental leaks or venting during refueling indoors or outdoors, followed by ignition. 

Recent CNG tank failures documented in the literature and internet illustrate potential hazards:

• In February 2011, a CNG explosion at Pierce Transit refueling depot (near Seattle) 

occurred during a CNG bus refueling (probably due to a compressor failure), destroying 

the bus.

411

• In February 2012, in Statesboro, Georgia, a university CNG bus caught fire and exploded 

when a small spark started a fire that reached the CNG tank, but the driver evacuated all 

passengers in time. Bus passenger fatalities and/or severe burn injuries could result from 

such an accident if timely evacuation were not possible. Worse consequences might

result if such an event occurred on a busy road at rush hour, causing a multi-vehicle chain 

of fire or explosions, and causing harm to passengers. According to industry best 

practices (standard NFPA 52) every CNG cylinder must be equipped with a pressure relief 

device (PRD) to safely vent the gas in the event of a vehicle fire, thus totally avoiding 

cylinder rupture. The explosion indicates that the PRD was not present, or malfunctioned. 

These incidents suggest that both manufacturing standards, and in-use safety standards for 

vehicle maintenance and condition are needed to ensure that necessary safety systems (i.e., 

PRD) are in place.

4.2.1.2 Rupture of Degraded or Expired CNG Tanks

NHTSA has been conducting active research on CNG tank failures, to ensure safety under 

FMVSS 304, "Compressed Natural Gas (CNG) Fuel Container Integrity."412 FMVSS No. 304 

and the ANSI NGV2 standard set minimum performance requirements for CNG containers used 

for fuel storage in motor vehicles. FMVSS 304 includes tank testing provisions which are 

necessary to determine whether a fuel storage system meets the performance requirements. 

Manufacturers’ labeling must indicate if a tank’s useful service life is 10, 15, 20 or 25 years.

413

FMVSS 304 requires that “This container should be visually inspected after a motor vehicle 

accident or fire and at least every 36 months or 36,000 miles, whichever comes first, for damage 

and deterioration.” NGV2414 also requires in-service visual CNG tank inspection every 3 years or 

36,000 miles, and its destruction and disposal after its service life. Based on limited experience 

to date with CNG buses and trucks, some SMEs estimated that the probability of such failure 

events due to aging, degraded CNG tanks used beyond their design life could be Medium, but the 

consequences potentially High.

When CNG tanks reach the end of their 15-25 years design life or service life, they should be 

decommissioned and replaced. There are reported explosions of expired or damaged CNG tanks 

 411 See incident at http://seattletransitblog.com/2011/02/28/breaking-gas-explosion-at-pt-facility 412 See B. Hennessey, Jan 2012, SAE/GIM presentation at 

www.sae.org/events/gim/presentations/2012/hennesseynhtsa.pdf

413 See www.ngvi.com/tag/ngv2/ 414 See www.transecoenergy.com/pages/CNG_Tank.htm
100

which remain in service, being unscrupulously resold through shops and online, e.g., on eBay; 

though these claims have not been substantiated by a Government agency.

415

Robert Zalosh, a fire and explosions prevention expert from Firexplo, cited several tank 

explosion incidents involving degraded, damaged or expired CNG tanks.416

A national inventory of all CNG tanks on trucks and buses could aid in enforcing proper tank 

end of life disposal. In addition, required NGV training for all automotive mechanics who 

service NGVs could help to ensure that damaged CNG tanks are not placed back in service 

without the necessary inspection.

4.2.1.3 Prevention and Mitigation of CNG Tank Failure and Refueling Hazards 

The regulatory requirement of FMVSS 304 (CNG Fuel Container Integrity) cited above is for

CNG tank inspection every 36 months, after a fire or reportable DOT accident, or the removal of

a tank from service after its expiration date. Compliance with FMVSS 301 (Fuel System 

Integrity) and FMVSS 303 (Compressed Natural Gas (CNG) System Integrity)417—which

currently apply only to vehicles below 10,000 lbs GVW and to school buses regardless of 

weight—could further enhance the safety of CNG fueled MD/HDVs if their domain of 

applicability were extended to Class 3 and above vehicles. 

The Clean Vehicle Education Foundation (CVEF) posts a long list of safety regulations, codes 

and standards applicable to CNG fueled vehicles, infrastructure and operations.

418 These include

SAE, NFPA and ANSI pressurized vessel voluntary consensus standards and recommended 

practices. The Foundation reviewed the limited calendar service life for ANSI NGV2 and 

FMVSS 304 NGV fuel containers, and developed a CNG tank Safety Training and Inspection 

program for owners/operators.

419 The foundation also posted safe CNG tank refueling, 

decommissioning, venting, post-crash, and proper procedures and guidance to prevent high-risk 

venting scenarios420 paying special attention to aftermarket conversions (see Figure 2-5).

 415 American Trucking Association. 2009. Expanding ATA’s Safety Agenda Executive Summary 

www.truckline.com/Newsroom/Policy%20Papers/Safety%20Task%20Force%20Report.pdf

416 Zalosh, R. 2009. CNG and Hydrogen Vehicle Fuel Tank Failure Incidents, Testing, and Preventive Measures 

www.mvfri.org/Contracts/Final%20Reports/CNGandH2VehicleFuelTankPaper.pdf and postings at 

www.firexplo.com/

417 See e-49CFR571 for FMVSS at www.ecfr.gov/cgi-bin/textidx?c=ecfr&tpl=/ecfrbrowse/Title49/49cfr571_main_02.tpl

418 NHTSA. 2010b. Tire Pressure Monitoring System Tests For Medium and Heavy Trucks and Buses 

www.nhtsa.gov/DOT/NHTSA/NVS/Vehicle%20Research%20&%20Test%20Center%20(VRTC)/ca/capubs/811314

.pdf 419 See www.cleanvehicle.org/technology/cylinder.shtml 420 Clean Vehicle Education Foundation. 2010. How Safe are Natural Gas Vehicles? 

www.cleanvehicle.org/committee/technical/PDFs/Web-TC-TechBul2-Safety.pdf, Clean Vehicle Education 

Foundation. 2012. Fatal accident removing cylinder solenoid valve 

www.cleanvehicle.org/technology/ValveFatality.pdf 
101

CVEF industry best practices to prevent and/or mitigate hazards related to vehicle tank or 

refueling infrastructure CNG leakage leading to potential fires and explosions include: adequate 

training of maintenance staff and operators, frequent inspections, emergency response planning 

and preparedness, and engineered controls. The literature recommends voluntary compliance 

with industry best practices, standards and guidelines for CNG tank failure and refueling hazard 

minimization, and avoidance measures. These include how to: 

• Provide methane gas and fire detectors both on-board the vehicle, and in depots and at 

fueling station. 

• Ensure that pressure relief devices (PRDs) are mounted on the CNG high-pressure tank 

and piping subsystem.

• Provide a manual CNG shut-off valve for back up and to safeguard and isolate the fuel 

storage system from the engine.

• Install fire extinguishers to respond to leaks and prevent or extinguish fires. 

• Safe vent CNG tanks for maintenance or replacement (using automated or manual 

venting) after a crash, and/or any piping and tubing that leaked.

• Ensure compliance with SAE best practices for CNG MDHD trucks, including SAE 

J2406 (Recommended Practices for CNG Powered Medium and Heavy-Duty Trucks), 

SAE J2343 (recommended practices for LNG-powered heavy-duty trucks).

• Comply with fire safety NFPA 52421 and NFPA 57422 codes applicable to any vehicle 

and/or facility that refuels CNG or LNG vehicles. 

• Install engineering controls including: methane detectors, visual/audible alarms, and 

other safeguards to prevent ignition and vehicle fires in NGV refueling depots. These are 

recommended over reliance on process controls, such as manually opening doors or 

vents whenever an NGV is brought into a shared indoor facility.

4.2.2 Safety of LNG-Fueled MD/HDVs and Refueling Systems

Cryogenic Liquefied Natural Gas (LNG) use as a transportation fuel is accelerating. As discussed 

in Chapter 2, LNG is better suited to over the road short haul HD operations than CNG (1 gal of

diesel has the same energy contents as 1.67 gals LNG, but 135 scf CNG). There are few 

refueling stations at present; these mostly are operated for corporate or city fleets. Currently 

there are only 6,000 LNG fueled vehicles in the United States, mostly transit buses in Dallas and 

Austin, TX; El Paso and Phoenix, AZ; and Los Angeles and Orange County, CA. Between 2010, 

the ports of Los Angeles and Long Beach will replace 800 drayage trucks with LNG trucks, with 

219 deployed by December 2012.423 Waste Management, Inc. has operated an LNG fueled 

refuse truck fleet for years in CA and PA, and just added its 1000th LNG truck to the fleet

 421 National Fire Protection Association. 2010. NFPA 52: Vehicular Gaseous Fuel Systems Code 

www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=52

422 National Fire Protection Association. 2002. NFPA 57: Liquefied Natural Gas (LNG) Vehicular Fuel Systems 

Code www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=57 423 See CARB, May 12, 2012, “Heavy Duty Natural Gas Truck Replacement Program” at 

www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2012/technology_integration/arravt045_ti_white_2012_

o.pdf
102

(Figure 5-3).

424 NREL has conducted several evaluations of the Waste Management, Inc. LNG 

truck fleet, showing emerging design and operational improvements over time.

There are a few commercial producers of LNG and some distribution firms (mostly via LNG 

tanker trucks) that supply LNG fueling stations. LNG Dedicated tanker truck operators include: 

Tri-Mac, Transgas, Southeast LNG, J.B. Kelley, and L.P. Transportation. 

The National Petroleum Council (NPC) Future Transportation Fuels (FTF) recent studies 

included a qualitative safety evaluation of LNG as an emerging transportation fuel, with 

associated risks relative to petroleum discussed in the literature and Chapter 2.

425

Figure 4-3: Refueling a Waste Management LNG Truck426

 424 See 2001 NREL report at Waste Management's LNG Truck Fleet Alternative Fuels Data at 

www.afdc.energy.gov/pdfs/waste_lng_final.pdf and www.wm.com/about/press-room/2011/20110712-wm-adds1000th-natural-gas-truck.jsp 425 National Petroleum Council. 2012. An Initial Qualitative Discussion on Safety Considerations for LNG Use in 

Transportation www.npc.org/FTF_Topic_papers/19LNG_Transportation.pdf
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This NPC white paper on LNG safety cited the excellent safety record for LNG transportation.

There were only 20-30 over-the-road incidents since 1971 that involved road crashes; less than 

half of which led to spills, and only one resulted in a fire with driver injury. In one crash, the

LNG vessel on a crashed tanker truck maintained integrity when diesel tanks of the other vehicle 

caught fire. Potential safety hazards include cryogenic liquid burns from drivers contacting 

spilled LNG and the potential for fire or explosion. However, engineered system safeguards, 

such as double walled steel container tanks with insulation, PRDs, and inspections to prevent 

steel tank embrittlement and cracking, reduce the risk of a boiling liquid expanding vapor 

explosion (considered a worst-case scenario in the event of a fire or crash). Multiple safeguards 

are used for refueling equipment and procedures. Fire suppression equipment and training for 

maintenance personnel and emergency responders could mitigate the consequences of such an 

LNG truck mishap.

4.2.3 Flammability of Lightweighting Materials and Toxicity of Burning Byproducts 

As discussed in Chapter 3.2.9, some SMEs raised potential safety concerns regarding the fire 

safety and flammability of some materials used for automotive light-weighting. Although there is 

little to no actual experience to date, the SMEs conjecture was that the probability of such 

occurrences, assuming larger scale deployment of lighter MD/HDVs, could be low to medium, 

but a fire related consequences could be medium to high. 

Various carbon-fiber reinforced polymers (CFRP), resins, and plastics are typically more 

flammable than steel, and lighter weight metals (high performance steels, aluminum and 

titanium). Use of CFRP and glass fiber composites in phenolic, with high stiffness to weight 

ratios, are some of the emerging structural materials used for vehicle mass reduction. Other 

material systems include honeycomb sandwich structures, high performance steels, and titanium 

and magnesium alloys to achieve “mass compounding” benefits that reduce overall MD/HD 

verse weight in order to enhance fuel efficiency.

NHTSA held several workshops to examine the relationship between vehicle mass and size and 

crash safety in 2013427 and 2011. Although lightweighted vehicles may offer improved 

maneuverability and shorter stopping distance, they could also result in reduced crash

survivability in certain crash scenarios and/or in vehicle fires. While there is no supporting data 

in the literature regarding the crashworthiness of light-weighted Class 2b to 8 vehicles, there are 

some general safety concerns about the materials. Knowledge of fire and shatter hazards 

associated with the composites used in aircraft and marine applications can be transferrable to 

MD/HDV composite applications. 

The fire safety and health hazards of byproducts from burning composites include the potential 

release of volatile organic compounds, toxic microscopic fibers, and combustion products were a 

 426 Wall Street Journal, May 23, 2012, “Will truckers ditch diesel?” at 

http://online.wsj.com/article/SB10001424052702304707604577422192910235090.html#printMode

427 See mass-size-safety May 2013 workshop presentations at www.nhtsa.gov/Laws+&+Regulations/CAFE+-

+Fuel+Economy/NHTSA+Vehicle+Mass-Size-Safety+Workshop; and the 2011 workshop transcript at 

www.nhtsa.gov/Laws+&+Regulations/CAFE+-+Fuel+Economy/NHTSA+Workshop+on+Vehicle+Mass-SizeSafety
104

secondary concern. Toxic fire byproducts, dispersal, and inhalation of fine fibers pose health 

hazards to emergency responders and bystanders.428 Carbon fiber and epoxy materials ignite 

easily and burn rapidly. Smoldering or burning composites produce toxic smoke containing 

irritants, combustion gases (HCl, HBr, NO2), soot particles and sharp fibers, which in turn can 

cause choking, cough and disorientation, as well as long-term health problems following 

exposure.429

Passive fire protection coatings such as Vermitex and fire suppression equipment in the cab 

could prevent and mitigate flammability hazard and toxicity of by-products for new MD/HDV 

materials. However, fire safety and environmental health issues for specific lightweighting

tractor cab and trailer bed materials would benefit from further study. Also, fire prevention and 

suppression best practices from light-weighted composite-rich aircraft skins and seating may be 

useful, if proven to be cost-effective.430

Brittle failure, or shattering of composite body panels in a crash- even without any fire- may also 

pose hazards from the release of fine particulates that irritate skin and lungs, and could require 

EMTs to wear respirators for protection. Materials Safety Data Sheets (MSDS) for specific fuel

efficient lighter weight truck and trailer materials could help in the evaluation of the potential 

severity of a crash with or without resulting fire. 

A 2007 Volpe Center study for NHTSA that developed a safety roadmap for future PCIVs

431 that 

included both structural and fire safety is also relevant to MD/HDV lightweighting materials 

options. Plastic foam filler used in interior seating and door padding could also release toxic 

volatile organic compounds and carbon monoxide in a crash-fire.

4.2.4 Safety Performance of Single Wide Base Tires and Low-Rolling Resistance Tires

Wide base single tires, also called “super-singles,” and LRRs are being increasingly adopted by 

MD/HDV fleets, as discussed in Chapter 2.7.2. In Chapter 3.2.7 we reported that some SMEs

perceived an increase in stopping distance or reductions in stability and control on slick 

highways when using LRR and/or wide base tires. In their opinion, this hazard probability could 

be medium but the consequence potentially medium to high.

As indicated in Chapter 2.7.2, this research identified no studies that document reduced safety 

performance for either LRR or wide-base tires. Indeed, EPA’s testing of conventional and LRR 

 428 Gandhi, S. 1999. Postcrash Health Hazards from Burning Aircraft Composites 

www.aviationfirejournal.com/aviation/library/VOL6-Hazards.pdf 429 Flight Safety Foundation. 2008. Downwind Debris http://flightsafety.org/asw/sept08/asw_sept08_p40-

43.pdf?dl=1

430 Gandhi, S., & Lyon, R. E. 1998. Health Hazards of Combustion Products From Aircraft Composite Materials 

www.tc.faa.gov/its/worldpac/techrpt/ar98-34.pdf 431 Brecher, A. 2007. A Safety Roadmap for Future Plastics and Composites Intensive Vehicles 

www.nhtsa.gov/DOT/NHTSA/NVS/Crashworthiness/Vehicle%20Aggressivity%20and%20Fleet%20Compatibility

%20Research/810863.pdf
105

Class 8 tractor and trailer tires found no significant correlation between the snow traction rating 

and the coefficient of rolling resistance.

432

Since a LRR wide base tire replaces twin tires, a high-speed blowout on the road of a wide base

tire, especially for heavy loads, could potentially affect the vehicle balance and stability at higher 

speeds. However, literature from the EPA’s SmartWay Program indicates that wide base tires

can improve the stability and control of combination trucks and tank trailers by allowing the tank 

to be mounted lower.

433 A NHTSA study of decreased tire pressure impacts on safety indicated 

that tire pressure monitoring is an important safety factor.

434 Although the NHTSA crash files 

did not contain direct evidence that points to low tire pressure as the cause of any particular 

crash, low tire pressure was considered a possible factor in crashes due to loss in traction and 

blowout failures of underinflated tires, as well as increased stopping distance.

The use of TPMS for wide base tires (or with any MD/HDV tires) may prevent under-inflation 

and reduce the risk of crashes due to flat tires and blowouts. 

4.2.5 Longer Combination Trucks . .

Several types of LCV configurations with extra length and gross vehicle weight (GVWR) 

ranging from 80,000 to over 120,000 lbs., have been allowed to operate (with special driver 

training requirements and varying restrictions) in 23 States to date (Figure 4-4). Operation is 

permitted on some interstate, rural two-lane, or city roads and interchanges. Advocates for 

Highway and Auto Safety strongly opposed lifting the 1991 restrictions on LCV highway 

operations, citing NHTSA, IIHS, and UMTRI data.435

The American Trucking Association (ATA) supports seven limited reforms to current truck size 

and weight regulations that would allow LCVs in order to increase the productivity, fuel 

efficiency, reduce truck traffic and environmental emissions.

436 The Owners Operator 

Independent Drivers Association opposes LCVs437 due to several safety concerns that may 

contribute to LCV crash hazards in mixed vehicle traffic, especially on urban freeways and

interchanges:

• Off-tracking at both low and high speeds, when tractor trailers turn in a curve or at 

intersections and encroach into shoulders and adjacent lanes, endangering both parallel 

and/or oncoming traffic. At low speeds, rear wheels track inside the path of the front 

 432 See “Snow Traction Performance of Low Rolling Resistance Drive Tires for Class-8 Tractor Trailers,” SAE 

Paper 2012-01-1918 at http://papers.sae.org/2012-01-1918/ 433 See www.epa.gov/smartway/documents/partnership/trucks/partnership/techsheets-truck/EPA-420-F10-041.pdf 434 See Tire Pressure Monitoring Final Rule (Part III-Safety problem) at 

www.nhtsa.gov/cars/rules/rulings/tirepresfinal/safetypr.html

435 See 2005 Factsheet “The Dangers of Large Trucks” at www.saferoads.org/issues/fs-trucks.htm 436 See “ATA Summary of Truck Size and Weight Studies” at 

www.transportationproductivity.org/Studies/ATA_Summary_of_TruckSizeandWeightStudies.pdf and 

www.truckline.com/AdvIssues/HighwayInf_Fund/Documents/Proposed%20Reforms%20to%20Federal%20Truck%

20Size%20and%20Weight%20Limits.pdf

437 See LCV issues at www.ooida.com/OOIDA Foundation/issues/LCVs.shtml
106

wheels while at high speeds they track outside the path, with widths varying with consist 

length and weight.

• Larger and heavier loaded LCV big rigs cause more and costly damage to infrastructure 

(pavements, bridges, and shoulders) that is not designed to sustain such loading, even 

though operators distribute the load over more axles. 

• Trailer sideways sway and jack-knifing motion on open road (even when not windy), 

which is amplified rearward for longer LCVs like triples and causes encroachments into 

adjacent lanes and traffic, especially in response to sudden steering.

• Lower stability of LCVs, which are more prone to rollover due to connections between 

the second and third trailer.

• Longer braking and stopping distance that can cause crashes in sudden braking, and 

require special and more frequent inspections and brake adjustments.

• Difficulties LCVs have when accelerating and merging with traffic at interchanges, due 

to the mismatch in speed and acceleration abilities of LCVs relative to other vehicles.

• Difficulty of LCVs in maintaining speed on inclined roads, both climbing and for downgrade, compared with other vehicles.

• Hazards of splash and spray from LCVs to vehicles when passing long LCVs in rainy, 

fog, windy, or snowy conditions (due to blinding spray on windshields, and side-winds).

• Difficulty of LCVs parking in rest areas and at weigh in motion (WIM) stations.

Figure 4-4: Three Types of LCV Configurations438

 438 Source: www.usroads.com/journals/rej/9708/re970806.htm
107

The SMEs interviewed in Chapter 3 believed that potential LCV hazards have a low probability; 

but consequence severity could be medium to high. However, several studies on the relative 

safety of LCV operations that compared to other trucks that were conducted over the past 20 

years indicate that such hazard assessment remains inconclusive because of insufficient data, and 

the small number of LCVs, as well as no causal linkage details in the crash data collected.439

The American Automobile Association (AAA) has conducted pilot studies in five States that 

allow LCV operations and determined that analysis is not possible due to inadequate detail in 

crash data collection, and lack of reliable data. California DOT (Caltrans) conducted LCV field 

tests in 1983 on a variety of roads and diverse speeds and determined that problems outweigh the 

benefits.440

The FHWA has conducted several Truck Size and Weight studies, including a 2000 scenario 

description of LCV nationwide traffic441 for the North American Free Trade Agreement 

(NAFTA), and a safety evaluation that required aggregating 6 years of crash data (1995-99) to 

get meaningful statistics for a safety comparison. These FHWA studies concluded that data were 

not adequate to make reliable risk predictions, and that fewer LCVs on the roads would reduce 

the overall travel risk exposure more than reducing truck crashes.442

The most comprehensive 2009 safety analysis of LCVs443 reviewed multiple Over-Size/Overweight (OS/OW) large truck crash data sets (TIFA/UMTRI, FARS, weigh station data-WIM, 

LTCCS, TRB) and concluded that as commercial vehicles become larger and heavier, crash rates 

decrease, but crash severity increases. However, due to lack of consistency and quality in crash 

data collections, findings on LCV relative safety are inconclusive.

The most recent cost-benefit reassessment of LCVs conducted by the University of Wisconsin

National Center for Freight and Infrastructure Research and Education (CFIRE) in 2012 

reviewed the literature for LCV pros and cons. It found as many claims for safer LCV 

operations, as for less safe impacts, and concluded that the benefits outweigh costs and warrant 

expanding nationwide LCV operations.444

In a potentially worst case, high consequence scenario, multiple vehicle crashes might occur on a 

two-lane highway when small vehicles are trying to overtake a long, slow-moving triple-trailer 

LCV-especially in low visibility hazardous weather- and crash with oncoming traffic, causing 

multiple fatalities, injuries and vehicle damage. No such worst case, pile-up crash scenario was 

 439 See AAA Foundation for Traffic Safety, “Longer Combination Vehicle Safety Data Collection,” 2000, at 

https://www.aaafoundation.org/sites/default/files/lcv.pdf

440 See CALTRANS Longer Combination Vehicles posting at 

www.dot.ca.gov/hq/traffops/trucks/exemptions/lcv.htm

441 See Chapter III, Scenario Descriptions at www.fhwa.dot.gov/reports/tswstudy/Vol3-Chapter2.pdf 442 See at Western Uniformity Scenario Analysis - Chapter 7 Safety: 

www.fhwa.dot.gov/policy/otps/truck/wusr/chap07.htm

443 “A Synthesis of Safety Implications of Oversize/Overweight Commercial Vehicles” by D. S Turner et al., 

University Transportation Center of Alabama (UTCA) report 07115, June 2009 at 

http://utca.eng.ua.edu/files/2011/10/07115-Final-Report.pdf 444 Adams, T., Kleinmaier, D., Marach, A., Helfrich, G., Levine, J., & Bittner, J. 2012. Longer Combination 

Vehicles: An Estimation of their Benefits and Public Perception of Their Use www.wistrans.org/cfire/.../CFIRE_05-

01_Final_Report.pdf 
108

found in a 2003 NHTSA/NCSA study of LCV crash types445 (see Table 15a, based on 1996-99 

crash data).

Countermeasures for potential LCV hazards, including both prevention and mitigation, could

include improved driver training and certification, restricted road operation, mandated safety 

appliances (antilock brakes (ABS), electronic stability control (ESC)) and automated trailer 

steering (ATS), which enables steerable multiple axles for improved LCV stability and control, 

potentially eliminating or reducing sway related hazards.446

4.2.6 Aerodynamic Components 

Aerodynamic components include a range of devices mounted onto a vehicle body, including 

those certified by the EPA SmartWay program447 as FE technologies:

• trailer gap reducer and trailer side skirts (used in combination with one another); 

• trailer boat tail and trailer side skirts (used in combination with one another);

• advanced trailer end fairing; and

• advanced trailer skirts.

For combination trucks, these components include cab extenders, trailer skirts, boat tails, and 

streamlined mirrors and bumpers. As discussed in Chapter 2, up to two-thirds of long-haul 

tractors and one-sixth of trailers in the United States are equipped with some aerodynamic FE

technologies. 

Some SMEs interviewed were concerned with potential incidents of aerodynamic components 

detaching while the vehicle traveled on a busy highway or urban roadway. Although trucks are 

more likely to strike stationary objects that damage a fairing while off-highway, e.g., at a loading 

or transfer facility, the maximum severity of a detachment incident would be on-highway at high 

speed. In a worst case high-consequence scenario, a multiple vehicle crash could occur if a small 

vehicle trailing a combination truck on a highway was struck by detached aerodynamic fairings.

In such a scenario, the detached fairing could penetrate the windshield of a trailing vehicle, 

leading to fatalities, injuries, and vehicle damage.

To date there are no known examples of such incidents reported in the literature. Thus, up to now 

the probability of occurrence for such detachment incidents appears to be very low. There is no a 

priori reason to expect that large scale adoption of aero devices on Class 8 tractor-trailers will 

lead to a higher probability of occurrence for highway fairing detachment incidents and 

potentially resulting crashes, though – in principle – the probability of individual aero 

detachment failures could scale to the number of devices deployed. 

Since aerodynamic fairings are designed to minimize weight, with typical trailer skirts weighing

approximately 200-pounds, they are unlikely to cause severe primary damage to other enclosed 

 445 See Report No. DOT HS 809 569 June 2003, An Analysis of Fatal Large Truck Crashes at wwwnrd.nhtsa.dot.gov/Pubs/809-569.pdf 446 See Drastic productivity gain for Large Truck Operations with Automated Trailer Steering, 2009 at 

www.engr.sjsu.edu/media/pdf/res/coe_res_facdev_grant_symp_040309_tsao_ppt.pdf 447 See www.epa.gov/smartway/technology/aerodynamics.htm
109

(i.e., non-motorcycle) vehicles on the highway in a detachment scenario. However, under special 

circumstances the resulting debris could result in vehicle crash hazards from avoidance 

maneuvers by nearby drivers.

While early designs of underbody fairings tended to be monolithic and were prone to snapping in 

a bottoming-out collision with a curb or railway crossing, newer designs employ composite 

materials (Figure 4-5) and are capable of flexing up to 90 degrees.448 Videos of various skirts 

depict the vehicle being driven over 2-3 foot tall snow banks and curbs without breaking or 

detaching the skirt.449 In a rear-end collision with a boat-tail equipped vehicle, the lower panel of 

the boat tail may be vertically aligned with the windshield of cab-over trucks, buses, or 

passenger vans.

450 Depending on the boat tail panel construction, a worst case crash scenario 

could involve penetration of the windshield and possibly fatal injuries to the trailing vehicle’s 

occupants. However, no reported incidents of this type could be documented by our literature 

and Internet search. 

Figure 4-5: Trailer Skirt Made From Resilient Composite Material451

International research sources reviewed and experience abroad 452,453,454,455,456,457 suggests that 

covering the cavity between the front and rear wheels of MD/HDVs can provide a safety benefit

 448 Freight Wing Incorporated. 2011. Aeroflex www.freightwing.com/aeroflex.php 449 Golsch, K. 2012. HD Vehicle Fuel Economy Improvements through Aerodynamic and Front End Airflow 

Technological Advances

450 Patten, J., Poole, G., Mayda, W., & Wall, A. 2010. Trailer Boat Tail Aerodynamic and Collision Study 

www.tc.gc.ca/eng/roadsafety/safevehicles-motorcarriers-environment-boat-tails-1307.htm 451 Freight Wing Incorporated. 2011. Aeroflex www.freightwing.com/aeroflex.php 452 SWOV – Dutch Road Safety Research Institute. 1996. Closed Sideguards on Trucks: Less Fuel Consumption for 

Trucks, Safer Feeling for Vulnerable Road Users

453 Cookson, R., & Knight, I. 2010. Sideguards on heavy goods vehicles 454 Globe and Mail, Scrapped Canadian study found early promise for safer truck design, 

www.theglobeandmail.com/news/national/scrapped-canadian-study-found-early-promise-for-safer-truckdesign/article10600215/
110

in side-impact collisions with non-motorists, such as bicyclists and pedestrians, by reducing the 

likelihood of side underrun by the rear wheels. For example, the fraction of fatal side-impact

collisions between bicyclists and trucks decreased 61 percent in the U.K. following a national 

implementation of cavity-covering sideguards on heavy trucks.

458 While sideguard 

implementation on U.S. trucks is nearly nonexistent,459 decades of both aerodynamic and nonaerodynamic sideguard implementation on trucks in the UK,

460 EU,461 Japan, and Brazil raise the 

question of whether North American truck skirts designed for aerodynamics and fuel economy 

could also potentially reduce the severity of under-ride crashes with non-motorists. At least one 

North American aerodynamic truck skirt manufacturer, Laydon Composites, already claims that 

its sideskirt meets the European lateral under-ride protection standard.462 A 2013 National 

Research Council Canada study funded by the Transport Canada EcoTechnology program463

examined the level of side underride protection potentially provided by three commercially 

available aerodynamic trailer skirt models.464 The preliminary 2010 NRC Canada report465

concluded: “The effectiveness of the sideguards on heavy vehicles has been demonstrated by a 

UK study.”

Another potential safety hazard scenario identified by SMEs as result of aerodynamic fairings 

adoption is the potential for making vehicle underbody inspection more difficult. The highest 

consequence scenario would be a failure to detect safety-critical vehicle issues, such as deficient 

brakes, leading to subsequent on-road failure. If brakes were to fail, a severe crash could result. 

However, the likelihood of this scenario was considered by SMEs to be low. No cases of 

inspection oversights that have caused subsequent crashes are known to be explicitly linked to 

aerodynamic components. (Of course, if an inspection fails to identify an existing issue that 

causes a later crash, this data may simply not exist.) However, interviews with truck mechanics 

and maintenance staff at a major fleet suggest that aero components do not hinder necessary 

 455 Riley, B. S., Penoyre, S., & Bates, H. J. 1985. Protecting Car Occupants, Pedestrians, and Cyclists in Accidents 

Involving Heavy Goods Vehicles by Using Front Underrun Bumpers and Sideguards 

http://trid.trb.org/view.aspx?id=237955

456 Lambert, J., & Rchnitzer, G. 2002. Review of Truck Safety: Stage 1: Frontal Side, and Rear Underrun Protection 457 I Knight et al. 2005. Integrated Safety Guards and Spray Suppression--Final Summary Report at 

www.roadsafetyobservatory.com/Evidence/Details/11356

458 Ibid. as 446 459 Limited municipal pilot deployments totaling approximately 40 vehicles have taken place in Portland, OR and 

Boston, MA since 2010, in addition to an unfunded sideguard mandate in Washington, D.C. These pilot programs 

are too recent and small-scale to have produced statistically significant national crash data. 460 Riley, B. S., Penoyre, S., & Bates, H. J. 1985. Protecting Car Occupants, Pedestrians, and Cyclists in Accidents 

Involving Heavy Goods Vehicles by Using Front Underrun Bumpers and Sideguards 

http://trid.trb.org/view.aspx?id=237955

461 European Union. 1989. European Council Directive 89/297/EEC http://eurlex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31989L0297:EN:HTML

462 National Research Council. 2010a. Technologies and Approaches to Reducing the Fuel Consumption of MD/HD 

Vehicles www.nap.edu/openbook.php?record_id=12845&page=R1 463 www.tc.gc.ca/eng/programs/environment-etv-menu-eng-118.htm 464 Globe and Mail, Scrapped Canadian study found early promise for safer truck design, 

www.theglobeandmail.com/news/national/scrapped-canadian-study-found-early-promise-for-safer-truckdesign/article10600215/

465 Patten, J.D. and Tabra, C.V. Side Guards for Trucks and Trailers Phase 1: Background Investigation, 2010. 

www.worldcat.org/title/side-guards-for-trucks-and-trailers-phase-1-background-investigation/oclc/797259567
111

access.466 In most European and Asian countries that adopted aerodynamic fairings as standard 

equipment on MD/HDVs normally present during inspections, access was not impeded. Clean 

carrier truck fleets, which are early adopters of aerodynamic packages, all demonstrated superior 

safety performance based on FARS and MCMIS databases versus conventional carriers (see 

Chapter 4). Moreover, the overall crash rate of trucks has not increased over the past decade,

even as the deployment of aerodynamic devices has greatly increased. 

To further reduce the likelihood of this scenario, potential countermeasures for inspection 

interference could include the use of mirrors by vehicle inspectors for inspecting certain items 

instead of crawling under the vehicle, making fairings removable, providing access hatches, or 

using hinged designs that provide on-demand access for inspectors. For visual inspection, certain 

fairings or wheel covers could also be made transparent.

4.2.7 Potential Fires from Overheated EV/PHEV Battery 

Longer-term experience with the use of high performance lithium ion batteries (LIB) in 

rechargeable energy storage systems (RESS) has accumulated from several thousands of 

advanced hybrid electric and electric transit buses and shuttles. By 2013, over 11 percent of 

transit bus fleets were hybrid and electric and more on order, according to APTA. Large transit 

agencies with safety operational experience include the New York MTA operating over 1600

hybrid buses, and the Washington Metrobus and King County DOT with hybrid fleets of over 

600 buses each. This operational fleet experience offers safety “lessons learned” relevant to 

emerging commercial HEV/EV/PHEV counterparts. LIBs may pose potential risks due to 

thermal runaway when overheated or explosion and fire when the cell is ruptured. Flammable 

lithium and vented hydrogen or oxygen may initiate and sustain fires, and corrosive or toxic 

electrolyte could leak if the battery is breached. Although there are very few documented fire 

incidents in hybrid-electric buses to date and some SMEs considered that although this hazard 

probability was low, they believed the consequence severity could be medium to high. Therefore, 

we documented reported incidents below to provide the context for such a scenario.

U.S. hybrid and electric transit buses models deployed and featuring LIBs integrated in high 

power and capacity RESS include: the Daimler Bus NA Orion VII, the DesignLine EcoSaver IV, 

the El Dorado National Axess, Gillig HEB models with Allison dual-mode compound split 

propulsion, North America Bus Industries (NABI) diesel electric with ISE/Bluways 

ThunderPower, the Proterra composite electric or fuel cell buses with TerraVolt and Altair 

Lithium Ion Titanate (LTO) LIBs, and the New Flyer Xcelsior.

The Orion VII diesel-electric hybrid buses with A123 Lithium Iron Phosphate (LiFePO4)

batteries467 have been in service since 2007, and have been documented as being both energyefficient and environmentally friendly.

468 These batteries have greater energy density and power 

and are 3,000 pounds lighter than the previously used lead acid Orion battery packs, thus 

 466 Navistar. 2012. Aerodynamic and Tire Technology Adopter Interviews 467 See www.a123systems.com/solutions-transportation.htm 468 See TRB/TCRP Report 132, “Assessment of Hybrid Electric Transit Bus Technology,” December 2009, 

www.trb.org/Main/Blurbs/Assessment_of_HybridElectric_Transit_Bus_Technolog_162703.aspx and “BAE/Orion 

Hybrid Electric Buses at New York City Transit: A Generational Comparison (NREL/TP-540-42217).”
112

improving bus fuel economy. They also have a stated six-year design life with lower operating 

and lifecycle costs. The BAE’s “next generation HybriDrive” may enable more flexible modular 

RESS configurations with electronically-controlled cooling and APU (fuel cells or other options) 

to be placed on the roof for easy access and maintenance, for fuel cell hybrid-electric buses. 

BAE’s HybriDrive469 is also compatible with other hybrid and electric bus platforms and RESS 

options: the New Flyer Industries Xcelsior (XDE40) 40-foot diesel HEB also uses the BAE 

HybriDrive with LiFePO4 batteries (from Lithium Technology Corporation). The LIB delivers 

200 kW peak power and is cooled with forced air. The Washington Metro Area Transit Authority 

(WMATA) received 152 such Xcelsior buses in 2011 and ordered 95 more in 2012 for 2013.470

Information on LIB failures and maintenance issues can also be found in NREL evaluations of 

hybrid and electric transit bus fleets upgraded with different LIB chemistries, which are 

operating in many U.S. cities.471, 472 These reports evaluate hybrid and electric bus fleets such as 

the New York City MTA’s BAE Orion diesel-hybrid bus fleet, Long Beach Transit’s gasolineelectric hybrids, King County Metro’s Allison hybrid-electric buses, and Knoxville Area 

Transit’s Ebus electric buses and trolleys. For instance, the NREL multi-generational comparison 

of Orion/BAE hybrid-electric buses operating in the New York City MTA’s transit fleet473

identified the key LIB performance, durability, and safety improvements needed for transit 

reliability, availability and durability. 474

On October 28, 2011, NHTSA’s Office of Defect Investigation issued a safety recall and 

corrective modifications or replacement of LIBs in Daimler Orion VII hybrid-electric buses. 

After several incident investigations due to accumulation of debris and moisture that potentially

breached electrical isolation,

475 1,300 Orion VII hybrid buses with the BAE HybriDrive and 

RESS using A123 LiFePO4 batteries (manufactured in November 2008), and some earlier 2006–

07 models retrofitted with LIBs were recalled to replace the battery modules on the bus roof. 

The BAE Systems HybriDrive476 integrated a LiFePO4 battery chemistry based on its superior 

thermal stability, energy capacity, and operational safety, as well its modular, compact, 

lightweight design, and longer cycle life (over 6 years). Even though the roof placement did not

require active battery cooling it allowed the debris and moisture accumulation to cause potential 

short-outs and fires.

 469 See www.hybridrive.com/ 470 See news at www.newflyer.com/index/2012_08_07_wmata_additional_order 471 See transit hybrid fleet reports posted at www.afdc.energy.gov/afdc/fleets/transit_experiences.html?print 472 See postings listed at www.afdc.energy.gov/afdc/fleets/transit_experiences.html 473 See “As Hybrid Buses Get Cheaper, Cities Fill Their Fleet,” New York Times, October 22, 2009, 

www.nytimes.com/2009/10/22/automobiles/autospecial2/22BUS.html?_r=1

474 See Clean Air Initiative: Infopool–Hybrid Bus postings at www.cleanairnet.org/infopool/1411/propertyvalue17735.html#h2_5

475 See Orion 7 Hybrid Bus Recall at http://recallcast.com/recalls/2011/oct/28/daimler-buses-north-america-incelectr-11v523000/ and at www.safercar.gov and http://recallcast.com/recalls/tag/orion/ 476 See www.hybridrive.com/hybrid-transit-bus.asp and www.hybridrive.com/lithium-ion-energy-storagesystem.asp
113

This recall affected more than 1,600 Orion buses operating in New York City. The 2011 Orion 

VII safety recall has already affected many urban transit fleets in the United States and 

contributed to Daimler’s discontinuing the manufacture and marketing of Orion VII hybrid buses 

in the United States and Canada in April 2012.477

The New Flyer manufacturer of Xcelsior buses, which also use the BAE HybriDrive RESS with 

A123 LIBs, notified NHTSA in March 2012 that it was recalling them to correct a similar RESS 

that could cause a LIB short and pose fire hazards. A total of 47 hybrid Xcelsior Metrobuses in 

the Washington, DC, area were also pulled from operation for inspections and corrective retrofits 

of LIBs by BAE Systems.478

To date, both the incidence and severity of hybrid and electric bus safety incidents involving 

LIBs overheating, degassing, or having electrical short-out incidents are very low. Only 10 

similar incidents have occurred in the United States and Canada in the past decade out of more 

than 2,200 operating hybrid buses, with no resulting fatalities or injuries. 

Although hybrid and electric delivery trucks and vans in cities have similar duty cycles,

centralized maintenance, and scheduled recharging to transit buses, long-haul trucks experience 

more demanding battery charge/discharge duty cycles. Potential hazards for this application 

could include the battery overheating or failing suddenly in severe weather condition, potentially 

causing fires or crashes. 

4.2.7.1 Electrical Hazards Prevention and Mitigation

After a Chevy Volt fire and defect investigation,479 NHTSA held a technical workshop on 

electric vehicle safety 480 (May 2012). In cooperation with the National Fire Protection 

Association (NFPA), issued an “Interim Guidance for Electric and Hybrid Vehicles Equipped 

with High Voltage Batteries”481, to assist owners/operators, first responders, and 

towing/recovery operators. The Guidance is applicable to all hybrid and electric vehicles, 

including MD/HDVs. 

At the May, 2012 Electric Vehicle Safety (EVS) workshop several presenters discussed the 

potential hazards specific to heavy-duty hybrids.

482 For instance, the high voltage cables and

 477 See www.newschannel6now.com/story/17736988/daimler-buses-reconfigures-operations-in-northamerica?clienttype=printable

478 See news item at washingtonexaminer.com/article/416416 479 PE11037; wwwodi.nhtsa.dot.gov/owners/SearchResults?searchType=ID&targetCategory=I&searchCriteria.nhtsa_ids=PE11037

480 See postings for May ’12 workshop and Jan ’12 Interim Guidance Report No. DOT HS 811 574 at 

www.nhtsa.gov/Vehicle+Safety/Electric+Vehicle+Safety+Symposium

481 See NHTSA “Interim Guidance for Electric and Hybrid Vehicles Equipped with High Voltage Batteries” and o 

postings at 

www.nhtsa.gov/About+NHTSA/Press+Releases/NHTSA+Statement+on+Conclusion+of+Chevy+Volt+Investigatio

n

482 See NHTSA EVS workshop summary and presentations posted at 

www.nhtsa.gov/About+NHTSA/Press+Releases/NHTSA+Hosts+Technical+Workshop+on+Electric+Vehicle+Safet

y
114

post-crash stranded energy in the high power hybrid and/or electric MD/HDV LIBs may pose 

hazards of electrocution and shock to maintenance technicians and/or emergency responders 

after a crash or fire incident. The probability of high voltage battery-related electrical shorts, 

electrocution and fire hazards can be reduced through compliance with FMVSS 305 (which 

currently applies only to vehicles lighter that 10,000 lbs GVW, and deals with electrolyte leakage 

and loss of electric isolation due to specific crash scenarios mandated by FMVSS 301), and 

adherence to the NHTSA interim Safety Guidance.

483

NHTSA is continuing to conduct research on failure scenarios and safety of RESS and LIBs. 

Such potential hazards may be preventable or mitigated through redundant engineering design, 

packaging, and abuse testing of batteries and the RESS subsystem. Effective prevention includes 

the use of a LIB battery management system (BMS) and thermal management system (TMS),

which monitor the voltage and temperature of power electronics, and which are programmed to 

detect faults and shutdown. In addition, crashworthy packaging prevents penetration and leakage 

of electrolyte from the battery in a crash.

OEMs are observing the voluntary SAE electrical safety and battery testing standards (e.g.,

J2929 and Battery Abuse Testing Handbook)484 as generally recommended best safety practice 

for emerging MD/HDV designers, manufacturers, owners and operators. There are numerous

other SAE existing or under development voluntary industry standards and recommended

practices for heavy-duty vehicle electrification safety485 (e.g., SAE J2910, J1654, J1673, J1742, 

J1797, J2344, J2464, J2758, J 2936, J2990.)

 483 Same as 470, and see postings at 

www.nhtsa.gov/About+NHTSA/Press+Releases/NHTSA+Statement+on+Conclusion+of+Chevy+Volt+Investigatio

n

484 See http://standards.sae.org/j2929_201102/ for “Electric and Hybrid Vehicle Propulsion Battery System Safety 

Standard - Lithium-based Rechargeable Cells” J2929, 2011-02-18

See J2929,2011-02-18 “Electric and Hybrid Vehicle Propulsion Battery System Safety Standard - Lithium-based 

Rechargeable Cells” at http://standards.sae.org/j2929_201102/ 485 See www.sae.org/standardsdev/vehicleelectrification.htm and http://ev.sae.org/standards/power-propulsion/items/
115

5 REGULATORY BARRIERS ANALYSIS 

5.1 Introduction

A review of the existing DOT multi-modal (NHTSA, FHWA, and FMCSA) regulatory 

framework was conducted to evaluate the potential impacts of state, Federal, and NAFTA 

regulations on the adoption of fuel efficiency-improving technologies and alternative fuels on 

commercial MD/HDVs. The focus was on identifying any regulatory barriers to rapid 

deployment of FE technologies and alternative fuels in the MD/HDV fleet, and a comprehensive 

list of regulations affecting FE adoption is summarized in Table 5-1. This regulatory review and 

analysis included a review only of technologies with identified relevant regulatory (both Federal

and state) barriers:

• Impacts on effective HD vehicle size and weight, cargo volume capacity, number of 

vehicles on road, infrastructure wear and tear;

• Aerodynamic technologies that may exceed applicable trailer length regulations;486

• Longer combination vehicles that may exceed applicable vehicle length regulations;

• Implications of Federal and State inch-width pavement weight laws for using wide based 

tires on non-tandem axle combination trucks;487

• National electric and natural gas transmission code standards, if applicable;488

• Potential conflicts with applicable safety equipment regulations raised by replacing side 

mirrors with cameras.489

The NACFE report on “Barriers to the Increased Adoption of Fuel Efficiency Technologies in 

the North American On-Road Freight Sector”490 focused primarily on market barriers, reporting 

only a few comments on incentives and barriers associated with existing and emerging 

regulations, such as the NHTSA and EPA fuel efficiency and GHG rule and the CARB tractortrailer GHG Rule. Similarly, the CalHEAT report491 on market barriers to truck industry 

adoption of FE technologies and alt-fuels mentioned in Sec.1 did not address remaining 

regulatory barriers.

 486 Transport Canada. 2009a. Aerodynamic Trailer Skirts www.tc.gc.ca/eng/programs/environment-ecofreight-roadaerodynamic-trailer-skirts-2540.htm 487 EPA. 2012. Low Rolling Resistance Tires A Glance at Clean Freight Strategies 

www.epa.gov/smartway/documents/partnership/trucks/partnership/techsheets-truck/EPA-420-F10-041.pdf 488 NREL. 2010a. Electric Vehicle and Infrastructure Codes and Standards Citations 

www.afdc.energy.gov/pdfs/48605.pdf, NREL. 2010b. Natural Gas Vehicle and Infrastructure Codes and Standards 

Citations www.afdc.energy.gov/pdfs/48611.pdf 489 Florida Department of Transportation. 2010. Evaluation of Camera-Based Systems to Reduce Transit Bus Side 

Collisions www.nctr.usf.edu/pdf/77905.pdf, Wierwille, W. W., Schaudt, W. A., Blanco, M., Alden, A. S., & 

Hanowski, R. J. 2011. Enhanced Camera/Video Imaging Systems (E-C/VISs) For Heavy Vehicles: Final Report

www.nhtsa.gov/DOT/NHTSA/NVS/Crash%20Avoidance/Technical%20Publications/2011/811483.pdf 490 See NACFE-ICCT 2013 report at www.theicct.org/sites/default/files/publications/ICCT-NACFECSS_Barriers_Report_Final_20130722.pdf

491 See CalHEAT “Market Barriers And Opportunities For Alternative Fuel-Hybrid Systems” June 2013 at 

www.calstart.org/Libraries/CalHEAT_2013_Documents_Presentations/CalHEAT_Market_Barriers_and_Opportunit

ies_for_Alternative_Fuel-Hybrid_Systems.sflb.ashx
116

The DOE/AFDC Fuels and Vehicles posts resources on Federal and State laws and regulations

that incentivize the vehicle hybridization and electrification492 and alt-fuels adoption legal 

incentives by state,

493 but an analysis of regulatory barriers is essential for their removal.

The 2002 FMCSA NAFTA rules that apply to Mexico and Canada trucks and buses arriving at 

border crossings and operating in the United States494 are enforcing safety regulations that apply 

to the U.S. truck fleet, but they address neither FE technologies nor alternative fuels. A recent 

Congressional Research Service (CRS) review of Federal freight policy and legislative 

initiatives495 mentions that the Moving Ahead for Progress in the 21st Century (MAP-21) 

legislation instructed DOT to perform a study of existing truck size and weight (TS&W) 

limits.496 The goal is to update Federal TS&W regulations applicable to the National Network of 

highways and arterial access roads (23 CFR 658) and to harmonize the current patchwork of 

State regulations discussed below and listed in Table 5-1. Federal TS&W limits apply only to the 

209,000 miles of interstate highways and principal access arterials, whereas States regulate and 

restrict LCVs operations on all other highways and roads. 

Below in Sections 5.2-5.9, are discussed potential regulatory barriers to the adoption of specific 

FE technologies. OMB Circular A-119 requires Federal agencies to adopt and adapt applicable 

voluntary consensus standards, where applicable, as a cost-effective alternative to rulemaking.497

The Society of Automotive Engineers (SAE), American National Standards Institute (ANSI), 

NFPA and other standards-developing organizations (SDOs) develop the safety standards and 

recommended practices for existing and emerging FE technologies and alternative fueled 

vehicles that are voluntarily adopted by OEMs and vehicle fleet owners and operators. These 

voluntary industry standards and guidelines may provide sufficient assurance of safe operability 

of MD/HDVs equipped with FE technologies and using alternative fuels, without requiring 

additional regulatory development.

5.2 Adoption of Alternative Clean Fuels

There are no major regulatory barriers for the adoption of CNG, LNG, or biodiesel as fuels in 

MD/HDV fleets. Indeed, there are more Federal and State regulatory and economic/tax

incentives for adopting cleaner alternative fuels than barriers to adoption. The DOE Alternative 

Fuels Data Center has compiled summaries of Federal and State laws and incentives by type of 

fuel and/or technology.498 The EPA’s Renewable Fuel Standards and State regulations and tax 

incentives for technology or fuel adoption to improve air quality (e.g., idle reduction equipment) 

are compiled and posted by the Transportation Environmental Resource Center.499

 492 See www.afdc.energy.gov/fuels/laws/ELEC/US 493 See www.afdc.energy.gov/laws and database on State incentives at www.dsireusa.org/ 494 FMCSA. 2007b. NAFTA Demonstration Project www.fmcsa.dot.gov/intl-programs/nafta-rules/new-mexrule.htm 495 Frittelli, J. 2012. Federal Freight Policy: An Overview www.fas.org/sgp/crs/misc//R42764.pdf 496 FHWA. 2012b. Moving Ahead for Progress in the 21st Century Act (MAP-21) 

www.fhwa.dot.gov/map21/summaryinfo.cfm

497 Composites World. 2007. New hybrid electric bus takes advantage of composites 

www.compositesworld.com/articles/new-hybrid-electric-bus-takes-advantage-of-composites 498 DOE Alternative Fuels Data Center. 2013a. Laws and Incentives www.afdc.energy.gov/laws/search 499 Tranportation Environmental Resource Center. 2013. Tranportation Environmental Resource Center List of State 

Regulations and Incentives for Alternative Fuels www.tercenter.org/pages/altfuelMA.htm
117

5.3 Hybridization and Electrification

5.3.1 Electric Drivetrain

Minimum speed limits may pose a challenge for all-electric technologies as MDHD vehicles 

equipped with electric drive systems may under some circumstances be unable to travel fast 

enough to meet minimum speed limits on high grade inclines, per SME input in Section 3.2.2. 

There are no Federal regulations establishing minimum speed limits, so each State regulates this 

independently.500 E-truck routes have been voluntarily “certified” by fleet managers for safe 

operation.

5.3.2 Hybrid Hydraulic Drivetrain

Excess weight added by hybrid hydraulic drivetrains may cause MD/HDVs to exceed applicable 

weight regulations. National weight standards apply to all commercial vehicle operations on the 

Interstate Highway System. The Intermodal Surface Transportation Efficiency Act of 1991 

(ISTEA) froze the maximum weight of vehicles on the Interstate System at 20,000 pounds per 

single axle, 34,000 pounds per tandem axle, and 80,000 pounds overall gross weight, or at the 

gross weight limit actually and lawfully in effect for such vehicles in a State on June 1, 1991 (in 

cases where that limit exceeds 80,000 pounds). Lower weight limits may be enforced where

derived by a bridge formula. States are not allowed to set weight limits on the interstate highway 

system at less than the Federal weight limits.501

Beyond the Interstate System, States have the flexibility to set their own weight limits, but 

Colorado is the only State to provide a weight exemption (of 1,000 pounds) for hybrid vehicles 

off the Interstate system, as described in the Colorado Revised Statutes 25-7-106.8 and 42-4-508.

5.4 ITS and Telematics

There are no major regulatory barriers identified for the adoption of ITS or telematics for 

MD/HDVs, as seen in Table 5-1. As discussed in Sections 2.4 and 3.2.3, there are both potential 

energy savings and efficiency benefits to their fleet-wide adoption. There may be barriers to 

adoption of telematics technologies by fleet owners and operators related to concerns with 

system and operational costs, or with drivers’ privacy protection502 , when fleet managers 

monitor the safety and efficiency performance of both assets and drivers that use GPS and ITSenabled devices. For instance, the FMCSA 2010 and 2011 NPRM related to Electronic OnBoard Recorders (EOBR) requirements for CMV operators and carriers faced litigation and was 

withdrawn and later modified in an MPRM requiring Electronic Logging Devices (ELD) and 

 500 Insurance Institute for Highway Safety. 2013. State Speed Limits www.iihs.org/laws/speedlimits.aspx 501 Vaidya, U. 2012. GATE Center of Excellence at UAB for Lightweight Materials and Manufacturing for 

Automotive Technologies 

www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2012/technology_integration/ti026_vaidya_2012_p.pdf

502 See www.worktruckonline.com/channel/safety-accident-management/article/story/2009/08/beware-the-legalpitfalls-in-telematics-monitoring-of-employee-drivers.aspx
118

Hours of Service (HOS) documentation to address concerns with potential drivers’ privacy and

harassment issues.

503

5.5 Human Factors and Operations

5.5.1 Idle Reduction

Adoption of idle reduction technologies by MD/HDVs504 is driven by compliance requirements 

with EPA Clean Air regulations, as well as with State and local mandates. There are many antiidling technology options verified by the EPA SmartWay program,

505 which applies to 

certification of new engines, rather than the whole vehicle. Excess weight from idle reduction 

technologies may cause MD/HDVs to exceed applicable weight regulations (for more detail on 

weight regulations, see Section 6.3.2). States may (but are not required to) allow MD/HDVs 

equipped with idle reduction technology to exceed the maximum gross vehicle weight limit and 

the axle weight limit by up to 550 pounds to compensate for the additional weight of the idle 

reduction technology.506

Most States have opted to allow weight exemptions for idle reduction technology. However, 

California, Hawaii, Kentucky, North Carolina, Rhode Island, and Tennessee and the District of 

Columbia do not allow weight exemptions. Most States and many cities limit the amount of time 

that MD/HDVs can idle, although the regulations in Table 6-1 do not address a specific antiidling technology. 

5.5.2 Speed limiters

There were no identified regulatory barriers preventing adoption of speed limiters for 

MD/HDVs. NHTSA published a notice in the Federal Register on January 3, 2011, granting 

petitions for rulemaking for an Engine Control Module Speed Limiter Device.

507

5.6 Longer Combination Vehicles and Truck Size and Weight Limits

Existing vehicle length and weight regulations may be limiting the adoption of LCVs. 

Regulatory restrictions for LCVs vary on a State-by-State basis, and this heterogeneity is 

preserved by Federal law. In 1991 ISTEA froze maximum weight and length limits for each 

State at their 1991 values.508 The Federal Highway Administration Office of Freight 

Management issued in December 2012 a notice soliciting bids for a new study on specific areas 

of Federal TS&W limits, their operation and their impacts, responding to a mandate in the MAP-

 503 See www.fmcsa.dot.gov/regulations/rulemaking/2011-8789 and 

www.fmcsa.dot.gov/regulations/rulemaking/2014-11244 504 See www.afdc.energy.gov/conserve/idle_reduction_heavy.html 505 See www.epa.gov/smartway/forpartners/technology.htm#tabs-4 506 Ibid. 507 NHTSA. 2011a. Federal Motor Vehicle Safety Standard; Engine Control Module Speed Limiter Device 

https://www.federalregister.gov/articles/2011/01/03/2010-33057/federal-motor-vehicle-safety-standard-enginecontrol-module-speed-limiter-device, Transport Canada. 2009a. Aerodynamic Trailer Skirts 

www.tc.gc.ca/eng/programs/environment-ecofreight-road-aerodynamic-trailer-skirts-2540.htm 508 Frittelli, J. 2012. Federal Freight Policy: An Overview www.fas.org/sgp/crs/misc//R42764.pdf
119

21 legislation.509 This requirement to further study the issue and complete the study by 

November 2014 was an outcome of contested proposals to allow six-axle truck/trailer 

configurations to gross 97,000 pounds.

As mentioned, ISTEA enforced a freeze limiting the use of the longer, heavier double- and 

triple-trailer combinations to those States in which they were already operating in 1991.510

Specifically, ISTEA imposed freezes on: 1) the maximum weight of LCVs, which consist of any 

combination of a truck tractor and two or more trailers or semitrailers which operate on the 

Interstate System at a gross weight over 80,000 pounds; and (2) the overall length of the cargo 

carrying units of combination vehicles with two or more such units where one or both exceed 

28.5 feet in length on the National Network. The maximum weight of LCVs and the maximum 

length of the cargo carrying units of combination vehicles is the weight or length in actual and 

legal operation in a State on June 1, 1991.511 

Weight limits governing trucking operations across the two international U.S. borders are very 

different. In crossing to Canada, all but one crossing for NHS highways have a GVW limit of 

more than 99,000 pounds; 9 of the 11 Interstate crossings have GVW limits of more than 

105,000 pounds. In crossing to Mexico, all four Interstate crossings are limited to a GVW of 

80,000 pounds, and six of nine other crossings on the NHS have a GVW of 84,000 pounds (with 

a permit from Texas).512 Harmonized operational restrictions, including route designations for 

LCVs, would help to foster their adoption. Figure 5-1 shows the current regulatory environment 

for LCVs in the United States. The existing environment for LCV operation is a State-by-State 

patchwork. 

Recent legislative initiatives include the 2011 two competing bills in Congress addressing 

TS&W limits, neither of which became law:513

• The Safe and Efficient Transportation Act (HR763, S747) to allow States to increase the 

weight limit from 80,000 lbs. to 97,000 with an extra axle and extra wheels, when 

appropriate. 

• The Safe Highways and Infrastructure Protection Act (HR1574, S876) to prohibit any 

weight limit increase and would extend Federal size and weight limits to the entire 

National Highway System. 

• In February 2013 the Safe and Efficient Transportation Act bill was introduced in the 

House (H.R. 612) to allow States to increase Interstate truck weights to 97,000 pounds, 

and require LCV tractor-trailers have a sixth axle to decrease per-tire weight and 

improve braking.

 509 FHWA. 2012a. MAP-21 Freight Provisions 

www.fhwa.dot.gov/planning/freight_planning/talking_freight/nov282012transcript.cfm

510 FHWA. 2000. Comprehensive Truck Size and Weight Study Volume 2, Chapter 3 

www.fhwa.dot.gov/reports/tswstudy/Vol2-Chapter3.pdf 511 Vaidya, U. 2012. GATE Center of Excellence at UAB for Lightweight Materials and Manufacturing for 

Automotive Technologies 

www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2012/technology_integration/ti026_vaidya_2012_p.pdf

512 FHWA. 2000. Comprehensive Truck Size and Weight Study Volume 2, Chapter 3 

www.fhwa.dot.gov/reports/tswstudy/Vol2-Chapter3.pdf 513 Frittelli, J. 2012. Federal Freight Policy: An Overview www.fas.org/sgp/crs/misc//R42764.pdf
120

• In May 2013 the Safe Highways and Infrastructure Preservation Act of 2013 (SHIPA, 

H.R. 1906 and S. 880) legislation514 was introduced in Congress, that would extend the 

current limit of 80,000 pounds on five axles in 53-foot trailers to the entire 220,000-mile 

National Highway System, beyond the current 44,000 miles of interstates. SHIPA is

intended to close a loophole used by States to allow longer, heavier trucks on certain 

Federally funded roadways. Both competing bills are still pending.

Figure 5-1: Regulatory Environment for Longer Combination Vehicles in the United States

5.7 Tire Technologies

5.7.1 Tire Pressure Management Systems and Automatic Tire Inflation Systems 

There are no major regulatory barriers for the adoption of TPMS or ATIS for MD/HDVs. The 

NHTSA regulations in 49 CFR Parts 571.138 and 585 (FMVSS 138) require TPMS for new 

vehicles under 10,000 pounds (LDVs) and stipulate that the system must include a warning 

 514 See www.truckingalliance.org/news/lautenberg-introduces-bill-to-restrict-truck-size-and-weight
121

signal to the operator when the pressure is outside a certain pressure range. MD/HDVs are not 

covered by the NHTSA regulations.

5.7.2 Single Wide Tires

Single wide tires meet the inch-width weight requirements for all States for dual axles but are 

restricted in certain States to 17,500 lbs. on a single axle at 500 lbs./inch width limit. They are 

disallowed on single axle positions on certain double and triple combination vehicles.515 The 

ATA has a spreadsheet available of the latest regulations for some States that limit “single tire” 

use and is working with these States to clarify regulations and resolve the limitations.516

5.7.3 Low Rolling Resistance Tires

By statute, NHTSA’s Tire Fuel Efficiency Consumer Information Program (TFECIP) only rates 

the fuel efficiency (i.e., rolling resistance), safety (i.e., wet traction), and durability (i.e., tread

wear life) of passenger car replacement tires. Some have claimed517 that low rolling resistance 

tires may increase stopping distances, and would therefore conflict with corresponding NHTSA 

tire safety standards (FMVSS 119)518 requirements, which apply to single wide tires. Lab and 

road testing on LRR performance on packed snow traction concluded that “the current 

generation of SmartWay verified LRR tires can offer a similar level of snow traction 

performance as non-SmartWay verified tires, while reducing fuel consumption and emissions.”

As mentioned in section 2.7.3, a NHTSA 2009 study of low and standard rolling resistance tires 

for light-duty vehicles demonstrated a strong and significant correlation between lower rolling 

resistance and increased wet slide, or the propensity to lose traction on wet road surfaces. 

However, this was most significant for vehicles without Anti-lock Braking Systems (ABS). For 

newer vehicles with ABS or electronic stability control (ESC) systems, the tradeoff was found to 

be less significant.519 The study did not address MD/HDVs, and prior findings may therefore 

have limited applicability. 

ABS has been required by NHTSA on all trucks and buses since 1997-1999 per FMVSS 121, 

which would suggest that it is unlikely that low rolling resistance tires would conflict with the 

maximum stopping distance for MD/HDV vehicles specified within that regulation. However, 

the 2009 study did not examine MD/HDVs and the effects on stopping distance may vary based 

 515New Generation of Wide-Base Tires: Impact on Trucking Operations, Environment, and Pavements, 2008, TRB 

at http://trid.trb.org/view.aspx?id=802211 516 Goodyear Truck Tires. 2013. Standards & Regulations www.goodyeartrucktires.com/pdf/resources/servicemanual/Retread_S13_V.pdf

517 Cited in Transport Canada Nov. 2012 study “Test Report Packed Snow Performance of Low Rolling Resistance 

Class 8 Heavy Truck Tires” at www.tc.gc.ca/media/documents/programs/nrc_test_reportpacked_snow_performance_eng.pdf

518 See FMVSS 119, New Pneumatic Tires for Vehicles Other Than Passenger Cars, 

www.nhtsa.gov/cars/rules/import/FMVSS/#SN119

519 Evans, L. R., MacIsaac, J. D., Harris, J. R., Yates, K., Dudek, W., Holmes, J., Popio, J., Rice, D., & Salaani, M. 

K. 2009. NHTSA Tire Fuel Efficiency Consumer Information Program Development: Phase 2 – Effects of Tire 

Rolling Resistance Levels on Traction, Treadwear, and Vehicle Fuel Economy 

www.nhtsa.gov/DOT/NHTSA/NVS/Vehicle%20Research%20&%20Test%20Center%20%28VRTC%29/ca/Tires/81115

4.pdf
122

on vehicle size and weight. Existing truck stopping distance requirements (49 CFR 571.121), as 

well as proposed ESC520 standards and a grant of petition for rulemaking on speed limiters

521

could create effective countermeasures for the potential degradation of braking or vehicle 

handling performance.

5.8 Aerodynamic Components

Aerodynamic technologies may conflict with applicable length and width regulations. However, 

according to FHWA regulations in 23 CFR Part 658.16 and Appendix D (see Table 5-1), 

aerodynamic devices that do not extend more than 5 feet beyond the rear of the vehicle are 

exempt from length limits, provided they do not have the strength, rigidity, or mass to damage a 

vehicle or injure a passenger and do not obscure tail lamps, turn signals, marker lamps, 

identification lamps, or any other required safety devices.522

Replacing side mirrors with cameras may conflict with applicable safety equipment regulations. 

NHTSA regulations in 49 CFR 571.111 require mirrors and do not include wording to allow 

cameras as an alternative. According to this regulation each multipurpose passenger vehicle, bus, 

or truck with a GVWR of more than 4,536 kilograms shall:

“Have outside mirrors of unit magnification, each with not less than 323 square centimeters 

of reflective surface, installed with stable supports on both sides of the vehicle. The mirrors 

shall be located so as to provide the driver a view to the rear along both sides of the vehicle 

and shall be adjustable both in the horizontal and vertical directions to view the rearward 

scene.”

5.9 Lightweight Components

The NHTSA FMVSS 121, Air Brake Systems523 regulation mandates 250-310 foot maximum 

stopping distances for heavy trucks at 60 mph. To provide more stopping power and counteract 

fade at high temperatures, brake manufacturers have increased the size and weight of their drum 

systems. However, one of several options for vehicle weight reduction is for brake systems to 

become smaller and lighter, aiding manufacturers in meeting fuel efficiency standards. Reducing 

weight and handling more work in order to meet required stopping distances are the dual forces 

driving the adoption of air-disc brakes. This does not create an actual adoption barrier but it is 

possible that future, more stringent fuel efficiency regulations could lead to potential conflict 

between these two purposes.

 520 https://www.Federalregister.gov/articles/2012/05/23/2012-12212/Federal-motor-vehicle-safety-standardselectronic-stability-control-systems-for-heavy-vehicles 521 https://www.Federalregister.gov/articles/2011/01/03/2010-33057/Federal-motor-vehicle-safety-standard-enginecontrol-module-speed-limiter-device. 522 Vaidya, U. 2012. GATE Center of Excellence at UAB for Lightweight Materials and Manufacturing for 

Automotive Technologies 

www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2012/technology_integration/ti026_vaidya_2012_p.pdf

www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2012/technology_integration/ti026_vaidya_2012_p.pdf

523 www.nhtsa.gov/DOT/NHTSA/Rulemaking/Rules/Associated%20Files/121_Stopping_Distance_FR.pdf
123

5.10 Findings and Conclusions Regarding Regulatory Barriers

There are few regulatory barriers to the adoption of FE technologies for MD/HDVs. Table 5-1

lists various categories of FE technologies and discusses the potential regulatory barriers that 

were investigated in this report. In each row a green “no” indicates that there are no documented 

regulatory barriers and a red “yes” indicates that there may be a barrier or conflict. 

Most of the potential barriers relate in some way to truck size and weight (TS&W) regulations. 

This may have implications for hybrid-electric and hybrid-hydraulic technologies, batteryelectric vehicles, idle reduction technologies, and LCVs. Size and weight regulations vary from 

State to State and the restrictions of one State may have indirect consequences on the use of FE 

technologies in surrounding States since truck operators must be compliant with regulations at 

every stage along a given route. In the case of LCVs, the Intermodal Surface Transportation 

Efficiency Act (ISTEA) preserved the heterogeneity of State regulations by enforcing a freeze 

limiting the use of the longer, heavier double- and triple-trailer combinations to those States in 

which they were already operating in 1991 (see Section 5.6).524 

However, in December 2012 the FHWA Office of Freight Management issued a notice soliciting 

bids for a new study on specific areas of Federal TS&W limits, their operation and their impacts, 

in response to a mandate in the MAP-21 legislation.525 This requirement for further study was 

an outcome of contested proposals to allow six-axle truck/trailer configurations to gross 97,000 

pounds. After further study FHWA may update the regulations on LCVs. A regulatory barrier for 

future consideration relates to the additional weight of some future (Phase 2) MD/HDV

technologies. The need for their proximity to the engine (e.g., waste heat recovery, turbocompounding) may bump tractor weights up above the 12,000 lb. front axle weight limit. This 

challenge was suggested in some DOE sponsored SuperTruck projects.

526

Another potential regulatory barrier relates to certain restrictions on the use of single wide tires. 

These limitations similarly vary from State to state. Organizations such as the ATA are working 

to resolve inconsistencies and to promote harmonized regulations across State boundaries for 

single wide tires. Also, as described in Section 5.8, replacing side mirrors with cameras may 

conflict with applicable safety equipment regulations. The NHTSA regulations in 49 CFR 

571.111 appear to require mirrors and do not include language to allow cameras as an 

alternative. 

There are a few areas where pending or proposed regulatory action may influence the adoption 

of FE technologies. For instance, NHTSA has begun a rulemaking process to consider speedlimiting devices on heavy-duty trucks, as described in Section 2.5.1.3. In addition, FMCSA will 

 524 FHWA. 2000. Comprehensive Truck Size and Weight Study Volume 2, Chapter 3 

www.fhwa.dot.gov/reports/tswstudy/Vol2-Chapter3.pdf 525 FHWA. 2012a. MAP-21 Freight Provisions 

www.fhwa.dot.gov/planning/freight_planning/talking_freight/nov282012transcript.cfm 526 See Navistar project weight per axle at 

www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2012/veh_sys_sim/vss064_jadin_2012_o.pdf
124

consider updated regulations regarding inspections of NGVs527 and has conducted research to 

identify relevant safety issues raised by stakeholders.528

Finally, OMB Circular A-119 requires Federal agencies to adopt and adapt applicable voluntary 

consensus standards, where applicable, as a cost-effective alternative to rulemaking.529 

Consensus standards by organizations such as SAE, ANSI, NFPA and others may provide 

sufficient assurance of safe operability of MDHDVs equipped with FE technologies and using 

alternative fuels.

 527 FMCSA. 2012a. Natural Gas Systems, Final Report, Kwan, Q. 2012. Safety Considerations Related to 

Commercial Vehicles Using Natural Gas www1.eere.energy.gov/cleancities/pdfs/ngvtf12_kwan.pdf 528 Transport Canada. 2009b. Summary Report - Assessment of a Heavy Truck Speed Limiter Requirement in 

Canada www.tc.gc.ca/eng/roadsafety/tp-tp14808-menu-370.htm 529 Composites World. 2007. New hybrid electric bus takes advantage of composites 

www.compositesworld.com/articles/new-hybrid-electric-bus-takes-advantage-of-composites, Office of Management 

and Budget. 1998. Circular No. A-119 Revised www.whitehouse.gov/omb/circulars_a119
125

Table 5-1: Summary of Potential Regulatory Barriers for the Adoption of MD/HDV Fuel 

Efficiency Technologies

Chapter 

index

FE technology Potential issue/conflict 

with existing 

regulations

Regulatory 

Entity

Regulation’s 

full name

Potential 

Barrier?

Description

2.8.3 Aerodynamic 

components

Aerodynamic 

technologies may 

exceed applicable 

trailer length 

regulations. 

FHWA 23 CFR Part 

658.16 - Length 

limit 

exemptions. 

(and appendix 

D to 23 CFR 

658)-

No Aerodynamic devices that do not extend more 

than 5 feet beyond the rear of the vehicle are 

exempt from length limits, provided they do 

not have the strength, rigidity, or mass to 

damage a vehicle or injure a passenger and do 

not obscure tail lamps, turn signals, marker 

lamps, identification lamps, or any other 

required safety devices.

3.2.8 Aerodynamic 

components

Replacing side mirrors 

with cameras may 

conflict with applicable 

safety equipment 

regulations

FHWA 23 CFR Part 

658.16 (and 

appendix D to 

23 CFR 658)

No FHWA: Certain devices are allowed to extend 

beyond the width limit, including rear-view 

mirrors, so switching to cameras will not alter 

the effective width limit. 

3.2.8 Aerodynamic 

components

Replacing side mirrors 

with cameras may 

conflict with applicable 

safety equipment 

regulations

NHTSA 49 CFR 571.111 Yes NHTSA: Regulation 571 appears to require 

mirrors and does not include wording to allow 

cameras as an alternative.

2.2.2.3 Biodiesel None No

2.2.1.3, 

5.2.1.1

CNG None No

3.2.2 Hybrid/electric MDHD vehicles may not 

be able to travel fast 

enough to meet 

minimum speed limits 

on high grade inclines. 

State No There are no Federal regulations establishing 

minimum speed limits. Each State regulates 

this independently. E-truck routes have been 

voluntarily “certified” by fleet managers for 

safe operation.

3.2.2 Hybrid/hydraulic Excess weight added by 

a series HH drivetrain 

may exceed applicable 

vehicle weight 

regulations.

FHWA 23 CFR 658.17, 

23 CFR 658.23

Yes National weight standards apply to all 

commercial vehicle operations on the 

Interstate Highway System. ISTEA froze the 

weights of truck tractors with two or more 

trailing units operating above 80,000 pounds 

on the Interstate System at the weight limits 

actually and lawfully in effect for such vehicles 

in a State on June 1, 1991. FHWA issued a final

rule on June 13, 1994. 

3.2.2 Hybrid/hydraulic Excess weight added by 

a series HH drivetrain 

may exceed applicable 

vehicle weight 

regulations.

State Colorado 

Revised 

Statutes 25-7-

106.8 and 42-

4-508

Yes Only one state, Colorado, provides a 1,000 lb. 

vehicle weight exemption for hybrid vehicles 

off the Interstate system.

2.5.2.3, 

3.2.5

Idle reduction 

technologies

Excess weight from idle 

reduction technology 

may exceed applicable 

vehicle weight 

regulations.

FHWA Federal: Public 

Law 112-141 

(MAP-21) and 

23 U.S. Code 

127(a)(12)

No Idle Reduction Technology Weight Exemption: 

States may (but are not required to) allow 

heavy-duty vehicles equipped with idle 

reduction technology to exceed the maximum 

gross vehicle weight limit and the axle weight 

limit by up to 550 pounds (lbs.) to compensate 

for the additional weight of the idle reduction 

technology. 
126

Chapter 

index

FE technology Potential issue/conflict 

with existing 

regulations

Regulatory 

Entity

Regulation’s 

full name

Potential 

Barrier?

Description

2.5.2.3, 

3.2.5

Idle reduction 

technologies

Extra weight from 

incorporation of idle 

reduction technology 

may exceed applicable 

vehicle weight 

regulations.

State Yes Most States allow heavy-duty vehicles 

equipped with idle reduction technology to 

exceed the maximum gross vehicle weight limit 

and the axle weight limit by up to 550 pounds 

(lbs.) to compensate for the additional weight 

of the idle reduction technology. However, six 

States (California, Hawaii, Kentucky, North 

Carolina, Rhode Island, Tennessee) and the 

District of Columbia do not allow weight 

exemptions. Most States and many cities limit 

the amount of time that MD/HDVs can idle, 

although 49-State and local regulations do not 

specifically address anti-idling technology. 

2.4.1.3 ITS/telematics None No

3.2.9, 

4.2.3

Lightweighting

materials

Stopping distance 

regulations could 

conflict with the desire 

to decrease weight

NHTSA FMVSS 121: Air 

Brake Systems

49 CFR Part 

571 

Yes NHTSA FMVSS 121 mandates 250-310 foot 

maximum stopping distances for heavy trucks 

at 60 mph. To provide more stopping power 

and counteract fade at high temperatures, 

brake makers have increased the size and 

weight of their drum systems. However, these 

design changes may decrease fuel efficiency 

instead of aiding OEMs in meeting the new 

GHG-FE rules. There is now greater demand for 

brake systems that are smaller and lighter yet 

still handle more work; this is driving the 

adoption of air-disc brakes.

2.2.1.4 LNG None No

2.6.3 Longer 

combination 

vehicles

Longer combination 

vehicles may exceed 

applicable vehicle 

length regulations.

FHWA 23 CFR 658.9, 

23 CFR 658.11, 

and Appendix 

A to 23 CFR 

Part 658

Yes ISTEA imposed freezes on: 1) on the maximum 

weight of LCVs, which consist of any 

combination of a truck tractor and two or more 

trailers or semitrailers which operate on the 

Interstate System at a gross weight over 80,000 

pounds; and (2) on the overall length of the 

cargo carrying units of combination vehicles 

with two or more such units where one or both 

exceed 28.5 feet in length on the National 

Network. The maximum weight of LCVs and 

the maximum length of the cargo carrying units 

of combination vehicles is the weight or length 

in actual and legal operation in a State on June 

1, 1991. Therefore, except for within the 

grandfathered States, LCVs cannot operate on 

Interstate highways. 

There is a patchwork of national (FHWA), 

international (Canamex), and State-based LCV 

restrictions. Harmonized operational 

restrictions, including route designations for 

LCVs, should be considered. 

2.6.3 Longer 

combination 

vehicles

Longer combination 

vehicles may exceed 

applicable vehicle 

length regulations.

State Yes Some States do not allow LCVs (See map 

included in chapter 6 text).
127

Chapter 

index

FE technology Potential issue/conflict 

with existing 

regulations

Regulatory 

Entity

Regulation’s 

full name

Potential 

Barrier?

Description

2.6.3, 

4.2.5

Longer 

combination 

vehicles

Longer combination 

vehicles may exceed 

applicable vehicle 

weight regulations.

FHWA 23 CFR 658.23 

LCV freeze 

(ISTEA freeze), 

including 

Appendix C 

(grandfathered 

state-wise LCV 

limits)

Yes ISTEA imposed freezes on: 1) on the maximum 

weight of LCVs, which consist of any 

combination of a truck tractor and two or more 

trailers or semitrailers which operate on the 

Interstate System at a gross weight over 80,000 

pounds; and (2) on the overall length of the 

cargo carrying units of combination vehicles 

with two or more such units where one or both 

exceed 28.5 feet in length on the National 

Network. The maximum weight of LCVs and 

the maximum length of the cargo carrying units 

of combination vehicles is the weight or length 

in actual and legal operation in a State on June 

1, 1991. 

2.7.3.3, 

4.2.4, 

3.2.7

Low rolling 

resistance tires

Low rolling resistance 

tires may conflict with 

stopping distance 

regulations.

NHTSA 49 CFR Part 

575 FMVSS 121 

- Air Brake

Systems -

Trucks, Buses, 

and Trailers 

No ABS has been required by NHTSA on all trucks 

and buses since 1997-1999 per FMVSS 121. The 

average age of Class 8 tractors, which stand to 

benefit the most from LRR tires, is 

approximately 9 years. FMVSS 121 also 

specifies maximum stopping distance for air 

brake systems on MDHDVs, regardless of tires. 

Potential countermeasures to degradation of 

braking or vehicle handling performance 

include electronic stability control and speed 

limiter implementation.

2.7.3.3, 

5.2.4, 

3.2.7

Low rolling 

resistance tires

Low rolling resistance 

tires may conflict with 

stopping distance 

regulations.

FMCSA 49 CFR 

393.52(a)(3) 

FMCSA brake 

performance.

No FMCSA regulates the maximum braking 

distance at 20 mph for GVW greater than 

10,000 lbs. Potential countermeasures to 

degradation of braking or vehicle handling 

performance include electronic stability control 

and speed limiter implementation. 

2.7.2.3 Single-wide tires Inch-width pavement 

weight laws may 

preclude the use of 

wide based tires on 

non-tandem axle 

combination trucks. 

FHWA No National weight standards apply to commercial 

vehicle operations on the Interstate Highway 

System, an approximately 40,000-mile system 

of limited access, divided highways that spans 

the Nation. Off the Interstate Highway System, 

States may set their own commercial vehicle 

weight standards.

2.7.2.3 Single wide tires State inch-width 

pavement weight laws 

may preclude the use of 

wide based tires on 

non-tandem axle 

combination trucks. 

State Yes Single wide tires meet the inch width-weight 

limits for all States, but are restricted in certain 

States to 17,500 lbs. on a single axle at 500 

lbs./inch width limit, and are disallowed on 

single axle positions on

certain double and triple combination vehicles 

(from a white paper developed by the 

American Trucking Association). The American 

Trucking Association has a spreadsheet 

available of the latest regulations for some 

States that limit “single tire” use. Work is 

ongoing with these States to clarify regulations 

and resolve the limitations.
128

Chapter 

index

FE technology Potential issue/conflict 

with existing 

regulations

Regulatory 

Entity

Regulation’s 

full name

Potential 

Barrier?

Description

2.5.1.3, 

3.2.4

Speed limiters None No NHTSA plans to publish a notice of proposed 

rulemaking in late 2013.530

2.7.1.3 TPMS/ATIS None NHTSA 49 CFR Parts 

571.138 and 

585: FMVSS 

138 Tire 

Pressure 

Monitoring 

Systems 

No NHTSA: TPMS is required for new vehicles 

under 10,000 lbs., and it must include a 

warning signal to the operator when the 

pressure is outside a certain pressure range. 

MDHDVs are not covered by the regulation. 

 530 See “Truck Speed Limiter Proposal Returned to NHTSA,” www.truckinginfo.com/channel/safetycompliance/news/story/2013/05/truck-speed-limiter-proposal-returned-to-nhtsa.aspx?prestitial=1
129

6 CONCLUSIONS AND RECOMMENDATIONS

The relevant safety findings from the literature review, summarized in Chapter 2, were 

complemented by inputs from focused interviews with experienced SMEs regarding the rapidly 

expanding use of alternative fuels, as discussed in Chapter 3. A cross-cutting measure for the 

safe deployment of new FE technologies and fuels is to provide improved CMV driver training. 

Safety concerns identified from the literature review and SMEs’ inputs can be prevented or 

mitigated by complying with applicable regulations, and safety standards and industry best 

practices and can be addressed by evolving ITS technologies and collision prevention devices. 

The key finding from this study is that there are no major safety hazards preventing the adoption 

of FE technologies or the increased use of alternative fuels and vehicle electrification by the

Nation’s MD/HDV fleet in order to achieve environmental and economic benefits while

complying with NHTS/EPA regulations. 

Chapter 2 safety-relevant fuel- or technology-specific specific findings include:

• Both CNG- and LNG-powered vehicles present potential hazards, and call for wellknown engineering and process controls to assure safe operability and crashworthiness. 

However, based on the reported incident rates of NGVs and the experiences of adopting 

fleets, it appears that NGVs can be operated at least as safely as diesel MD/HDVs. Using 

natural gas instead of diesel fuel may help fleets comply with the MD/HD greenhouse gas 

rules that require up to 20 percent emissions reduction by 2018. 

• There are no safety contraindications to the large scale fleet adoption of CNG or LNG 

fueled heavy-duty trucks and buses, and there is ample experience with the safe operation 

of large public transit fleets. Voluntary industry standards and best practices suffice for 

safety assurance, though improved training of CMV operators and maintenance staff is 

needed to ensure natural gas safety of equipment and operating procedures. Sound 

design, manufacture and inspection of natural gas storage tanks will further reduce the 

potential for leaks, tank ruptures, fires, and explosions.

• Biodiesel blends used as drop-in fuels have presented some operational safety concerns

depending on blending fraction, such as material compatibility, bio-fouling sludge 

accumulation, or cold-weather gelling. However, best practices for biodiesel storage, and 

improved gaskets and seals that are biodiesel resistant, combined with regular 

maintenance and leak inspection schedule for the fuel lines and components enable the 

safe use of biodiesel in newer MD/HDVs.

• Propane (LPG, or autogas) presents well-known hazards due to overpressure in tank 

overfill, and unintended ignition (due to leaks or a crash) that are preventable by using

Overfill Prevention Devices (OPD) that supplement the automatic stop-fill system on the 

fueling station side, and pressure release devices (PRD). Established best practices and 

safety codes (e.g. NFPA) have proven that propane fueled MD/HDVs can be as 

operationally safe as the conventionally-fueled counterparts. 

• As the market penetration of hybrid and electric drivetrain accelerates, and the capacity 

and reliability of lithium ion batteries used in Rechargeable Energy Storage Systems 

improve, associated potential safety hazards (e.g., electrocution and stranded energy, 

thermal runaway leading to battery fire) have become well understood, preventable and 

manageable. Existing and emerging industry technical and safety voluntary standards,

compliance with applicable NHTSA regulations and guidance, as well as growing 
130

experience with the operation of hybrid and electric MD/HDVs will enable the largescale adoption of safer and more efficient power-train electrification technologies. 

Key safety findings from the literature review pertaining to the specific FE technologies 

implemented to date in the MD/HDV fleet include:

• Telematics, integrating on-board ITS sensors, video and audio alerts for MD/HDV 

drivers, offer potential improvements in both driver safety performance and fuel 

efficiency. Both camera and non-camera based telematics setups are currently integrated 

with available crash avoidance systems (such as ESC, RSC, LDWS, etc.) and appear to 

be well accepted by MD/HDV fleet drivers. 

• Both experience abroad and the cited U.S. studies of trucks equipped with active speed 

limiters indicated a safety benefit, as measured by up to 50 percent reduced crash rates, in 

addition to fuel savings and other benefits, with good CMV driver acceptance. Any 

negative aspects were small and avoidable if all the speed limitation devices were set to 

the same speed, so there would be less need for overtaking at highway speeds.

• No literature reports of adverse safety impacts were found regarding implementation of 

on-board idle-reduction technologies in MD/HDVs (such as automatic start-stop, directfired heaters, and APUs).

• There was no clear consensus from the literature regarding the relative crash rates and 

highway safety impacts of LCVs, due to lack of sufficient data and controls and

inconsistent study methodologies. Recent safety evaluations of LCVs and ongoing MAP21 mandated studies will clarify and quantify this issue.

• Tire technologies for FE (including ATIS, TPMS, LRR and single-wide tires) literature 

raised potential safety concerns regarding lower stability or loss of control, e.g., when 

tires pressure is uneven or a single wide tire blows out on the highway. However, systems 

such as automated tire monitoring systems and stability enhancing electronic systems

(ABS, ESC, RSC) promise to compensate and mitigate any adverse safety impacts. 

• Aerodynamic technologies that offer significant fuel savings have raised potential 

concerns about vehicle damage or injury in case of detached fairings or skirts, although 

there were no documented incidents of this type in the literature. Conversely, there is a 

potential safety benefit from skirts that also function as side underride guards to protect 

VRUs in multimodal operating environments.

• The increasing use of light weighting materials may pose some safety and 

crashworthiness hazards, depending on their performance in structural or other vehicle 

subsystem applications (chassis, power-train, crash box or safety cage). Some composites

(fiberglass, plastics, CFRC, foams) may become brittle on impact or due to weathering 

from UV and extreme cold, or consumed by fires. No examples of such lightweight 

material failures on MD/HDVs were identified in the literature. Industry has developed 

advanced, high performance lightweight material options tailored to their automotive 

application, e.g., thermoplastics resistant to UV and weathering. 

Chapter 3 summarized the safety inputs offered by a representative group of SMEs regarding 

fleet adoption of FE technologies and alternative fuels. SMEs indicated that there was a learning 

curve to the process: design and engineer novel subsystems; integrate them safely in the vehicle;

inspect, maintain and refuel fleet vehicles for safe operability; and train staff to safely operate a 

fleet of such new MD/HDVs. Although some SMEs raised specific safety concerns, their
131

experience demonstrates that system- or fuel-specific hazards can be prevented or mitigated by 

observing applicable industry standards, and by training managers, operators and maintenance 

staff in safety best practices.

Specific safety insights mentioned by the SMEs, based on their experience included:

• Alternative fuels did not raise major safety concerns but generally required better 

education and training of maintenance staff and operators. There was anecdotal safety 

concern regarding high pressure (4,000 psi) CNG cylinders that could potentially explode

in a crash scenario or if otherwise ruptured. However, aging CNG fuel tank safety can be 

assured by enforcing regulations such as FMVSS 304 and by periodic tank inspections,

end-of-life disposal and replacement. A propane truck fleet manager found the fuel to be 

as safe as or safer than gasoline, and reported no safety issues with the company’s 

propane, nor with hybrid gasoline-electric trucks. 

• OEMs of drivetrain hybridization and electrification systems, including batteries for the

rechargeable energy storage system indicated that they undergo multiple safety tests and

are designed with fail-safes for various misuse and abuse scenarios. A potential safety 

risk was the integration of hybrid components downstream by bodybuilders in retrofits, 

as opposed to new vehicles. Another potential safety concern raised was the uncertain 

battery lifetime due to variability of climate, operation duty-cycles and aging

degradation. Without state-of-charge indicators, vehicles could become underpowered or 

stranded if the battery degrades and is not serviced or replaced in a timely manner. 

• ITS and telematics raised no safety concerns; on the contrary, fleet managers stated that

“efficient drivers are safer drivers.” Monitoring and recording of driver behavior, 

combined with coaching, appeared to reduce distracted and aggressive driving and 

provided significant FE and safety benefits. 

• Tire technologies: Wide base tires safety concerns stemmed from the decrease in tire 

redundancy in case of a tire blowout at highway speeds. In the case of LRR, a concern 

was that these tires could negatively affect truck stopping distance and stability control.

• Speed-limiter safety concerns could be encountered when trucks pass other vehicles on 

the highway instead of staying in the right-hand lane behind other vehicles. However, by 

combining speed limiters with driver training programs the truck fleet safety performance 

could actually improve. 

• Aerodynamic systems safety to date was satisfactory, with no instances of on-road 

detaching of components. However, covering the underside or other safety-critical 

components with aerodynamic fairings can make them harder to inspect, (e.g., worn lugs,

CNG relief valve shrouds, wheel covers, and certain fairings). Drivers and inspectors 

need to be able to see through wheel covers and to be able to access lug nuts through 

them. These covers must also be durable to withstand frequent road abuse. 

• For lightweighting materials, safety concerns raised were potentially lower

crashworthiness (debonding, or brittle fracture in impact), and potential for decreased 

survivability in vehicle fires depending on the specific composite or other material choice

and its application. 

The scenario-based deterministic hazard analysis presented in Chapter 4 reflected not only 

literature safety findings and SME’s safety concerns, but also reported truck or bus mishaps that 

have occurred. Key hazard analysis scenarios included: CNG-fueled truck and bus vehicle fire or 
132

explosion due to CNG tank rupture, when pressurized fuel tanks degraded due to aging or PRDs 

failed; LNG truck crashes leading to fires, or LNG refueling-related mishaps; the flammability or 

brittle fracture issues related to lightweighting materials in crashes; reduced safety performance 

for either LRR or wide-base tires; highway pile-ups when LCVs attempt to pass at highways 

speeds; aerodynamic components detaching while the vehicle traveled on a busy highway or 

urban roadway; and fires resulting to overheated lithium ion batteries in electric or hybrid buses. 

All these hypothetical worst case scenarios were shown to be preventable and possible to 

mitigate by observing safety regulations and voluntary standards, or with engineering and 

operational best practices. 
133

7 APPENDICES

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134

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135

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Gehm, R. 2012a. Greater connectivity coming to heavy trucks www.sae.org/mags/tbe/10927/

Gehm, R. 2012b. Putting electrification to work http://ev.sae.org/article/10917/

Gentry, W. 2012. Statement of William Gentry, President, Gentry Trailways and on behalf of 

American Bus Association and United Motorcoach Association Subcommittee on 

Highways and Transit, House Transportation and infrastructure Committee, U.S. House 

of Representatives 

http://republicans.transportation.house.gov/Media/file/TestimonyHighways/2012-09-13-

Gentry.pdf

Grant, C. 2012. Emergency responder related electric vehicle research 

www.sae.org/events/nevss/summit/2012_EV_Safety_Summit_grant.pdf

GreenRoad. 2012. http://greenroad.com/

Grenzeback, L. R. 2011. Freight Transportation Challenge: More Trucks, More Fuel, More 

GHGs? 

http://its.ucdavis.edu/events/outreachevents/asilomar2011/presentations2/session%205/G

renzeback_Asilomar2011_Freight_v2.pdf

Greszler, A. 2009. Technologies and Policies for Improving Truck Fuel Efficiency & Reducing 

CO2 www.aceee.org/files/pdf/conferences/30th/transport/Greszler.pdf

Greszler, A. 2011. Heavy-Duty Truck Technologies for Reducing Fuel Consumption and GHG 

Emissions 
136

http://its.ucdavis.edu/events/outreachevents/asilomar2011/presentations2/session%205/G

renszler_Asilomar%202011%20Volvo.pdf

Hollick, P. 2012. Sustainability: Green fleets, Squaring the environmental circle www.busex.com/article/sustainability-green-fleets

House Transportation and Infrastructure Committee. 2012a. Duncan holds hearing focused on 

improving truck and bus safety program 

http://transportation.house.gov/News/PRArticle.aspx?NewsID=1724

House Transportation and Infrastructure Committee. 2012b. Subcommittee on Highways and 

Transit Hearing on “Evaluating the Effectiveness of DOT's Truck and Bus Safety 

Program” http://democrats.transportation.house.gov/hearing/subcommittee-highwaysand-transit-hearing-%E2%80%9Cevaluating-effectiveness-dots-truck-and-bus

IIHS. Speed Limiters in Trucks Would Serve 2 Purposes 

www.iihs.org/externaldata/srdata/docs/sr4508.pdf

Industrial Technology. Festo picked for groundbreaking fifthwheel development 

www.industrialtechnology.co.uk/index.php?pageId=47&aid=490

Integrys Trillium CNG. 2012. CNG gaseous fuels: a successful alternative 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26500

Johnston, J. 2007. Comments of the owner-operator independent drivers association, inc in 

response to notice and request for comments Motor Vehicle and Carrier Safety Standards 

[Docket No. NHTSA-2007-26851] 

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nts%20final%203-27-07.pdf

JOST. 2011. How is lack of maintenance to the fifth wheel affecting your bottom line? 

www.jost.co.za/bulletins/24-3-2011-10835-07-

2010%20jost%20jsk37c%20%20standard%20ajustment%20procedure..pdf

Kenney & Conley PC. 2011. NHTSA Focusing on Speed-Limiter Rule 

http://knowledgebase.findlaw.com/kb/2011/Oct/361377.html

Koch, J. 2012. Technologies for increasing fuel efficiency and eliminated fuel waste 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26464

Lay, F. 2012. We have a line of sight to a 50% gallons reduction by 2020 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26434

Lockridge, D. 2010. FMCSA talks safety technology www.truckinginfo.com/news/newsdetail.asp?news_id=70422

Ly, H. 2011. Closing the Safety Loop Secures NGV Fleet Investment 

www.ngvglobal.com/closing-the-safety-loop-secures-ngv-fleet-investment-0309

McCartt, A. T. 2007. IIHS Letter re: Motor Vehicle and Carrier Safety Standards; Devices to 

Limit the Speed of Certain Trucks Request for Comments; Docket No. NHTSA-2007-

26851 www.iihs.org/laws/comments/pdf/nhtsa_fmcsa_ds_atm_032707.pdf

Menzies, J. 2013. The future potential of truck telematics www.trucknews.com/news/the-futurepotential-of-truck-telematics/1002000560

Miller, E. 2013. Speed, Lack of Seat Belts Cited By U.S. in Truck Fatality Spike 

www.ttnews.com/gateclient/logonPC.aspx?storyid=31050&t=Speed-Lack-of-Seat-BeltsCited-By-U.S.-in-Truck-Fatality-Spike

Morgenstern, M. 2012. Propane tank explodes in McDonald's drive-through 

www.theblaze.com/stories/propane-tank-explodes-in-mcdonalds-drive-through/
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Mugno, S. 2012. Written Statement of American Trucking Associations, Inc. Before the 

Committee on Transportation and Infrastructure Subcommittee on Highways and Transit 

U.S. House of Representatives Hearing "Evaluating the effectivenes of DOT's truck and 

bus safety program" 

www.trucking.org/Newsroom/Testimony1/FINAL%20ATA%20Testimony%20CSA%20

Sept%202012.pdf

National Shippers Strategic Transportation Council. 2012. Competing Bills on Truck Size and 

Weight Limits http://freightadvocacy.org/key-issues/truck-size-and-weight-limits

NGV Journal. 2011. National NGV Fleet Forum forms in the United States 

www.ngvjournal.com/en/vehicles/item/5048-national-ngv-fleet-forum-forms-in-theunited-states

NHTSA. 2008a. National Motor Vehicle Crash

Causation Survey: Databook of NMVCCS Variables www-nrd.nhtsa.dot.gov/Pubs/811052.pdf

NHTSA. 2008b. National Motor Vehicle

Crash Causation Survey www-nrd.nhtsa.dot.gov/Pubs/811059.PDF

NHTSA. 2010. Grant of petition for rulemaking: Federal Motor Vehicle Safety Standard; Engine 

Control Module Speed Limiter Device http://origin.www.gpo.gov/fdsys/pkg/FR-2011-

01-03/html/2010-33057.htm

NHTSA. 2011. Fatality Analysis Reporting System (FARS) Analytical Users Manual 1975-2010 

www-nrd.nhtsa.dot.gov/Pubs/811529.pdf

North American Council for Freight Efficiency. 2010. Executive Report – Wide Base Tires 

http://nacfe.org/LZdmQ/wp-content/uploads/2010/12/NACFE-ER-1002-Wide-BaseTires-Dec-2010.pdf

North American Council for Freight Efficiency. 2011. Speed limiters save money and fuel 

without significant productivity loss http://nacfe.org/wpcontent/uploads/2011/04/NACFE-ER-1003-Speed-Limiters-Mar-2011.pdf

Owings, S. 2012. Statement of Steve Owings, father of Cullum Owings, killed in a truck crash

involving an inattentive and speeding trucker and president and co-founder, Road Safe 

Amercica (RSA) before the Subcommittee on Highways and Transit, Committee on 

Transportation and Infrastructure, U.S. House of Representatives 

http://republicans.transportation.house.gov/Media/file/TestimonyHighways/2012-09-13-

Owings.pdf

Owner-Operator Independent Drivers Association. 2010. Comments of the owner-operator 

independent drivers association, inc in response to a notice of intent, request for scoping 

comments [Docket No. NHTSA-2010-0079] 

www.ooida.com/Issues&Actions/Regulatory/issues/docs/U.S.DOTNHTSA%20fuel%20mileage%20comments.pdf

Palmer, D. L. 2012. Statement of Assistant Chief David L. Palmer President of Commercial 

Vehicle Safety Alliance before the Highways and Transit Subcommittee of the House 

Transportation and Infrastructure Committee on "Evaluating the effectivenes of DOT's 

truck and bus safety program" 

http://republicans.transportation.house.gov/Media/file/TestimonyHighways/2012-09-13-

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138

Parish, R., Chen, P., & Gallo, J.-B. 2012. How drive cycle affects hybrid fuel economy and 

vehicle cost effectiveness 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26444

Patton, O. 2011. NHTSA clears path for speed-limiter proposal 

www.truckinginfo.com/news/news-detail.asp?news_id=72581

Purchase, E. 2011. Trucking Speed Governors: PA Semi Accident Lawyers on NHTSA Study 

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Renaud, P. S., & Coleman, D. 2012. Telematics: real-time fleet management and improved fuel 

economy www.ntea.com/worktruckshow/index.aspx?id=25469

Road Safe America. 2012. The Problem: Physics 

www.roadsafeamerica.org/docs/RSASpeedLimiterWhitePaper_0039.pdf

Rocky Mountain Institute. RMI overview of Walmart Truck Fleet http://move.rmi.org/marketsin-motion/case-studies/trucking/wal-mart-s-truck-fleet.html

Samba, D., Park, B. B., & Gardner, B. 2011. Evaluation of Large-Truck Transportation 

Alternatives with Safety, Mobility, Energy, and Emissions Analysis 

http://trid.trb.org/view.aspx?id=1092002

SmartDrive Systems. 2012. SmartDrive www.smartdrive.net/

Technical Advisory Service for Attorneys. 2010. Trucks and Truck Accidents: Considerations 

for Litigators 

www.tasanet.com/knowledgeCenterDetails.aspx?docTypeID=1&docCatID=4&docID=5

5

Toledo, T., Albert, G., & Hakkert, S. 2007. A Simulation-Based Evaluation of the Impact of 

Active Speed Limiters on Traffic Flow and Safety 

http://pubsindex.trb.org/view.aspx?id=801530

Transport Canada. 2009. Summary Report - Assessment of a Heavy Truck Speed Limiter 

Requirement in Canada www.tc.gc.ca/eng/roadsafety/tp-tp14808-menu-370.htm

Transport Topics. 2011. NHTSA to Propose Truck Speed Limiters 

www.ttnews.com/articles/basetemplate.aspx?storyid=25848

Trucker News Services. 2012. Speed limiter report: Trucks with devices had 50% lower ‘speedlimiter relevant’ crash rate 

www.thetrucker.com/News/Stories/2012/3/27/SpeedlimiterreportTruckswithdeviceshad5

0lowerspeed-limiterrelevantcrashrate.aspx

TruckingInfo. 2012. Survey suggests 'connected fleet' is becoming the norm 

www.truckinginfo.com/news/news-detail.asp?news_id=76592

Tsao, H.-S. J. 2009. Drastic Productivity Gain for Large-Truck Operations with Automated 

Trailer Steering 

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Vertical Alliance Group Inc. 2012. CSA 2010 BASIC Definitions and Data Sources for Carriers 

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Vise, A. 2010. NHTSA to consider speed limiter rule in 2012 www.ccjdigital.com/nhtsa-plansto-consider-speed-limiter-rule-in-2012/

Walkowicz, K. 2012. Medium and Heavy-Duty Vehicle Evaluations 

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Wall Street Journal. 2012. New York’s Schumer: GPS Guides Trucks into Crashes 

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Walmart Trucking Fleet Overview. www.walmartstores.com/Sustainability/7674.aspx

Whistler, D. 2012. Researchers sift through crash statistics 

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7.2 Regulatory Bibliography

American Transportation Research Institute. 2012. Compendium of Idling Regulations 

www.atri-online.org/research/idling/ATRI_Idling_Cab_Card.pdf

California Air Resources Board. 2009. Heavy-Duty Vehicle Idling Emission Reduction Program 

www.arb.ca.gov/msprog/truck-idling/truck-idling.htm

California Air Resources Board. 2012a. Facts About Tractor-Trailer Greenhouse Gas Regulation 

Reducing Climate Change Emissions from Tractor-Trailers 

www.arb.ca.gov/cc/hdghg/fact_sheets/HDGHG_Genl_Fact_Sheet.pdf

California Air Resources Board. 2012b. Implementation Guidance for the Tractor-Trailer GHG 

Regulation www.arb.ca.gov/cc/hdghg/documents/modaeroguidev1.pdf

California Department of Transportation. 2012. Truck Route Classifications in California 

www.dot.ca.gov/hq/traffops/trucks/routes/truck-routes.htm

Casey, C. 2012. Standards Update for Commercial Vehicles 

www.sae.org/events/nevss/summit/2012_EV_Safety_Summit_casey.pdf

Chart. 2009. Codes, Standards and Advisories Applicable to Natural Gas Vehicles 

www.nexgenfueling.com/t_codes.html

Clift, T. 2012. Chrysler, Ford Join Toyota in Opposing Mexico's Fuel Economy, Emissions 

Proposal 

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DOE Alternative Fuels Data Center. 2013. Laws and Incentives 

www.afdc.energy.gov/laws/search

DOE EERE. 2006. Fact #411: February 13, 2006 States that Allow Longer Combination 

Vehicles www1.eere.energy.gov/vehiclesandfuels/facts/2006_fcvt_fotw411.html

DOE EERE. 2013a. Federal Laws and Incentives for Idle Reduction 

www.afdc.energy.gov/fuels/laws/3259/U.S.

DOE EERE. 2013b. IdleBase http://cleancities.energy.gov/idlebase

EPA. 2012. EPA and NHTSA Joint MD/HD Planning Meetings

FHWA. 2000a. Comprehensive Truck Size and Weight Study Volume 2, Chapter 2 

www.fhwa.dot.gov/reports/tswstudy/Vol2-Chapter2.pdf

FHWA. 2000b. Comprehensive Truck Size and Weight Study Volume 2, Chapter 3 

www.fhwa.dot.gov/reports/tswstudy/Vol2-Chapter3.pdf

FHWA. 2003. Commercial Vehicle Size and Weight Program 

www.ops.fhwa.dot.gov/freight/sw/overview/index.htm
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FHWA. 2004a. Federal Size Regulations for Commercial Motor Vehicles 

http://ops.fhwa.dot.gov/freight/publications/size_regs_final_rpt/size_regs_final_rpt.pdf

FHWA. 2004b. Questions and Answers About Vehicle Size and Weight 

www.ops.fhwa.dot.gov/freight/sw/faqs/index.cfm

FHWA. 2005. Aerodynamic Device Compliance with 23 CFR 658.16(b) (4) 

www.ops.fhwa.dot.gov/freight/sw/aerodevice23cfr65816.htm

FHWA. 2010. Freight Facts and Figures 2010 

http://ops.fhwa.dot.gov/freight/freight_analysis/nat_freight_stats/docs/10factsfigures/figu

re3_4.htm

FHWA. 2012. MAP-21 Freight Provisions 

www.fhwa.dot.gov/planning/freight_planning/talking_freight/nov282012transcript.cfm

FMCSA. 2002. DOT Sets Safety Requirements For Mexican Trucks, Buses in United States 

www.fmcsa.dot.gov/about/news/news-releases/2002/031402.htm

FMCSA. 2007. NAFTA Demonstration Project www.fmcsa.dot.gov/intl-programs/naftarules/new-mexrule.htm

FMCSA. 2012. Research on the Safety Impacts of Speed Limiter Device Installations on 

Commercial Motor Vehicles: Phase II www.fmcsa.dot.gov/facts-research/researchtechnology/report/Speed-Limiters.pdf

Frittelli, J. 2012. Federal Freight Policy: An Overview www.fas.org/sgp/crs/misc//R42764.pdf

Fuetsch, M. 2013. U.S. could spend $340 million for natural gas tax incentives 

http://trid.trb.org/view.aspx?id=1245149

Goodyear Truck Tires. 2013. Standards & Regulations 

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Hosier, F. 2009. Compressed natural gas explosion injures workers; what safety regulations 

apply? www.safetynewsalert.com/compressed-natural-gas-explosion-injures-three-whatosha-rules-apply/

Insurance Institute for Highway Safety. 2013. State Speed Limits 

www.iihs.org/laws/speedlimits.aspx

Jacobs, R. 2012. Megabus faces challenge over weight limits 

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Kilcarr, S. 2012. Weighing in on disc brakes www.equities.com/news/headline-story?dt=2012-

11-08&val=691808&cat=tech

Knight, C. 2010. What Happens When Your CNG Tanks Expire? www.governmentfleet.com/article/print/story/2010/05/what-happens-when-your-cng-tanks-expire.aspx

Kwan, Q. 2012. Safety Considerations Related to Commercial Vehicles Using Natural Gas 

www1.eere.energy.gov/cleancities/pdfs/ngvtf12_kwan.pdf

Mexico Trucker Online. 2002. NAFTA - U.S.DOT Regulations 

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National Research Council. 1990. Appendix D Recommendations from Truck Weight Limits 

Study http://nap.edu/openbook.php?record_id=10382&page=256

National Shippers Strategic Transportation Council. 2012. Competing Bills on Truck Size and 

Weight Limits http://freightadvocacy.org/key-issues/truck-size-and-weight-limits

NHTSA. 1995. FMVSS 303: Fuel System Integrity of Compressed Natural Gas (CNG) Vehicles 

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NHTSA. 2004. FMVSS 138: Tire Pressure Monitoring Systems; Controls and Displays. 49 CFR 

Part 571. 

www.nhtsa.gov/DOT/NHTSA/Rulemaking/Rules/Associated%20Files/TPMSfinalrule_6.

pdf

NHTSA. 2007a. 49 CFR Parts 571 & 585 

www.nhtsa.gov/DOT/NHTSA/Rulemaking/Rules/Associated%20Files/ESC_FR_03_200

7.pdf

NHTSA. 2007b. Laboratory test procedure for FMVSS No. 139 New Pneumatic Radial Tires for 

Light Vehicles 

www.nhtsa.gov/DOT/NHTSA/Vehicle%20Safety/Test%20Procedures/Associated%20Fil

es/TP-139-03.pdf

NHTSA. 2009a. Compressed Hydrogen Cylinder Research and Testing In Accordance With 

FMVSS 304 

www.nhtsa.gov/DOT/NHTSA/NVS/Crashworthiness/Alternative%20Energy%20Vehicle

%20Systems%20Safety%20Research/811150.pdf

NHTSA. 2009b. FMVSS 121: 49 CFR Part 571 Air Brake Systems 

www.nhtsa.gov/DOT/NHTSA/Rulemaking/Rules/Associated%20Files/121_Stopping_Di

stance_FR.pdf

NHTSA. 2010. Grant of petition for rulemaking: Federal Motor Vehicle Safety Standard; Engine 

Control Module Speed Limiter Device http://origin.www.gpo.gov/fdsys/pkg/FR-2011-

01-03/html/2010-33057.htm

NHTSA. 2011. FMVSS 305: Federal Motor Vehicle Safety Standards; Electric-Powered 

Vehicles; Electrolyte Spillage and Electrical Shock Protection 

www.nhtsa.gov/DOT/NHTSA/Vehicle%20Safety/Test%20Procedures/Associated%20Fil

es/TP-305-01.pdf

NHTSA. 2012. Notice of proposed rulemaking: Electronic Stability Control Systems for Heavy 

Vehicles www.nhtsa.gov/staticfiles/nvs/pdf/136_Hvy_Veh_ESC_NPRM.pdf

NHTSA. 2013. Tire Regulations www.nhtsa.gov/Laws+&+Regulations/Tires

NHTSA and EPA. 2011. Greenhouse Gas Emissions Standards and Fuel Efficiency Standards 

for Medium- and Heavy-Duty Engines and Vehicles; Final Rule 

www.gpo.gov/fdsys/pkg/FR-2011-09-15/html/2011-20740.htm

NREL. 2010a. Electric Vehicle and Infrastructure Codes and Standards Citations 

www.afdc.energy.gov/pdfs/48605.pdf

NREL. 2010b. Natural Gas Vehicle and Infrastructure Codes and Standards Citations 

www.afdc.energy.gov/pdfs/48611.pdf

Office of Management and Budget. 1998. Circular No. A-119 Revised 

www.whitehouse.gov/omb/circulars_a119

Raj, P. K., Hathaway, W. T., & Kangas, R. 1996. Design Guidelines for Bus Transit Systems 

Using Compressed Natural Gas as an Alternative Fuel 

http://trid.trb.org/view.aspx?id=461266

Schmidt, J. 2011. Could U.S.-Mexico agreement lead to cleaner and more efficient vehicles in 

Mexico? http://grist.org/article/2011-03-04-could-us-mexico-agreement-on-cross-bordertrucking-lead-to/

State of New Hampshire. 2013. AN ACT relative to weight of commercial vehicles with idle 

reduction devices www.gencourt.state.nh.us/legislation/2013/HB0196.html
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Tranportation Environmental Resource Center. 2013. Tranportation Environmental Resource 

Center List of State Regulations and Incentives for Alternative Fuels 

www.tercenter.org/pages/altfuelMA.htm

Trucking.org. 2008. State Weight Exemptions (As of March 2008) 

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%20Table.pdf

UnsafeTrucks.org. 2008. NAFTA Regulations on Safety http://unsafetrucks.org/nafta.htm

Wall Street Journal. 2012. New York’s Schumer: GPS Guides Trucks into Crashes 

http://blogs.wsj.com/drivers-seat/2012/09/25/new-yorks-schumer-gps-guides-trucks-intocrashes/

7.3 Green Vehicle Deployment Bibliography

ABI Research. 2009. Commercial Hybrid Vehicles: A Global Analysis of Medium- and HeavyDuty Hybrid Trucks and Buses www.abiresearch.com/research/product/1009888-hybridand-electric-vehicles/

Addison, J. 2010. UPS Fleet Hybrid Delivery Trucks and CNG www.cleanfleetreport.com/upsfleet-hybrid-delivery-trucks/

Alliance Autogas. 2012. Alliance Autogas: powered by propane right here, right now 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26458

American Public Transportation Association. 2012. Transit on the Cutting Edge of Clean 

Technology www.apta.com/resources/statistics/Documents/Transit-CleanTechnology.pdf

Barnitt, R. 2011. FedEx Express Gasoline Hybrid Electric Delivery Truck Evaluation: 12-Month 

Report www.nrel.gov/docs/fy11osti/48896.pdf

Bennett, S. 2008. Alliant Switches Fleet Vehicles to Biofuels 

www.worktruckonline.com/Channel/Fuel-Management/Article/Story/2008/07/AlliantSwitches-Fleet-Vehicles-to-Biofuels.aspx?prestitial=1

Bertram, M. R. W. A. B. K. 2010. Propane Vehicles: Status, Challenges, and Opportunities 

www1.eere.energy.gov/cleancities/pdfs/propanepaper09_final.pdf

Boesel, J. 2012. Why now is the right time to diversify your portfolio...and help make the nation 

stronger and better 

www.ntea.com/WorkArea/linkit.aspx/LinkIdentifier=id%26ItemID=26495&ei=NFXzT4

6ZKsjZ6wHFqNyqCA&usg=AFQjCNFrhMcO367FTlOU.S.am1zicJn3vvyg

Bolton, P. 2012. New York State-wide Alternative Fuel Vehicle Program for Vehicles and 

Fueling Stations 

www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2012/technology_integration

/arravt053_ti_bolton_2012_o.pdf

Business Wire. 2012. CleanFUEL U.S.A and Collins Bus Corporation Deliver More than 100 

Propane Autogas-Powered Buses 

www.businesswire.com/news/home/20121114005626/en/CleanFUEL-U.S.A-CollinsBus-Corporation-Deliver-100

California Air Resources Board. 2012. Heavy-Duty (Tractor-Trailer) Greenhouse Gas 

Regulation www.arb.ca.gov/cc/hdghg/hdghg.htm
143

California Natural Gas Vehicle Coalition. 2012. CEC Awards More than $8.5 Million: AB 118 

buy-down funds go largely to NGVs for vehicles in all classes; more funds may be in the 

offing www.cngvc.org/pdf/newsletters/CalNGVNews_040212.pdf

CalStart. 2012a. Best Fleet Uses, Key Challenges, and the Early Business Case for E-Trucks 

www.calstart.org/Libraries/ETruck_Task_Force_Documents/Best_Fleet_Uses_Key_Challenges_and_the_Early_Busin

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CalStart. 2012b. Leading Fleets Take Delivery of World’s First Commercially Available Hybrid 

Hydraulic Parcel Delivery Trucks – Hybrids Without Batteries 

www.calstart.org/news_and_publications/CALSTART-in-the-news/CALSTART-PressReleases/worlds-first-hybrid-hydraulic-delivery-trucks.aspx

CalStart. 2012c. Market Demand Analysis For Hybrid and Electric Trucks 2012-2016

Carbon War Room Research & Intelligence Group. 2012. Unlocking fuel-saving technologies in 

trucking and fleets 

http://carbonwarroom.com/sites/default/files/reports/Unlocking%20Fuel%20Saving%20

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Chandler, K., Norton, P., & Clark, N. 2001. Waste Management's LNG Truck Fleet Final Results 

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Clay, K. 2012. Transforming Transportation Globally The Role of Natural Gas 

www.sae.org/events/gim/presentations/2012/kclay.pdf

Coca-Cola. 2011. 2010/2011 Sustainability Report www.thecocacolacompany.com/sustainabilityreport/downloads.html

Composites World. 2007. New hybrid electric bus takes advantage of composites 

www.compositesworld.com/articles/new-hybrid-electric-bus-takes-advantage-ofcomposites

Cornils, H. 2009. Hybrid Solutions for MD Commercial Vehicles 

www.erc.wisc.edu/documents/symp09-Cornils.pdf

County of Fairfax. 2012. County of Fairfax, VA fleet summary 

www.ntea.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=26463

DOE Alternative Fuels Data Center. 2012. Maine Fleets Make Progress with Propane 

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DOE EERE. 2011a. Project Results: Evaluating FedEx Express Hybrid-Electric Delivery Trucks 

www.nrel.gov/docs/fy11osti/50042.pdf

DOE EERE. 2011b. Project Startup: Evaluating Coca-Cola’s Class 8 Hybrid-Electric Delivery 

Trucks www.nrel.gov/docs/fy11osti/49621.pdf

DOE EERE. 2012a. Largest U.S. Port Complex Embraces LNG for Heavy-Duty Trucks 

www.afdc.energy.gov/case/1203

DOE EERE. 2012b. National Clean Fleets Partnership 

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DOE EERE. 2013. Propane: Liquefied Petroleum Gas (LPG) 

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Ebert, R. D. 2012. Long Island Regional Energy Collaborative: Promoting a Green Economy 

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Energy Information Administration. 2010. How many alternative fuel and hybrid vehicles are 

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