i 

Bridge Curb/Railing and Approach Treatment 

for Extremely Low Volume Roads 

FINAL REPORT

FHWA PUBLICATION NUMBER: FHWA-FLH-23-003b

December 2022
ii

Notice

This document is disseminated under the sponsorship of the U.S. Department of 

Transportation in the interest of information exchange. The U.S. Government 

assumes no liability for the use of the information contained in this document. 

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers' names appear in this report only because they are considered 

essential to the objective of the document. 

Quality Assurance Statement 

The Federal Highway Administration (FHWA) provides high-quality information 

to serve Government, industry, and the public in a manner that promotes public 

understanding. Standards and policies are used to ensure and maximize the 

quality, objectivity, utility, and integrity of its information. FHWA periodically 

reviews quality issues and adjusts its programs and processes to ensure 

continuous quality improvement.
iii

TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

FHWA-FLH-23-003b

2. Government 

Accession No.

3. Recipient's Catalog No.

 

4. Title and Subtitle

Guide for Bridge Curb/Rail and Approach Treatment for Extremely Low Volume 

Road

5. Report Date

December 2022

6. Performing Organization 

Code

7. Author(s) 

Carrigan, C., Plaxico, C. 

8. Performing Organization 

Report No.

9. Performing Organization Name and Address

VHB 

940 Main Campus Drive, Suite 500 

Raleigh, NC 27606

RoadSafe, LLC

12 Main St.

Canton, ME 04221 

10. Work Unit No.

11. Contract or Grant No.

DTFH6116D00040

12. Sponsoring Agency Name and Address

U.S. Department of Transportation

Federal Highway Administration 

1200 New Jersey Avenue, SE

Washington, DC 20590

13. Type of Report and 

Period

Final Guide, August 2020 – 

December 2022

14. Sponsoring Agency 

Code: FHWA

15. Supplementary Notes. 

16. Abstract

Bridges cross over a variety of features ranging from drainage ditches to deep valleys and serve a variety of functions. 

There are several competing considerations when evaluating bridge railings: the risk of injury and death for vehicle 

drivers and occupants, the structural integrity of the bridge itself, and the service provided to the principal users of the 

bridge. 

The objective of this research was to develop a guide for bridge rails and approach treatments for extremely low-volume 

roads and low-speed roads. This report describes the background research and information used to develop the resulting 

guide: Guide for Bridge Curb/Rail and Approach Treatment for Extremely Low Volume Roads (the Guide).

17. Key Words: Bridge rails, low-volume roads, approach 

treatments

18. Distribution Statement

No restrictions.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages: 

47 

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized
iv
v 

TABLE OF CONTENTS

Introduction ..................................................................................................................................... 1

Literature and Policy Search ........................................................................................................... 1

Design of Bridge Rails ................................................................................................................ 2

AASHTO Guide Specification for Bridge Railings ................................................................ 3

NCHRP Report 350 ................................................................................................................. 3

Manual for Assessing Safety Hardware .................................................................................. 4

Longitudinal Barriers for Passenger Vehicle Containment ..................................................... 5

Guidelines and Specifications ..................................................................................................... 6

FHWA and AASHTO ............................................................................................................. 6

State Departments of Transportation ....................................................................................... 9

Relevant Research ..................................................................................................................... 12

Bridge Curb and Railings for Extremely Low Volume Roads.............................................. 12

Approach Guardrail for Bridges on Low-Volume Roads ..................................................... 13

Analysis Methods used in Roadside Design Guidance Development ................................... 13

Forest Service Manual FSM 7700 ......................................................................................... 16

Manual on Uniform Traffic Control Devices ............................................................................ 17

Summary ................................................................................................................................... 17

Parameters for Use of Bridge Rails and Associated Hardware .................................................... 18

Establishing Safety Performance Goals .................................................................................... 18

Bridge Railings .......................................................................................................................... 18

Approach Terminals and Transitions ........................................................................................ 23

New Hardware Developed ............................................................................................................ 24
vi 

LIST OF FIGURES

Figure 1. Equation. NCHRP 15-65 governing equation. .............................................................. 14

Figure 2. Equation. Continuous features. ...................................................................................... 15

Figure 3. Equation. Direct features. .............................................................................................. 15

Figure 4. Equation. ICER. ............................................................................................................. 16

Figure 5. Equation. Risk of proposed to existing crashes. ............................................................ 18

Figure 6. Equation. Relative risk of bridge rail to no bridge rail. ................................................. 19

Figure 7. Equation. Relative risk reduction of bridge rail to no bridge rail. ................................. 19

Figure 8. Equation. The condition probability of a fatal or serious injury given that a crash 

occurs. ........................................................................................................................................... 20

Figure 9. Graphic. Engineering drawing of the MASH TL-1 WVBR bridge rail. ....................... 26

Figure 10. Graphic. FEA model of the MASH TL-1 WVBR bridge rail test article (oblique 

view). ............................................................................................................................................ 27

Figure 11. Graphic. Design alternative 1 for modified WVBR for use on 4-by-12 plank decks. 28

Figure 12. Graphic. Design alternative 2 for modified WVBR for use on 4-by-12 plank decks. 28

Figure 13. Graphic. Design alternative 3 (recommended) for modified WVBR design for use on 

4-by-12 plank decks. ..................................................................................................................... 29

Figure 14. Graphic. Engineering drawing for 12-inch timber curb/rail used in analysis. ............ 31

Figure 15. Graphic. Sequential views of impact at 15 degrees and 15 mph. ................................ 32

Figure 16. Graphic. Sequential views of impact at 15 degrees and 20 mph. ................................ 32

Figure 17. Graphic. Sequential views of impact at 15 degrees and 25 mph. ................................ 33

Figure 18. Graphic. Sequential views of impact at 25 degrees and 10 mph. ................................ 33

Figure 19. Graphic. Sequential views of impact at 25 degrees and 15 mph. ................................ 33

Figure 20. Graphic. Sequential views of impact at 25 degrees and 20 mph. ................................ 33
vii 

LIST OF TABLES

Table 1. Crash test acceptance equivalencies from FHWA.(13) ...................................................... 4

Table 2. Approximate crash test acceptance equivalencies.(18, 19) ................................................... 5

Table 3. Summary of State practices for bridge rail and approach rail on low-volume roads. ... 12

Table 4. Association of maintenance levels and MASH test levels.(47) ....................................... 17

Table 5. Crash severity proportions for bridge features at various posted speed limits. .............. 21

Table 6. RRR for bridge railings on bridges with posted speed limits of 45 mph or less and 50 

vpd or less. .................................................................................................................................... 22

Table 7. TL selection for bridge rails on roads with ADT ≤ 50 vpd and posted speed ≤ 45 mph. 23

Table 8. Crash severity proportions for terminals, transitions, and unshielded bridge rail ends at 

various posted speed limits. .......................................................................................................... 24

Table 9. RRR for terminals, transitions, and unshielded bridge ends on bridges with posted speed 

limits of 45 mph or less and 50 vpd or less................................................................................... 24

Table 10. Occupant risk metrics computed using TRAP software for the baseline and modified 

mount cases. .................................................................................................................................. 30

Table 11. Summary of results for low-profile curb/rail cases. .................................................... 32
viii

LIST OF ACRONYMS

AADT annual average daily traffic

ADT average daily traffic 

AASHO American Association of State Highway Officials 

AASHTO American Association of State Highway and Transportation Officials 

B/C benefit-cost 

BCAP Benefit-Cost Analysis Program 

DOT department of transportation

FEA finite element analysis 

FHWA Federal Highway Administration 

FLMA Federal Land Management Agency 

FLTP Federal Lands Transportation Program 

FMVSS Federal Motor Vehicle Safety Standards 

GSBR Guide Specifications for Bridge Railings

ICER incremental cost effectiveness ratio 

I.M. instructional memorandums 

ISPE in-service performance evaluation 

KA fatal and serious injury 

LON length of need 

MASH Manual for Assessing Safety Hardware 

MoDOT Missouri Department of Transportation 

mph miles per hour 

MUTCD Manual on Uniform Traffic Control Devices 

MwRSF Midwest Roadside Safety Facility 

NBI National Bridge Inventory 

NCHRP National Cooperative Highway Research Program 

NHS National Highway System 

NHTSA National Highway Traffic Safety Administration 

OIV occupant impact velocity

ORA occupant ridedown accelerations 

PL1 Performance Level 1

PL2 Performance Level 2

PG performance goal 
ix

RDG Roadside Design Guide 

RRR relative risk reduction 

RSAP Roadside Safety Analysis Program 

SUT single unit truck 

TAC technical advisory committee 

TL1 Test Level 1

TTI Texas A&M Transportation Institute 

UNL University of Nebraska-Lincoln 

USBR United States Bureau of Reclamation 

VLV very low volume 

vpd vehicles per day 

WVBR West Virginia Timber Curb-Type Bridge Rail 
1 

INTRODUCTION

Bridges cross over a variety of features ranging from drainage ditches to deep valleys and serve a 

variety of functions. There are several competing considerations when evaluating bridge railings: 

the risk of injury and death for vehicle drivers and occupants, the structural integrity of the 

bridge itself, and the service provided to the principal users of the bridge. 

The objective of this research was to develop a guide for bridge rails and approach treatments for 

extremely low-volume roads and low-speed roads. This report describes the background 

research and information used to develop the resulting guide: Guide for Bridge Curb/Rail and 

Approach Treatment for Extremely Low Volume Roads (the Guide). 

The Guide is applicable to both single- and two-lane bridges. For the purpose of the Guide, 

extremely low-volume roads were defined as roads with less than 50 vehicles per day (vpd) and 

posted speed ranges equal to 5-15 miles per hour (mph), 16-30 mph, and 31-45 mph. The Guide 

assesses the need for and practicality of providing bridge rails to contain passenger vehicles 

while accommodating other vehicle types like agricultural equipment, timber harvesting 

equipment, recreational vehicles, and other associated service vehicles on the subject bridges. 

The Guide leads inspectors and engineers to practical, low-cost solutions based on the 

characteristics of the bridge, anticipated traffic, and the intended use of the bridge. The outcome 

of using the Guide is the determination of whether the existing bridge rails, associated roadside 

hardware, and other conditions can be left as-is; when improvements should be considered to 

achieve the safety performance goal; and what potential solutions might be implemented to 

achieve the safety performance goal. When new hardware is recommended, the Guide provides 

construction details for the new hardware. The Guide also includes an inspection checklist that 

inspectors can use to collect information from an inspection. Evaluating the need to provide 

roadside barriers, such as w-beam for shielding obstacles on the roadside, beyond the approaches 

to the bridge, was beyond the scope of this project and the Guide. 

This report documents the literature and policy search and the development of parameters for the 

use of bridge rails, associated hardware and delineation on the extremely low volume and low 

speed roads. This report also documents the research conducted to modify the Manual for 

Assessing Safety Hardware (MASH) Test Level 1 (TL-1) low profile West Virginia Timber 

Curb-Type Bridge Rail (WVBR) for installation on a lower strength bridge structure (e.g., 4 ft by 

12 ft wood plank bridge deck). 

Multiple concepts were considered for the Guide, including a variety of alternatives for format 

and content, however, only the final concept is documented herein. This research also included 

the compilation of drawings and technical information for a variety of bridge rails, transitions, 

and terminals. The Guide contains the associated drawings. 

LITERATURE AND POLICY SEARCH

The United States Bureau of Reclamation (USBR) is a Federal Land Management Agency 

(FLMA). The Federal Lands Transportation Program (FLTP) was established in 23 U.S.C. 203 

to improve the transportation infrastructure owned and maintained by FLMAs. FLTP is one 

potential funding source for improvement projects for the 315 bridges within the National Bridge 
2 

Inventory (NBI) for which USBR is responsible. This USBR inventory is dominated by lowspeed and low-volume bridges. 

Although low-volume roads make up approximately 80 percent of roadway mileage in the 

nation’s transportation system, these low-volume roads only represent approximately 20 percent 

of the vehicle miles traveled.

(1) Low-volume roads, however, “have a fairly high bridge density, 

averaging approximately 9 bridges every 100-centerline kilometers (14 bridges every 100-

centerline miles.”(2) 

There has been interest for several decades in developing selection guidance for the multiple 

performance, service, or test levels of bridge rails. The recent completion of the National 

Cooperative Highway Research Program (NCHRP) 22-12(03) (Recommended Guidelines for the 

Selection of Test Levels 2 through 5 Bridge Rails) provided this guidance, but did not address 

extremely low-volume roads nor low-speed roads. (3)

Design of Bridge Rails

Historically, design of bridge rails has followed guidance contained in the American Association 

of State Highway Officials (AASHO) Standard Specifications for Highway Bridges. Prior to 

1965, the AASHO specification required very simply that “substantial railings along each side of 

the bridge shall be provided for the protection of traffic.”(4, 5) It was specified that the top

members of bridge railings be designed to simultaneously resist a lateral horizontal force of 150 

lb/ft and a vertical force of 100 lb/ft applied at the top of the railing. The design load on lower 

rail members varied inversely with curb height, ranging from 500 lb/ft for no curb to 300 lb/ft for 

curb heights of 9 inches or greater. It was further specified that the railing have a minimum 

height of 27 inches and a maximum height of 42 inches above the roadway surface.(4) These 

loads are only a fraction of what is used today. 

Revised railing specifications were subsequently published in 1965 in the 9th edition of the 

AASHO Standard Specifications for Highway Bridges.

(6) It required that rails and parapets be 

designed for a transverse load of 10,000 lbs divided among the various rail members using an 

elastic analysis. The force was applied as a concentrated load at the mid-span of a rail panel with 

the height and distribution of the load based on rail type and geometry. The height of the railing 

was required to be no less than 27 inches and railing configurations successfully crash tested 

were exempt from the design provisions.(6)

Olson was perhaps the first to systematically examine the performance requirements for bridge 

railings in 1970. His results, documented in NCHRP Report 86, suggested use of appropriate 

transitions, evaluation through crash testing, ability to minimize bridge rail penetration, and other 

things that are considered standard objectives of bridge railing design today.(7)

Crash testing is the most direct means of assessing barrier impact performance. Full-scale crash 

testing is typically used to verify the ability of a barrier to contain an impacting vehicle. The 

traffic volumes, geometrics, and operational characteristics, which can affect the likelihood of a 

crash, are not addressed by crash testing. The next section summarizes the different performance 

and test levels that have been used over the years to evaluate crash test performance. A section

on guidelines for when to use barriers follows. 
3 

AASHTO Guide Specification for Bridge Railings 

In 1989, AASHTO published the Guide Specifications for Bridge Railings (GSBR) to provide a 

more comprehensive approach for the design, testing, and selection of bridge rails than that 

contained in the AASHTO Standard Specifications for Highway Bridges.

(8, 9) The GSBR

emphasized the need for full-scale crash testing to verify that a given bridge rail design meets the 

desired impact performance criteria. The document defined three bridge rail performance levels 

and associated crash tests to be used in qualifying the railing. The crash test matrices for each 

performance level were described by crash test conditions defined in terms of vehicle type, 

vehicle weight, impact speed, and impact angle. Two passenger vehicles—a small 1,800-lb 

passenger car and a 5,400-lb pickup truck—were common to all three performance levels. Test 

conditions associated with Performance Level 1 (PL1) included an 1,800-lb small car and a 

5,400-lb pickup truck impacting at an angle of 20 degrees and a speed of 50 mph and 45 mph, 

respectively. For Performance Level 2 (PL2), the speed of the small car and pickup truck tests 

were increased to 60 mph. 

Although the 1989 GSBR recommended crash testing as the basis for bridge rail evaluation and 

acceptance, it did provide the bridge engineer with suggested design information including the 

magnitude, distribution, and vertical location of railing design loads for each performance level. 

The transverse loads were derived from two related research studies in which vehicle impact 

forces were measured using instrumented concrete walls.(10, 11)

NCHRP Report 350

In 1993, NCHRP Report 350 was published, superseding the previous crash testing guidelines 

contained in NCHRP Report 230.

(12) One major change in Report 350 is that six different test 

levels for roadside hardware were added for bridge railings. The intent was to provide test 

guidelines for developing a range of longitudinal barriers, including bridge railings and 

transitions, that could be used in different situations. TLs 1 through 3 relate to containment of 

passenger vehicles (e.g., small passenger cars and pickup trucks) and vary by impact speed, with 

increasing impact speeds defined for increasing test levels: 31 mi/hr for TL-1, 44 mi/hr for TL-2, 

and 62 mi/hr for TL-3.)

 While Report 350 provided the testing recommendations, only general 

guidance was provided about what field conditions would indicate the need for a particular test 

level bridge railing. 

The Federal Highway Administration (FHWA) established approximate equivalences between 

the multiple service levels, performance levels, and test levels in a memorandum to FHWA 

Regional Administrators in 1997.(13) The equivalencies set out by FHWA are summarized in 

table 1. 
4 

Table 1. Crash test acceptance equivalencies from FHWA.(13)

Bridge Railing

Testing Criteria

Acceptance Equivalencies

Report 350 TL1 TL2 TL3 TL4 TL5 TL6

Report 230 MSL-1 

MSL-2† 

AASHTO Guide Spec PL1 PL2 PL3

AASHTO LRFD Bridge Spec PL1 PL2 PL3

† This is the performance level usually cited when describing a barrier tested under NCHRP Report 230. It is close 

to TL3 but adequate TL3 performance cannot be assured without a pickup truck test.

Manual for Assessing Safety Hardware 

Since the publication of NCHRP Report 350 in 1993, changes have occurred in vehicle fleet 

characteristics, operating conditions, and technology. NCHRP Project 22-14(2), Improvement of 

Procedures for the Safety-Performance Evaluation of Roadside Features, was initiated to take 

the next step in the continued evolution of roadside safety testing and evaluation. The results of 

this research effort culminated in the 2009 AASHTO MASH.(14) MASH was updated again in 

2016.(15)

MASH includes essentially the same test level approach with some changes to the impact 

conditions for the higher test level longitudinal barrier tests and the vehicles used for testing. 

Aside from these changes to the test vehicles, TL1 and TL2 remained largely the same as they 

were in NCHRP 350. 

The weight and body style of the pickup truck test vehicle changed from a 2,000 kg (4,409 lb), 

3⁄4-ton, standard cab pickup to a 2,270 kg (5,000 lb), 1⁄2-ton, 4-door pickup. This change in 

vehicle mass of approximately 15 percent was deemed to produce an impact condition that was 

similar to, and possibly more severe than the TL4 single unit truck (SUT) test from NCHRP 

Report 350. 

Crash testing conducted at the Midwest Roadside Safety Facility (MwRSF) at the University of 

Nebraska-Lincoln (UNL) under NCHRP Project 22-14(2) and the Texas A&M Transportation 

Institute (TTI) under NCHRP Project 22-14(3) demonstrated that taller barriers will be required. 

Dobrovolny et al. recently developed MASH equivalencies of NCHRP Report 350 bridge 

railings.

(16) Ray and Carrigan considered the MASH equivalencies in NCHRP Web Only 

Document 307, Recommended Guidelines for the Selection of Test Levels 2 Through 5 Bridge 

Rails.

(17) The equivalency results from these research efforts and the previously discussed table 1

Report 350 equivalencies are summarized in table 2. 
5 

Table 2. Approximate crash test acceptance equivalencies.

(18, 19)

Bridge Railing Testing Criteria Acceptance Equivalencies

MASH TL1 TL2 TL3 TL4 TL5 TL6

Report 350 

• Solid Concrete 

• Metal beam and post (on ≥24 

in concrete parapet)

TL2 TL3 & 

TL4

TL5

Report 350 

• Concrete Beam and Post

• Metal Beam and Post (deck 

mounted) 

• Metal Beam and Post (on 

curb) 

TL2, TL3, 

& TL4

TL5

Report 230 

MSL-1 

MSL-2† 

AASHTO Guide Spec PL1 PL2 PL3

AASHTO LRFD Bridge Spec PL1 PL2 PL3

† This is the performance level usually cited when describing a barrier tested under NCHRP Report 230. It is close 

to TL3 but adequate TL3 performance cannot be assured without a pickup truck test.

Longitudinal Barriers for Passenger Vehicle Containment

While most of the design and testing effort under Report 350 and MASH involved designing 

higher test level bridge railings, a number of TL1 and 2 bridge railings were also developed. 

Faller, for example, designed and tested three TL1 bridge railings for timber bridge decks as well 

as three curb railings for very low speed (i.e., 15 mi/hr) applications.(20, 21) These and other plans 

for timber rails on concrete decks were subsequently documented in the USDA Forest Products 

Laboratory plan sets.(22)

A number of bridge rail, transitions, and terminals were found in the literature which have been 

crash tested to demonstrate passenger vehicle containment at low-impact speeds. In addition to 

these systems, the literature also includes suggestions for using long-span guardrail on short 

bridges and small radius guardrail on approaches. Appendix A of the Guide provides a 

culmination of the hardware found through the literature and developed under this research 

effort. Construction drawings and associated technical details to constructing this hardware were 

developed and are summarized in the Guide appendices along with the source materials. 
6 

Guidelines and Specifications

FHWA and AASHTO 

AASHTO Guide Specification for Bridge Railings 

Engineers have recognized that bridge rail needs can vary significantly from site to site based on 

a multitude of factors. For the first time, the 1989 AASHTO GSBR provided bridge engineers 

guidance for determining the appropriate railing performance level for a given bridge site.(8)

Selection tables were provided that estimated the appropriate railing performance level for a 

given bridge site based on highway and site characteristics like highway type (e.g., divided, 

undivided, etc.), design speed, traffic volume, percent trucks, and bridge rail offset. The tables 

applied to bridges that were on tangent, level roadways with deck surfaces approximately 35-ft 

above the underlying ground or water surface. It was further assumed that there was low 

occupancy land use or shallow water under the bridge structure. Correction factors were 

provided to permit the engineer to adjust the traffic volume for horizontal curvature, vertical 

grade, different deck heights, and different densities of land use beneath the bridge. These 

selection procedures were developed using a benefit-cost (B/C) analysis combined with 

engineering judgment. While much thought and effort went into the derivation of the bridge rail 

section tables and performance levels, an independent review of the bridge rail performance 

selection guidelines found that some of the data and assumptions used in the B/C analysis were 

faulty.(23) Consequently, the selection guidelines were never implemented on a national basis. 

The 1989 GSBR did not address lower volume, lower speed bridge railings.(8)

AASHTO Roadside Design Guide 

The AASHTO Roadside Design Guide (RDG) was first published in 1989, the most recent, 

fourth edition, was published in 2011.(24, 25) Chapter 7 of the RDG covers the selection and 

placement of bridge railings and transitions. Most of the RDG is dedicated to guidance regarding 

high-speed and/or high-volume roadways; however, some guidance is provided for low-speed, 

low-volume roadways. The 2011 RDG states “full treatment may not be cost-effective on bridgelength culverts, and alternate treatments should be considered. Such treatments could include 

extending the structure and leaving the edges unshielded or using a less expensive, semi-rigid 

type railing.”(24) Additionally, Chapter 12 of the RDG covers guidance for low-volume roads and 

streets. In this chapter the guidance is tailored to low-cost countermeasures. For this reason, 

signage and delineation is discussed as the most cost-beneficial improvement to low-volume, 

low-speed roads. The 2011 RDG guidance for bridge railings on low-volume roads includes a 

discussion of approach railings. The RDG observes that approach railing, when used, should not 

direct an impacting vehicle into the parapet. The RDG also provides that “improving a deficient 

[bridge] railing by extending the approach guardrail completely across the structure is often used 

and very effective countermeasure. Although such a modified design may not meet full design 

standards, it will provide continuity of the rail if struck by errant motorists.”(24)

NCHRP Project 22-12(03) 

While a variety of bridge railings have been developed at various test levels, what had not been 

established were the criteria for selecting which test level should be used based on specific traffic 

and site conditions. The AASHTO RDG recognizes the multiple test level approach but gives 

only very general guidance about why different test level bridge railing might be used.(24) At 

present, highway agencies must make decisions on which test level is appropriate for each site 
7 

based on an ad hoc basis. The objective of NCHRP Project 22-12(03), Recommended Guidelines 

for the Selection of Test Levels 2 through 5 Bridge Rails, was to develop such guidelines.(18) The 

conditions where TL2 through TL5 bridge railings should be considered were examined in that 

research. A selection procedure was developed that was based on the risk of a vehicle penetrating 

bridge railing and resulting in a fatal or serious injury crash. The procedure accounts for the 

traffic volume, posted speed, land use under the bridge, and road characteristics (e.g., grade, 

curvature, lane width, etc.). The traffic and site characteristics are quantified by the designer and 

used to predict the risk of a fatal or serious injury crash. NCHRP Project 22-12(03) was limited 

to TL2 through TL5 bridge railings and addressed neither extremely low-volume roads nor lowspeed roads.

AASHTO Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT ≤ 400) 

AASHTO’s Guidelines for Geometric Design of Very Low-Volume Roads (VLV Guide) 

observes “roadside design is the one major determinant of safety on very low-volume local 

roads.” (26) The VLV Guide also observes, however, that “roadside clear zones provide very little 

benefit, and that traffic barriers are not generally cost-effective, on roads with very low traffic 

volumes.” In addition, the VLV Guide indicates that “the roadside design guidelines for very 

low-volume local roads provide great flexibility to the designer in exercising engineering 

judgement to decide where it is appropriate to provide improved roadsides.”(26)

FLH Barrier Guide for Low Volume and Low Speed Roads 

The 2005 FLH Barrier Guide defines low-volume roads as those roads with an average daily 

traffic (ADT) of less than 2,000 vehicles per day vpd. The FLH Barrier Guide outlines 

challenges faced by roadway engineers when designing low-volume roads:

(2)

• The fatal crash rate is estimated to be three times higher on rural minor roads.

• These roads typically have more restricted rights-of-way, little or no clear zones, and 

substandard design features.

• High bridge density and issues associated with bridges including restricted conditions and 

rigid rails.

• Corrective measures can be difficult to justify economically. 

The FLH Barrier Guide observes that low travel speeds and driver familiarity can mitigate some 

of the challenges mentioned above, however, fatal and serious injury crashes can and do still 

occur on low-speed roads. 

Locating and designing barrier systems, as well as properly designing and locating approach 

rail/transition sections are covered in Chapter 4 of the FLH Barrier Guide. The design method 

outlined in the RDG is the chosen method for locating longitudinal barriers (including approach 

rail). Values for determining the length of need (LON) presented in the FLH Barrier Guide cover 

lower speeds and lower traffic volumes than what is published in the 2002 RDG.(2, 27) It is not 

clear where these values for lower speed and lower traffic volume came from.

Bureau of Reclamation Guidelines for Determining the Adequacy of Existing Bridge Safety 

Features at Reclamation’s Bridges

The 2006 Bureau of Reclamation Guidelines for Determining the Adequacy of Existing Bridge 

Safety Features at Reclamation’s Bridges is used in combination with the Bridge Inventory and 

Inspection Program Directives and Standards (Reclamation Manual Directives and Standards 
8 

FAC 07-01) “with the purpose of aiding Reclamation's bridge inspectors in determining the 

adequacy of bridge traffic safety features on Reclamation's bridges.”(28) 

This Guide classifies bridges into two categories. “Class A bridges (Low-Speed, Low-Volume) 

are those bridges which meet all of the following: 

• With an approximate ADT volume (estimated at time of inspection, if no traffic counts 

are available) less than or equal to 50 vpd, 

• And with estimated traffic speeds approaching and across the bridge of 15 mph or less, 

and 

• The structure is a standard width one lane bridge (i.e., 16 ft or less). 

Class B bridges are those bridges (greater than Class A), that do not meet the criteria for a Class 

A bridge.”(28) Bridges are further divided by type where Type 1 bridges are open to the public 

and Type 2 bridges are not. Recommendations are made regarding bridge rail and approach rails 

based on the class and type of the bridge. 

It is suggested within the guidance that Type 1, Class A bridges should have, at a minimum, a 

curb-type rail (crash tested or equivalent). Other factors where a higher level of protection might 

be warranted for Type 1, Class A bridges are listed as follows:

• “Historical data, including accident history at the site.

• Approximate vehicle speed approaching and across the bridge. 

• Approximate traffic volume.

• One-way versus two-way traffic.

• Number of traffic lanes.

• Type of vehicles using the bridge, i.e. trucks, RVs, etc. 

• Bridge surface (i.e., slippery vs. non-slippery).

• Approach roadway vertical and horizontal alignment. 

• Approach roadway width vs. bridge clear deck width. 

• Approach roadway surface (i.e., gravel, pavement, etc.). 

• Approach roadway embankment steepness. 

• Bridge features (i.e., length, width, deck crown, etc.). 

• Waterway features (i.e., height of bridge above waterway, depth of water, etc.). 

• Typical weather conditions (i.e., snow, ice, high winds, fog, etc.).

• Nighttime use.

• Familiarity of users with bridge (i.e., local traffic only, tourists, etc.).

• Probable severity of injury from a bridge run-off due to bridge height, depth of water, 

vehicle speed, etc.

• Pedestrian use.
9 

• Projected future use of the bridge. 

• Aesthetic concerns.”

Further guidance is provided for approach rails and terminals for these classes and types of 

bridges. 

State Departments of Transportation

Many State DOTs refer designers to Chapter 13 of the AASHTO LRFD Bridge Design 

Specifications for bridge rail strength and geometric requirements, NCHRP 350 for crash test 

criteria, and the AASHTO Guide Specification and RDG for guidance on design and installation 

of bridge rail and approach guardrail. These national documents are supplemented in many 

States with State-specific guidance in which the States detail the use of specific railings under 

certain situations. A sample of State-adopted guidance for bridges carrying low-volume roads is 

presented in this section. 

Illinois Department of Transportation 

Chapter 35 of the Illinois Department of Transportation (DOT) published Bureau of Local Roads 

and Streets Manual allows for the elimination of approach rail on the bridge rail end closest to 

traffic if one or more of the following apply: 

• “The posted speed limit is less than 25 mph on an urban curbed section. 

• The ADT is less than 150; the bridge is approach roadway width; and the bridge is on

tangent alignment. 

• A township or road district bridge is wider than the approaching roadway and the bridge 

is on tangent alignment.”(29)

Iowa DOT

The Iowa DOT publishes instructional memorandums (I.M.) to assist local public agencies with

transportation-related topics. I.M. 3.213 provided guidance for determining the need for traffic 

barriers at roadway bridges and culverts through 2016. Bridge rail upgrade needs were 

determined using a point system that assigns points for various characteristics for the bridge such 

that if the sum of points is less than 25, the bridge rail need not be updated. The categories that 

are used to assess points include: crash history, annual average daily traffic (AADT), bridge 

width, bridge length, and existing bridge rail type. The guidance limited the use of approach 

guardrail “if the following conditions exist:

• AADT is less than 400 vpd, 

• The structure is at least 24-ft wide, 

• The structure is on a tangent road section, 

• The B/C ratio is less than 0.8, and 

• The bridge width is wider than the approach roadway width.”(30)
10 

In 2018, substantive changes were made to I.M. 3.213 and it was re-numbered to I.M. 3.230. The 

2018 I.M. 3.230 provides for shorter approach guardrail in locations with less than 400 vpd or 

45-mph speed limit. However, “if ALL [emphasis is original] of the following conditions exist, 

the local public agency may elect not to install [approach] guardrail:

• Current ADT at structure is less than 400 vpd. 

• Structure width (curb-to-curb) is 24 ft or greater, and is wider than the approach roadway 

width. 

• Structure is on tangent alignment.”(31)

I.M. 3.230 goes on to provide guidance for the installation of crash cushions or approach 

guardrail that is not considered crashworthy in some locations where the site conditions do not 

allow for the approach guardrail sections. 

With respect to bridge rail on roadways with ADT of 400 vpd or less, I.M. 3.230 recommends 

that the Iowa DOT Bridge Standards be used to guide construction of bridge rail. If the designer 

decides not to use the Iowa DOT Bridge Standards on roads with less than 400 vpd then “a 

bridge rail considered to be crashworthy shall be used, meeting a minimum of TL-1 in NCHRP 

350 or MASH.”(31) Additionally, on short bridges, I.M. 3.230 allows designers to extend the 

guardrail using the Iowa DOT long span standard plan (BA-211) across the bridge rather than 

using a typical bridge rail, parapet, and approach section system. 

Kansas DOT 

In 2014, Kansas DOT performed a study to support guardrail and bridge rail recommendations 

for very low-volume local roads in Kansas. The bridges reviewed in this study were ones whose 

ADT was less than 50 vpd and when the bridge length was less than 50 ft. The study was a 5-

year period from 2008 through 2012. Of the 306,056 crashes in Kansas during the study period, 

only 74 crashes occurred on the very low-volume roads that fit the inclusion criteria. Three of the 

crashes were coded as fatal crashes, there were 2 serious-injury crashes, and 69 crashes of other 

severities. Further, crashes on these short, VLV roads account for only 0.05 percent of the total 

fatal and serious-injury crashes and 0.0016 percent of all crashes in Kansas over the study 

period.(32) The authors go on to point out that: 

Due to the very small number of crashes with low-volume bridges the proportion of fatal 

crashes should be considered cautiously. Statistically the confidence interval for this small 

group is very large... As a result, one cannot interpret the data to indicate that a crash with a 

low-volume bridge is more likely to result in a fatality than any crash occurring in the 

State.

(32) 

Although there is support, both from the Kansas 2014 study and AASHTO guidance documents 

to eliminate the requirement for bridge rail on very low-volume roads with lengths of 20-50 ft an 

acknowledgement is made that some benefits of bridge rail cannot be captured by the study 

design. Examples of these benefits are that they provide delineation and even non-crashworthy 

barriers can provide some protection at low-angle and or low-speed crashes. Therefore, the final 

recommendation is that a non-tested bridge rail without approach guardrail design could be used 

on new or rehabilitated bridges that “meet all of the following conditions:

• The bridge is located on a road functionally classified as a Local Road.

• Traffic volume is less than or equal to 50 vpd. 
11 

• The approach roadway is a two-wheel path road.

• Roadway surface of approaches is gravel, sand, or dirt.

• Maximum length of the bridge is 50 ft.

• The new structure shall be no less than 24-ft wide.

• Bridge is not located on or adjacent to a curve or intersection.

• A Type 3 object marker shall be installed at each end of the bridge rails.”(32)

In 2017 Kansas DOT performed a follow-up study to look at roadways with ADT over 50 vpd up 

to 100 vpd and bridges over 50-ft long up to 100-ft long. The study period was extended by two 

years to be from 2008 to 2014. Of the 424,494 crashes of all severity types that occurred over the 

study period 103 matched the inclusion criteria outlined above. Two of the crashes were 

categorized as fatal, 4 were serious-injury crashes, and 97 of the crashes were of other severities. 

The conclusion of the updated study is that: 

Although the crash risk is greater than the baseline risk found for bridges investigated in 

Report No. KS-14-16, the risk under any of these scenarios is very low. In addition, the costs 

of installing a crash tested bridge rail and properly installed approach guardrail section on 

these bridges cannot be justified because the return on the investment, measured in reduced 

crash costs, is very low.(33)

The updated recommendation for installation of non-tested bridge rails in the 2017 report are 

very similar to those from the 2014 study.(32, 33) The differences are highlighted below using bold

type: 

• “The bridge is located on a road functionally classified as a Local Road.

• Traffic volume is less than or equal to 100 vpd. 

• The approach roadway is a two-wheel path road.

• Roadway surface on the approaches is gravel, sand, or dirt. 

• Maximum length of the bridge is 100 ft.

• The new structure shall be no less than 24-ft wide.

• The bridge is not located on or adjacent to a curve or intersection. 

• A Type 3 object marker shall be installed at each end of the bridge rails.”(33)

Missouri DOT (MoDOT)

The MoDOT Engineering and Policy Guide covers guardrail, including bridge end treatments, in 

Section 606.1.(34) In this section, delineation is permitted in place of approach guardrail on 

bridges where the speed limit is less than 60 mph and the traffic volume is less than 400 vpd. The 

Engineering and Policy Guide goes on to recommend delineation-only on bridge replacements or 

rehabilitations where the existing structure was unshielded, or the existing roadway cannot 

reasonably accommodate the installation of approach guardrail. Further, the Guide dis-allows the 

delineation-only in the following situations: (34)

• On major roads and the National Highway System (NHS). 

• For bridges in areas of poor geometry. 

• In areas with crash history higher than the statewide average for similar roads.
12 

South Dakota DOT

The South Dakota DOT Local Roads Plan published in 2011 includes design criteria for bridge 

rail on rural roads.(35) The plan allows turned down rail end treatments of 15 degrees or flatter 

and NCHRP 350 TL2 or better on rural collectors and local rural roads with ADT of less than 

150 vpd.(35)

State Summary 

Table 3 is a summary of the bridge rail and approach guardrail guidelines discussed in the 

previous sections. All of the States that the project team reviewed for this report have a traffic 

volume requirement to allow exceptions from typical bridge rail and approach guardrail design 

and placement guidance. Other conditions that States consider for exceptions include speed limit, 

structure width, and site layout. There is limited guidance for the installation of hardware on 

geometrically constrained sites.

Table 3. Summary of State practices for bridge rail and approach rail on low-volume roads. 

State Option Condition

Illinois Eliminate approach 

guardrail 

ADT < 150; tangent; and/or bridge width ≥ roadway 

width or Owned by Twp; tangent; and/or bridge width 

≥ roadway width.

Iowa

Shorter approach 

guardrail 

ADT < 400; tangent; bridge width ≥ roadway width; 

and bridge width ≥ 24 ft.

Crash cushion or nontested approach guardrail Above conditions and site layout does not allow 

shorter approach guardrail. 

Use TL-1 or better 

bridge rail or Long Span 

Standard Plan 

ADT < 400. 

Kansas Non-tested bridge rail 

w/o approach railing 

ADT < 100; Local road; 2-wheel path; gravel, sand or 

dirt approach; bridge length ≤ 100’, bridge width ≥ 24 

ft; not in proximity of curve or intersection; and use 

Type 3 object marker. 

Missouri Delineation in lieu of 

approach guardrail 

ADT < 400; PSL < 60 mph; not on NHS or major road; 

areas of poor geometry; or high crash history. 

South 

Dakota 

Turn down rail end 

treatments and TL-2 or 

better bridge rail

ADT < 150; rural collectors and local rural roads. 

Relevant Research

Bridge Curb and Railings for Extremely Low Volume Roads 

Bigelow et. al performed a statistical analysis of low-volume bridges in Iowa and found that 

there were “fewer than 350 crashes over an 8-year period” that occurred on the over 17,000 lowvolume bridges in the State.

(36) Of those 350 crashes, 31 (10 percent) were fatal or serious-injury 

crashes. Bigelow et al.’s statistical analysis found that the crash rate increased with decreasing 

bridge width and decreasing traffic volume. The crash rate for bridges with fewer than 50 vpd 

was twice that of bridges with 200 to 400 vpd. Similarly, bridges less than 20-ft wide had a crash 
13 

rate twice that of bridges that were between 24- and 28-ft wide. Bigelow et al. also found that a 

B/C analysis on a system-wide basis never resulted in a benefit-cost ratio greater than one where 

the alternative was to bring the railing “up to standard.” 

Seitz examined the issue of selecting bridge railing and approach guardrails for very low-volume 

roadways in Kansas.

(32) Of the 3,600 deficient bridges in Kansas, 2,500 (70 percent) were bridges 

with traffic volumes less than 50 vpd. There were 74 crashes on bridges located on rural minor 

collectors or rural local roads in Kansas in the period 2008 through 2012. Almost seven percent 

of these crashes were fatal or serious injury. Like Bigelow et al., Seitz found that a B/C analysis 

where the alternative was updating to w-beam bridge rail was never greater than one on a 

systemwide basis. 

Approach Guardrail for Bridges on Low-Volume Roads 

Gates et. al performed a statistical analysis of:

Ninety-six run-off-the-road crashes that occurred on the approach or departure to 68 county 

State-aid highway bridges in 10 Minnesota counties over a 15-year period.... None of the 33 

crashes with approach guardrail resulted in a fatality or severe injury, whereas roughly onequarter of the 63 crashes with a roadside bridge rail end resulted in a fatality or severe 

injury.(37)

The roadways in the analysis ranged from 16 to 41,524 vpd; with an average of 1,320 vpd and 

325 vpd for bridges with and without approach railing, respectively. Additionally, 75 percent of 

the bridges were located on roads with a speed limit of 55 mph, 14 percent were located on roads 

with 40-50 mph speed limits and 11 percent were located on roads with speed limits between 30-

35 mph.(37) Gates et. al concluded that the presence of approach guardrails lowers the severe 

crash rate except at bridges with the very lowest ADTs where crashes were infrequent. 

Analysis Methods used in Roadside Design Guidance Development

Encroachment-based modeling has a rich, long history in roadside safety for the consideration 

and comparison of design alternatives. The 1977 Barrier Guide presented a hand-calculation 

method which AASHTO updated in 1989 when the first RDG was published.

(25) Costeffectiveness procedures which were based on the encroachment probability model were 

provided in Appendix A of the 1989 AASHTO RDG along with a computer program called 

Roadside. The Benefit-Cost Analysis Program (BCAP) followed the program Roadside with a 

focus on selecting bridge railings using a cost-benefit encroachment estimation procedure.(25)

Advancements to encroachment-based modeling and computing culminated in the Roadside 

Safety Analysis Program (RSAP) in 2003.(38) Field-collected encroachment trajectories and 

updates to crash severity modeling lead to a third update to RSAP in 2012, RSAPv3.(39) Most 

recently, advancements in statistical modeling have permitted the development of a simplified 

representation of encroachment trajectories leading to the simplification of encroachment-based 

computer modeling codes and ultimately a reduction in simulation time.(40) NCHRP Project 15-

65, Development of Safety Performance Based Guidelines for the Roadside Design Guide, 

presents the current state-of-the-practice for encroachment-based modeling.(41)

The AASHTO RDG implies that the goal of roadside design is to reduce fatal and serious injury 

(KA) crashes. The objective of NCHRP Project 15-65 was to develop performance-based 
14 

roadside safety guidance to address high-priority needs that support quantitative design 

decisions, and that promote consistency in interpretation and implementation.

(41) NCHRP Project 

15-65 developed a methodology which estimates the risk of fatal and serious injury crashes for 

use in evaluating roadside designs.(41)

The NCHRP Project 15-65 framework provides a governing equation to represent the sequence 

of run-off-the-road events and subevents to develop roadside designs which minimize the 

OUTCOME (e.g., risk, cost, etc.) of a crash as shown below in figure 1.(41)

Figure 1. Equation. NCHRP 15-65 governing equation.

Where:

• OUTCOMES = The total number of crashes with the specified outcome on the segment 

involving all features on the segment. 

• OUTCOMEj = The number of crashes with the specified outcome involving feature j 

(e.g., the number of serious injury or fatal crashes involving impacts with a bridge curb) 

per edge mile per year.

• j = Feature number from 1 to n where n is the total number of features evaluated on the 

segment.

• BEFS = The expected annual number of encroachments expected on a segment in edge 

encroachments/mi/yr assuming base conditions as a function of traffic volume (AADT).

• EAFS = Highway and traffic characteristic encroachment adjustment factors for the 

highway segment of interest.

• LS = Segment length in feet. 

• Pcj = The conditional probability of a vehicle striking an object given an encroachment 

occurs. The length ratios are the probability of leaving the roadway in the given 

proportion of the roadway under the assumption that encroachments are equally likely 

anywhere on the segment. The form of Pcj depends on the type of object as shown below: 

Continuous Features (e.g., guardrails, curbs, edge of bridge, etc.) shown in figure 2.
15 

Figure 2. Equation. Continuous features. 

Discreet Features (e.g., trees, poles, bridge piers, water bodies, etc.) shown in figure 3. 

Figure 3. Equation. Direct features.

• PSEVj = The conditional probability of observing the severity of interest given that there is 

an interaction with roadside feature j.

• THRj = The conditional probability of passing through feature j given the vehicle 

interacts with feature j.

• δj = One if all interactions with the feature do not lead to an increase in harm (e.g., 

terrain); zero if all interactions with the feature lead to an increase in harm (e.g., 

longitudinal barrier, edge of bridge). 

• PSLs = Posted speed limit on the segment in mi/hr.

• Lj = The effective length of an individual feature j along the segment in ft.

• Py(Yj) = Cumulative probability density function of the lateral extent of encroachment 

when lateral offset y = Y.

• Px(Xj) = Sum of the cumulative probability density function of the maximum longitudinal 

extent of encroachment.

• WBj = The distance in feet from the edge of the traveled way measured laterally to the

farthest point of feature j plus VW cos(θ15). 

• WFj = The distance in feet from the edge of the traveled way to the closest face (i.e., 

traffic side) of feature j.

• LTMax = The length in ft of the longest trajectory in the data base of trajectories used to 

calculate Px(Xj) and Py(Yj) (i.e., 1,000 ft). 

• Vw = Typical passenger vehicle width in ft (e.g., 6.5 ft).

• θ15 = The 15th-percentile encroachment angle in degrees (e.g., 5 degrees(42)).

• θ85 = The 85th-percentile encroachment angle in degrees (e.g., 22 degrees(42)).

More details on the derivation of figure 1 can be found in the NCHRP Project 15-65 final 

report.(41) Recalling the goal of roadside design involves minimizing the frequency of KA runoff-road crashes, there are a variety of approaches that can be used after the frequency of KA 

crashes for each alternative has been determined using figure 1. 

Risk is the proportion of poor outcomes to all outcomes. Relative risk can be used to determine if 

the risk of the null alternative is greater or less than the risk of the alternatives under 

consideration.

(43-45) The null alternative is the existing conditions of the roadside being evaluated. 
16 

The B/C method used by programs like Roadside, BCAP, RSAP, and RSAPv3 estimate the 

societal cost of crashes through estimating the risk of crashes for the modeled alternatives. The 

societal costs of crashes are then compared to the agency costs such as construction, 

maintenance, and repair for each alternative over the life of the project. The benefit is usually 

considered to be the reduction in societal costs associated with each alternative compared to the 

direct costs (i.e., the construction, maintenance, and repair costs). A B/C ratio greater than one 

indicates the alternative should be considered. 

Cost effectiveness analysis is similar to B/C analysis but instead of monetizing the societal cost 

of the crash reduction (i.e., the benefit), the number of fatal and serious injury crashes avoided is

used. An incremental cost effectiveness ratio (ICER) compares the annualized direct costs of the 

design alternative (i.e., construction, maintenance, repair) to the annual reduction in the number 

of KA crashes. The ICER is defined as follows in figure 4:

Figure 4. Equation. ICER.

Where:

• ICERi/j = Incremental cost effectiveness ratio of alternative j with respect to alternative i.

• POi, POj = Performance outcome for alternatives i and j over the project life.

• DCi , DCj = Annualized direct costs for alternatives i and j.

The ICER can be thought of as the annual cost of the design alternative required to avoid one KA 

crash. For example, one bridge rail alternative may have an annualized cost of construction, 

maintenance, and repair of $5,000 to avoid one KA crash per year. Another alternative may have 

an annualized cost of $10,000. The alternative with the lower ICER would be the preferred 

alternative since it accomplishes the objective of avoiding one KA crash at a lower cost. The 

selected alternative may or may not be cost-beneficial but choosing the lowest ICER promotes 

the best use of scarce agency funds. The ICER could be calculated for various bridges and used 

to prioritize improvements to those bridges. 

Forest Service Manual FSM 7700

Forest Service requirements for the design and construction of bridges are included in Chapter 

7720 (Transportation System Development) of the Forest Service Manual 7700 (Travel 

Management) and Chapter 72 (Design Requirements) of FHS 7709.56b.(46, 47) FSM 7722.03 

indicates that all bridge designs be consistent with the AASHTO LRFD bridge design 

specifications.

(48) Requirements for traffic barriers and bridge railings are discussed in FSM 

7722.12 and FSH 7709.56b Section 72.3. 

FSM 7722.12 (Traffic Barriers and Approach Guardrails) indicates that the AASHTO guidelines 

for VLV roads and RDG be used. (24, 26) Further, Section 7722.12 states that “all road bridges 

must be designed with a traffic barrier system that has either been successfully crash-tested to a 

currently acceptable TL per the 2009 AASHTO MASH.” (14) Bridges are categorized into 

maintenance levels 1 through 6 which are then associated with MASH test levels as shown in 
17 

table 4. Approach guardrails are not required for the TL1 systems but must be used for TL2 and 

TL-3 if the roadway approach geometry allows. 

Table 4. Association of maintenance levels and MASH test levels.

(47)

Maintenance Level (ML) Minimum Required Test Level (TL)

MLs 1 and 2 TL 1

ML 3 with design speed ≤ 30 miles per hour TL 1

ML 3 with design speed > 30 miles per hour TL 2

MLs 4 and 5 TL 3

Section 72.3 of FSH 7709.56b (Roads Bridge Traffic Barriers) also discusses bridge railings. 

Like FSM 7722.12, it references the AASHTO LRFD bridge design specifications, the VLV

Guide, RDG, and MASH.(14, 24, 26, 41, 49) Crash tested bridge railings are required and the test 

level and maintenance level associates shown in table 4 are reiterated. Approach guardrail for 

bridges is discussed in Section 72.4 (Approach Guardrail for Road Bridges).

As discussed above, the Forest Service requirements explicitly cite the relevant AASHTO 

documents. 

Manual on Uniform Traffic Control Devices 

Chapter 5 of the Manual on Uniform Traffic Control Devices (MUTCD) addresses traffic control 

devices for low-volume roads. (50) Chapter 5 provides guidance for bridge signage for narrow 

bridges, one-lane bridges and object markers, and barricades for the ends of roads. (50)

When objects are located adjacent to the roadway, such as bridge abutments or the ends of traffic 

barriers, the MUTCD requires that a Type 3 marker be used. Type 3 markers are “vertical 

rectangles with alternating black and retroreflective yellow stripes sloping downward at an angle 

of 45 degrees towards the side of the obstruction on which traffic is to pass.”(50) The MUTCD 

provides the following guidance for object markers mounted on posts: when marking objects 

within the roadway or closer than 8 ft from the shoulder, the minimum mounting height from the 

surface of the road way to the bottom of the object marker should be 4 ft. It goes on to provide 

the option that “when object markers are applied to an obstruction that by its nature requires a 

lower or higher mounting, the vertical mounting height may vary according to need.”(50)

Type 3 objects markers are applied to roadside appurtenances. Type 3 object markers may be 

mounted directly to a bridge abutment or rail end or mounted on a signpost. If mounted directly 

on the appurtenance the marker height will be dictated by the geometry of the appurtenance, if 

mounted on a sign post the height to the bottom of the object marker should be 4 ft. 

Summary

The proceeding review of literature and crash data studies compiles the existing guidance for 

bridge rail design on low- and VLV roads. There is some variety in the ways each State 

addresses low-volume roads. Guidance for extremely low volumes (i.e., less than 50 vpd) and 

low-speed roads is limited. One observation that is repeated throughout this literature review, 

however, is that upgrading bridge rails on bridges servicing low-volume roads is generally not 
18 

cost-beneficial. Conversely, however, there are crash risks associated with these bridges which 

must be balanced with the costs of improvements to these bridges. 

Available, crash tested hardware appropriate for use in low-speed situations for containing 

passenger vehicles was also gathered. While not overwhelming in scope, there is an assortment 

of rails, transitions, and terminals available for these low-speed situations. The Guide includes 

new construction drawings of these available solutions. 

PARAMETERS FOR USE OF BRIDGE RAILS AND ASSOCIATED HARDWARE

After developing the structure and layout for the Guide, the project team made recommendations 

for the various parameters of the Guide. This section documents the development of those 

parameters. 

Establishing Safety Performance Goals

A safety performance goal measures the reduction in risk to passenger vehicle occupants 

resulting from installing or improving a roadside feature. Roadside hardware should only be 

installed if it results in a reduced risk of fatal or serious passenger vehicle crashes (KA). 

Shielding a shallow drainage ditch with several hundred feet of guardrail and terminals, for 

example, may actually increase the risk since the guardrail is more likely to be struck because it 

is much longer than the culvert. On the other hand, installing a bridge railing to prevent vehicles 

from leaving the bridge and falling from a large height into a water-filled channel likely will 

reduce the risk of a fatal or serious injury crash. A safety performance goal (PG) is the amount of 

risk reduction necessary to justify implementing a proposed improvement. 

The project team considered relative risk reduction and benefit cost PGs. Given the previously 

observed challenges with using a B/C approach, the project team recommended a relative risk 

reduction approach. 

Bridge Railings 

This project was concerned with evaluating bridge railings on extremely low-volume, low-speed 

roadways. In particular, three speed categories were considered (i.e., 5-15 mph, 16-30 mph, and 

31-45 mph) and all the bridges experienced a traffic volume of 50 vpd or less. Ray et al. recently 

developed a risk-based method for examining roadside designs.(51) The method predicts the 

number of expected fatal and serious-injury crashes for a roadside design based on statistical 

models that incorporated the traffic characteristics and placement of roadside features. The 

expected number of fatal and serious injury crashes are called the OUTCOME of the design. If a 

proposed roadside design results in fewer fatal and serious-injury crashes than the existing 

design, the following equation will hold true (as shown in figure 5): 

Figure 5. Equation. Risk of proposed to existing crashes.

 
19 

where

OUTCOME = The expected annual frequency of fatal and serious-injury crashes for 

the roadside design.

ENCR = The expected number of vehicle encroachments onto one roadside 

edge. 

CRASH = The conditional probability of a crash with a roadside feature given an 

encroachment occurs.

SEVERITY = The conditional probability of a fatal or serious injury given that a 

crash occurs.

Each of the three terms (i.e., ENCR, CRASH, and SEVERITY) are estimated by detailed 

mathematical and statistical models presented in the NCHRP 15-65 final report. (51) In the case of 

evaluating the need for a bridge railing, the ENCR and CRASH terms will be identical for both 

the existing design (e.g., no bridge railing) and the proposed design (e.g., install a crash tested a 

bridge railing). The ENCR term involves characteristics like the traffic volume, posted speed 

limit, horizontal and vertical curvature, and other similar roadway characteristics. Since none of 

these are changed in assessing the need for a bridge railing the ENCRPROPOSED will be equal to 

ENCREXISTING. Similarly, the CRASH term involves the location and type of roadside features. 

Since the bridge railing is located at the edge of the bridge deck both CRASHPROPOSED and 

CRASHEXISTING will be equal. Evaluating the need for bridge railing, then, is only a function of 

the ratio of the SEVERITYPROPOSED to SEVERITYEXISTING. The risk reduction (RR) ratio of 

OUTCOMEs for evaluating bridge railings is simply (as shown in figure 6): 

Figure 6. Equation. Relative risk of bridge rail to no bridge rail.

This ratio for the case of evaluating bridge railings is the relative risk and should be less than 

unity to consider installing a bridge railing. Another related term is the relative risk reduction

(RRR) which is given by figure 7: 

Figure 7. Equation. Relative risk reduction of bridge rail to no bridge rail.

The PG is the particular value of RRR needed to justify the construction of a bridge railing. 

Calculating the RRR for evaluating bridge railing need involves calculating the crash severity of 

the following:

• A vehicle falling from an unshielded bridge deck onto a dry or water-covered surface 

below the bridge from various heights. 

• A vehicle striking and penetrating through a bridge railing onto a dry or water-covered 

surface below the bridge from various heights.

• A vehicle striking a crash tested bridge railing.
20 

In addition, the following simplifying assumptions were made in the analysis:

• A bridge railing that has not been evaluated in full-scale crash tests is assumed to be 

effective in containing passenger vehicle crashes half the time.

• A crash tested bridge railing will always contain and redirect a passenger vehicle (i.e., the 

chance of penetrating a crash tested bridge railing is so small that it may be neglected). 

It is recognized that neither of these assumptions will be true under all impact conditions, 

however, the assumptions were necessary due to the lack of data to quantify either effectively. 

SEVERITY is expressed as the proportion of fatal and serious injury crashes (KA) with respect 

to all crashes involving the feature. The proportion of fatal and serious crashes (KA) are adjusted 

to a baseline speed of 65 mph based on a method proposed by Ray and Carrigan as shown in 

figure 8: (52)

Figure 8. Equation. The condition probability of a fatal or serious injury given that a crash occurs. 

where

KA = The proportion of fatal and serious injury crashes involving feature j 

with respect to all crashes with feature j adjusted to a baseline speed 

of 65 mph as per. (52)

PSL = Posted speed limit in mph. 

Carrigan found that the proportion of fatal and serious-injury crashes for rigid concrete bridge 

rails and median barriers was 0.0159 normalized to a posted speed limit of 65 mph. (53) There are 

no in-service evaluations of test TL-1 or TL-2 bridge railings, so this value is used as a estimate 

for bridge railings on low-speed, low-volume roadways. Similarly, Ray found that the proportion 

of fatal and serious injury crashes when vehicles were immersed in water was 0.0486 based on 

the 2002 to 2006 Washington State crash data. (39) In a study examining selection criteria for 

highway bridges, Ray proposed a risk of serious and fatal injury crashes of 0.0589 for 

environments under a bridge where only the vehicle occupants were at risk.

(17)

These values are shown in the bottom row of table 5. The severity values for bridge rail crashes 

(KABR) are scaled by the highest posted speed limit in the three speed limit categories as shown 

in the second column of table 5. It has been assumed that immersion in water or a drop of 20 ft or 

more is not a function of posted speed so all the severity values for water surfaces in the fourth 

and last columns of table 5 are the same. The severity value for a dry surface condition under the 

bridge for drops of 20 ft or less and a posted speed limit of 31-45 mph was found by scaling the 

low-risk environment value by the posted speed. 

Under bridge conditions with 6 or more inches of water have a higher risk than dry under bridge 

conditions when the drop is 20 ft or less because trapped vehicle occupants have been known to 

drown in water channels with as little as 6 inches of water.
21 

Table 5. Crash severity proportions for bridge features at various posted speed limits.

Posted 

Speed 

Limit 

(mph)

KABR KADRY KAWATER

Drop Height - H (ft)

H ≤ 20 20 < H ≤ 50 H > 50 H ≤ 20 20 < H ≤ 50 H > 50 

 5 - 15 0.0002 0.0002 0.2501 0.5000 0.0486 0.2743 0.5000 

16 - 30 0.0016 0.0099 0.2549 0.5000 0.0486 0.2743 0.5000 

31 - 45 0.0053 0.0195 0.2598 0.5000 0.0486 0.2743 0.5000 

 65 0.0159 0.0589 - - 0.0486 - - 

There are no data available that can be used to estimate the crash severity of passenger vehicles 

falling from a bridge deck onto a surface. Weckbach has summarized recent medical research on 

survivability in free falls from high drops as follows:

“The American College of Surgeons' Committee on Trauma (ACS-COT) defines 

a critical threshold for a fall height in adults as > 20 feet (6 meters), as part of the 

field triage decision scheme for transport to a designated trauma center. A 

retrospective analysis of 101 patients who survived vertical deceleration injuries 

revealed an average fall height of 23 feet and 7 inches (7.2 meters), confirming 

the notion that survivable injuries occur below the critical threshold of a falling 

height around 20-25 feet. A more recent study on 287 vertical fall victims 

revealed that falls from height of 8 stories (i.e., around 90-100 feet) and higher, 

are associated with a 100% mortality. Thus, a vertical falling height of more than 

100 feet is generally considered to constitute a "non-survivable" injury”. (54)

Turgut et al. also summarized the survivability of free falls from heights finding similar results. 

(55) Interestingly, the velocity of a free fall from 20 ft is ft/s which is just under 

the 40 ft/s maximum allowable occupant impact velocity used in crash testing guidelines like 

Report 350 and MASH. (12, 15) These results indicate that a person may survive jumping from a 

bridge with a 20-ft drop although there will likely be injuries. If a 20-ft drop is survivable for a 

person jumping off a height, a vehicle occupant should be no more at risk since they have the 

added benefit of passive safety systems (e.g., front and side airbags, seat belts, roof-crush 

protection, etc.). A vehicle dropping off a bridge 20 ft or less above a dry surface at a low posted 

speed limit (i.e., 5-15 mph) would likely not result in a fatal or serious injury based on the 

summaries of the biomedical literature presented by Weckbach and Turgut.(54) 

Lapostolle et al. also examined factors affecting survivability of falls from heights and found that 

the median height for a fatal fall was 49 ft and virtually all victims are fatally injured in a fall of 

85 ft or more. (56) Based on the median height for fatal injuries from Lapostolle et al., severity of 

passenger vehicles falling off bridges with drops of 50 ft or higher over either wet or dry surfaces 

was taken to be 0.5000 as shown in table 5. Ray examined bridge rail crash data from 
22 

Pennsylvania, Ohio, and Nebraska and found 38 bridge rail penetrations.

(17) The 38 bridge-rail 

penetrations involved: 

• 0 motorcycles. 

• 26 passenger cars. 

• 12 heavy vehicles. 

The 38 bridge-rail penetrations resulted in 5 fatal crashes, 13 A-injury crashes, 5 B-injury 

crashes, 6 C-injury crashes, 8 PDO crashes, and 1 crash of unknown severity. This resulted in 

crash severity of 0.4737 (i.e., (5+13)/38=0.4737). This value found by Ray (0.4737) confirms the 

recommendation of Lapostolle et al. from the biomechanics literature. Independently, the values

agree quite well. Values shown in an italic font in table 5 are interpolated from the values to the 

left and right. Given these crash severity estimates, risk reduction ratios can be calculated as 

shown in table 6.

Table 6 shows the relative risk reductions for bridge railings on low-speed, low-volume bridges. 

If the PG is that the RRR must be greater than 0, a bridge railing meets the PG for all conditions 

except bridges with a 20 ft or smaller drop to a dry under bridge environment. 

Table 6. RRR for bridge railings on bridges with posted speed limits of 45 mph or 

less and 50 vpd or less.

Posted 

Speed 

Limit 

(mi/hr)

Dry Environment Under Bridge 

(Condition A)

Water Under Bridge 

(Condition B)

Drop Height - H (ft)

H ≤ 20 20 < H ≤ 50 H > 50 H ≤ 20 20 < H ≤ 50 H > 50 

 5 - 15 0% 100% 100% 88% 100% 100% 

16 - 30 98% 99% 100% 88% 99% 100% 

31 - 45 97% 98% 99% 88% 98% 99% 

A = An ephemeral or intermittent stream channel with water depth less than six inches during the majority 

of the time the bridge is in use. 

B = Any stream channel with year-round flow or ephemeral or intermittent stream channels with water 

depths greater than or equal to six inches for at least three months.

Virtually all the situations studied indicate a bridge rail would satisfy the PG, except for heights 

equal to 20 ft or less, over a drive environment at posted speed limits of 5-15 mph. Using the 

posted speed limit grouping and the impact speeds used when developing the various Test levels, 

the selection guidelines shown in table 7 indicate which test level of rail is appropriate for which 

speed. 
23 

Table 7. TL selection for bridge rails on roads with ADT ≤ 50 vpd and posted 

speed ≤ 45 mph.

Posted 

Speed 

Limit 

(mph)

Dry Environment Under Bridge 

(Condition A)

Water Under Bridge 

(Condition B)

Drop Height - H (ft)

H ≤ 20 20 < H ≤ 50 H > 50 H ≤ 20 20 < H ≤ 50 H > 50 

5 - 15 NRB 12-in curb rail or TL-1 

16 - 30 TL-1 or higher bridge rail

31 - 45 TL-2 or higher bridge rail

Note: Surface condition under bridge: 

A = An ephemeral or intermittent stream channel with water depth less than six inches during the majority 

of the time the bridge is in use. 

B = Any stream channel with year-round flow or ephemeral or intermittent stream channels with water 

depths greater than or equal to six inches for at least three months.

NRB =Not risk beneficial.

Approach Terminals and Transitions

Establishing a, PG for the terminals and transitions on the approach to an extremely low-volume, 

low-speed bridge follows a similar path to a bridge railing PG. 

There are only a few MASH TL-1 and TL-2 guardrail terminals and there are no in-service 

performance evaluations (ISPEs) to determine the appropriate crash severity. Crash severity of 

Report 350 TL-3 terminals has been examined in a few studies. Ray found that less than five 

percent of crashes with Report 350 tangent guardrail terminals resulted in fatal or serious 

injuries.

(57) Similarly, Carrigan and Ray found that a little over five percent of crashes with 

Report 350 TL-3 flared guardrail terminals resulted in fatal or serious injuries.

(58) A 65-mph 

baseline crash severity of 0.0500 appears to be a reasonable assumption for TL-1 and 2 terminals 

based on these studies. 

The National Highway Traffic and Safety Administration (NHTSA) requires passenger vehicles 

meet the requirements of the Federal Motor Vehicle Safety Standards (FMVSS).

(59) Several of 

the FMVSS requirements are crash tests. FMVSS 208 requires that passenger vehicles pass 

safety requirements in a 35-mph full frontal crash into a flat rigid wall. Since the vehicle safety 

systems are designed to protect vehicle occupants in collision with rigid objects up to an impact 

speed of 35 mph no terminal is necessary to shield the bridge rail end in the lower two speed 

categories (i.e., posted speed limits 30 mph or less).

There are no ISPEs of transitions but the severity of strong-post w-beam guardrail normalized to 

65 mph has been found to be 0.0071.(60) It is assumed that crash tested transitions have risk 

values similar to strong-post w-beam guardrails. One of the most rigid roadside features on the 

roadway are bridge piers. Ray found that the severity of bridge pier crashes normalized to 65 

mph is 0.0656.(49) The risk value for a rigid pier is used to conservatively represent striking the 
24 

end of a bridge railing. These 65-mph baseline risk values are shown in the bottom row of table 

8. 

Table 8. Crash severity proportions for terminals, transitions, and unshielded bridge rail ends at 

various posted speed limits.

Posted Speed 

Limit 

(mi/hr)

KAEXPOSED END KATERM KATRANS

 5 - 15 0.0000 0.0006 0.0000 

16 - 30 0.0000 0.0049 0.0000 

31 - 45 0.0218 0.0166 0.0024 

 65 0.0656 0.0500 0.0071 

As was done earlier for bridge railings, the relative risk reduction can be calculated from the 

values in table 8 as shown below in table 9 where the indication “NRB” indicates that a terminal 

or transition is not risk beneficial for the conditions.

If the posted speed limit is 30 mph or less and the rail is terminated at a post when a post and rail 

bridge rail system is used, the installation of new terminals and/or transitions is generally not risk 

beneficial. If the posted speed limit is greater than 30 mph, the installation of terminals and 

transitions are risk beneficial. 

Table 9. RRR for terminals, transitions, and unshielded bridge ends on bridges with posted speed 

limits of 45 mph or less and 50 vpd or less.

Posted Speed 

Limit 

(mi/hr)

Terminals Transition

 5 - 15 NRB NRB

16 - 30 NRB NRB

31 - 45 24% 89% 

 NRB = Not risk beneficial.

NEW HARDWARE DEVELOPED

One of the objectives of this project was to modify the MASH TL-1 low profile WVBR for 

installation on a lower strength bridge structure (e.g., 4x12 wood plank bridge deck), and then 

assess crash performance under MASH TL-1 conditions using finite element analysis (FEA).(15)

A summary of that effort is provided below. 

The WVBR is a 19.75-inch tall timber bridge rail that was developed and full-scale crash tested 

to MASH TL-1 by MwRSF in 2008.(61) Figure 9 shows an engineering drawing for the MASH 

TL-1 WVBR bridge rail (additional drawing details can be found in the Guide). The full-scale 

test involved the WVBR bridge rail mounted onto a transverse, nail-laminated timber deck

constructed from 2-by-6 pine boards. The impact conditions included a 2002 Dodge Ram 1500 

Quad Cab pickup with a gross static mass of 5,179-lb impacting the railing at 30.8 mph and 

impact angle of 26.1 deg. 
25 

A detailed FEA model of the baseline WVBR bridge rail including a portion of the bridge 

superstructure and deck was developed by the project team, as illustrated in figure 10. The 

project team then validated the FEA model against a full-scale crash test WVBR-1 using the 

procedures outlined in NCHRP Web-Document 179. Once the model was validated, the mount 

design was modified to accommodate installation of the WVBR bridge rail on lower strength 

bridge decks. 

The project team communicated with the project’s technical advisory committee (TAC) to 

identify the critical bridge deck design. The TAC included representatives from the Bureau of 

Reclamation, the Bureau of Indian Affairs, Bureau of Land Management, and National Parks 

Service (Great Smoky Mountain Park). Each of these organizations have an interest in bridge 

rails used on extremely low-traffic, low-speed timber bridges. The project team found that the 2-

by-4 laminated deck was the most common deck type among the user agencies, but that the 4-by12 plank deck was more critical for bridge rail mounting. Both decks are 3.5 inches thick. 

The project team then developed a finite element model of a portion of a bridge superstructure 

design including a 4-by-12 timber plank deck. The mount for the baseline WVBR was modified 

to meet connection strength requirements for the lower strength deck design. The scope of this 

effort was limited to developing a modified mount; thus, the baseline bridge rail design (i.e., rail, 

scupper blocks and hardware) was not modified. It was determined from review of damages to 

the bridge deck during the static testing that the overall strength of the post-mount connection 

was likely reduced due to the malleable washers crushing into to deck boards during loading. 

Thus, a primary aspect of the modified design was to include a load distributor plate on the 

bottom surface of the deck to spread the forces over a larger area to prevent these local damages. 
26 

Source: FHWA

Figure 9. Graphic. Engineering drawing of the MASH TL-1 WVBR bridge rail.
27 

Source: FHWA

Figure 10. Graphic. FEA model of the MASH TL-1 WVBR bridge rail test article (oblique view).

Three alternative mount designs were developed and are illustrated in figures 11, 12, and 13. 

Each or these designs were shown to meet all performance criteria in MASH for TL-1 impact 

conditions. The barrier successfully contained and redirected the pickup with minimal damage to 

the system. There were no detached elements from the barrier that showed potential for 

penetrating the occupant compartment or presenting undue hazard to other traffic. The vehicle 

remained upright and very stable throughout impact and redirection. The occupant impact 

velocity (OIV) and maximum occupant ridedown accelerations (ORA) values were within 

recommended limits specified in MASH. Table 10 shows the results for the occupant risk 

calculations and includes additional metrics specified by the European Committee for 

Standardization. MASH evaluation criteria recommends that OIVs be less than 30 ft/s, and that 

ORAs be less than 15 g. 

Accordingly, the WVBR bridge rail with the each of the modified mount designs is considered 

applicable to other bridge structures with similar or greater deck-strength at the bridge rail 

mounting points (e.g., thicker wood decks, stronger materials, etc.). Design 3, which is shown in 

figure 13, provides the greatest strength and is the recommended mounting option for bridges 

with transverse plank decks. 
28 

Source: FHWA

Figure 11. Graphic. Design alternative 1 for modified WVBR for use on 4-by-12 plank decks.

Source: FHWA

Figure 12. Graphic. Design alternative 2 for modified WVBR for use on 4-by-12 plank decks.
29 

Source: FHWA

Figure 13. Graphic. Design alternative 3 (recommended) for modified WVBR design for use on 4-

by-12 plank decks. 
30 

Table 10. Occupant risk metrics computed using TRAP software for the baseline and modified 

mount cases. 

Occupant Risk Factors MASH 1-11 

Baseline D1 D2 D3 (26" OH)

Occupant 

Impact 

Velocity 

xdirection 11.8 11.5 11.8 11.5 

(ft/s) ydirection 13.8 13.5 13.5 13.1 

 at time at 0.1609 seconds on 

right side of interior

at 0.1638 seconds on right 

side of interior

at 0.1639 seconds on right 

side of interior

at 0.1550 seconds 

on right side of 

interior

THIV 18.0 17.1 17.4 17.7 

(ft/s) at 0.1554 seconds on 

right side of interior

at 0.1579 seconds on 

right side of interior

at 0.1581 seconds on right 

side of interior

at 0.1550 seconds 

on right side of 

interior

Ridedown 

Acceleration xdirection

-5.4 -5.9 -5.8 -5.2 

(g's) (0.5422 - 0.5522 

seconds) 

(0.1865 - 0.1965 

seconds) (0.2224 - 0.2324 seconds) (0.5535 - 0.5635 seconds) 

 ydirection-3.3 -3.2 -3.9 -3 

 (0.6418 - 0.6518 

seconds) 

(0.2269 - 0.2369 

seconds) (0.2004 - 0.2104 seconds) (0.5496 - 0.5596 seconds) 

PHD 5.7 6 6.2 5.7 

(g's) (0.5422 - 0.5522 

seconds) 

(0.1866 - 0.1966 

seconds) (0.2226 - 0.2326 seconds) (0.5495 - 0.5595 seconds) 

ASI

0.46 0.45 0.45 0.47 

(0.0508 - 0.1008 

seconds) 

(0.0507 - 0.1007 

seconds) (0.0502 - 0.1002 seconds) (0.0517 - 0.1017 seconds) 

Max 50-ms 

moving avg. 

acc. xdirection

-3.3 -3.2 -3.2 -3.3 

(g's) (0.0183 - 0.0683 

seconds) 

(0.0184 - 0.0684 

seconds) (0.0187 - 0.0687 seconds) (0.0185 - 0.0685 seconds) 

 ydirection-3.4 -3.4 -3.4 -3.6 

 (0.0864 - 0.1364 

seconds) 

(0.0901 - 0.1401 

seconds) (0.0839 - 0.1339 seconds) (0.0930 - 0.1430 seconds) 

 zdirection

-3.3 -3 2.6 -2.4 

 

(0.5140 - 0.5640 

seconds) 

(0.5468 - 0.5968 

seconds) (0.5462 - 0.5962 seconds) (0.5397 - 0.5897 seconds) 

Maximum 

Angular 

Disp.

(deg) 

Roll 8.4 8.2 8.6 8.2 

(0.7132 seconds) (0.7268 seconds) (0.7947 seconds) (0.7326 seconds)

Pitch -3.5 -3.7 -3.6 -3.8 

(0.7330 seconds) (0.6669 seconds) (0.7947 seconds) (0.5592 seconds)

Yaw -35.5 -34 -33.6 -35 

(0.5842 seconds) (0.6098 seconds) (0.6158 seconds) (0.5559 seconds)
31 

The project team also performed supplemental evaluations to determine the performance 

threshold for a 12-inch-tall timber railing regarding successful containment and redirection of a 

5,000-lb pickup. Although these lower profile railings do not meet MASH TL-1, it was of 

interest to the TAC to determine the limits for impact conditions at which these low-profile 

barriers will successfully contain and redirect the 2200P vehicle (i.e., 5,000-lb quad-cab pickup). 

This information may be useful for bridge owners in assessing “risk” for bridge design options 

(e.g., to widen bridge or not) for these low-profile railing/curbs which are used on many narrow, 

single-lane bridges to support farming and logging industry. These bridges often include lowprofile curbs with no other railing features. Taller curbs or railing designs are not applicable in 

these cases because they would make direct contact with the oversized loads which are often 

hauled across these bridges.

For the evaluations, the WVBR bridge rail was modified by reducing the height of the scupper 

blocks to achieve an overall height of 12 inches for the bridge rail measured from the deck 

surface to the top of the railing, as illustrated in figure 14. The 4-by-12 plank deck (i.e., the 

critical deck type) was also used for these evaluations.

Source: FHWA

Figure 14. Graphic. Engineering drawing for 12-inch timber curb/rail used in analysis.

Due to the low impact severity for these cases, the only safety metric of concern was the 

tendency for the vehicle to override the barrier. Table 11 shows the results of each analysis case 

indicating a pass (i.e., vehicle containment) or an override condition. Sequential views for select 

analysis cases are shown in figures 15 through 20. Additional results including sequential views 

are provided in the Guide. 
32 

Table 11. Summary of results for low-profile curb/rail cases.

Impact Speed

Impact Angle

15 Degrees 25 Degrees

10 mph Pass Expected*

Partial Redirection/

Partial Override

15 mph Pass Complete Override

20 mph Pass Complete Override

25 mph Borderline Pass Complete Override 

Expected* 

30 mph Override Expected* Complete Override 

Expected* 

* Analysis not conducted 

Source: FHWA

Figure 15. Graphic. Sequential views of impact at 15 degrees and 15 mph.

Source: FHWA

Figure 16. Graphic. Sequential views of impact at 15 degrees and 20 mph. 
33 

Source: FHWA

Figure 17. Graphic. Sequential views of impact at 15 degrees and 25 mph.

Source: FHWA

Figure 18. Graphic. Sequential views of impact at 25 degrees and 10 mph.

Source: FHWA

Figure 19. Graphic. Sequential views of impact at 25 degrees and 15 mph.

Source: FHWA

Figure 20. Graphic. Sequential views of impact at 25 degrees and 20 mph.
34 

The results indicated that the 12-inch-tall curb/railing would successfully contain and redirect the 

5,000-lb pickup at impact speeds of 20 mph or less when the impact angle was less than or equal 

to 15 degrees. For the 15-degree impact cases that resulted in successful containment of the 

vehicle, the maximum impact severity was 4,475 ft-lb. The analyses also indicated that the 12-

inch-tall curb/railing would successfully contain and redirect the pickup when impact speeds 

were less than 10 mph for an impact angle of 25 degrees. For the 25-degree impact cases that 

resulted in successful containment, the maximum impact severity was 2,983 ft-lb. It was 

therefore concluded that impact severity does not appear to correlate well with the pass/fail 

conditions for the low-profile barrier. 

This is likely because the stiffness and strength of the barrier were not governing factors for 

successful containment of the vehicle in the evaluations. Success or failure was predominately 

determined by the tendency for the tire to climb the barrier during impact, which occurred more 

prevalently at higher impact angles due to the tire tread having more contact with the railing. It is 

expected that other tire related factors will also affect the tendency for vehicles to override low 

profile curb/railing barrier. For example, larger diameter tires and/or tires with lower inflation 

pressure would likely have higher tendency to override. The project team suggests that additional 

testing and analysis be performed to further validate the steer response for the model and to also 

verify the results presented herein regarding the performance limits of the 12-inch-tall 

curb/railing. 
35 

REFERENCES

1. Coghlan, G.T., Opportunities for Low-Volume Roads, in TRB A5002: Committee on LowVolume Roads. 1999, TRB. 

2. Stephens, L.B., I. PerformTech, and F.H. Administration, Barrier Guide for Low Volume 

and Low Speed Roads. 2005. p. 151. 

3. Ray, M., et al., Developing Selection Tables for Bridge Railing. 2013: researchgate.net.

4. AASHTO, Standard Specifications for Highway Bridges. 1961: Washington DC. 

5. AASHTO, Standard Specifications for Highway Bridges. 1965, Washington D.C.: 

American Association of State Highway and Transportation Officials.

6. AASHTO, Standard Specifications for Highway Bridges. 1965: Washington D.C. 

7. Olson, R.M. and E.R. Post, Tentative Service Requirements for Bridge Rail Systems. 

1970, Transportation Research Board, National Cooperative Highway Research Program: 

Washington, D. C. . 

8. AASHTO, Guide Specifications for Bridge Railings. 1989, American Association of 

State, Highway and Transportation, Officials: Washington, D.C. p. 49. 

9. AASHTO, Standard Specifications for Highway Bridges. 14th edition ed. 1989, 

Washington, D. C.: American Association of State Highway and Transportation Officials. 

XLVI+420P. 

10. Noel, J.S., et al., Loads on Bridge Railings. Transportation Research Record, 1981. 796: 

p. 31-35. 

11. Beason, W.L. and T.J. Hirsch, Measurement of Heavy Vehicle Impact Forces and Inertia 

Properties,. 1989, Texas Transportation Institute,: College Station, TX. 

12. Ross, H.E., et al., Recommended Procedures for the Safety Performance Evaluation of 

Highway Features. 1993, Transportation Research Board, National Cooperative Highway 

Research Program: Washington, D.C. p. 142. 

13. Horne, D.W., ACTION: Crash Testing of Bridge Railings, F.H. Administration, Editor. 

1997, U. S. Department of Tranpsportation: Washington, D.C. 

14. AASHTO, Manual for Assessing Safety Hardware. 2009, American Association of State 

Highway and Transportation Officials: Washington, D.C. p. 259. 

15. AASHTO, Manual for Assessing Safety Hardware. 2016, American Association of State 

Highway and Transportation Officials: Washington, D.C. p. 259. 

16. Silvestri Dobrovolny, C., et al., Development and Evaluation of Concrete Barrier 

Containment Options for Errant Motorcycle Riders. Transportation Research Record: 

Journal of the Transportation Research Board, 2019. 2673(10): p. pp 14-24.

17. Ray, M.H. and C.E. Carrigan, Recommended Guidelines for the Selection of Test Levels 2 

Through 5 Bridge Rails. Vol. Web-Only Document 207. 2014, Washington, D. C.: 

Transportation Research Board, National Cooperative Highway Research Program.
36 

18. Ray, M.H. and C.E. Carrigan, Recommended Guidelines for the Selection of Test Levels 2 

Through 5 Bridge Rails. Vol. (pending). 2014, Washington, D. C.: Transportation 

Research Board, National Cooperative Highway Research Program.

19. Chiara Silvestri-Dobrovolny, N.S., Sana Moran,Tyler Skinner, Roger Bligh, William 

Williams, MASH EQUIVALENCY OF NCHRP Report 350-APPROVED BRIDGE 

RAILINGS. 2017, Transportation Research Board: National Cooperative Research 

Program.

20. R. K. Faller, B.T.R.a.M.D.F., Top-Mounted W-Beam Bridge Railing for Longitudinal 

Glulam Timber Decks Located on Low-Volume Roads. 1996, University of Nebraska. 

21. R. K. Faller, e.a., Railing Systems for Use of Timber Deck Bridges. Transportation 

Research Record, Transportation Research Board, Washington, D.C.,, 1996. 1656. 

22. M. A. Ritter and R. K. Faller, e.a., Plans for Crash-Tested Bridge Railings for 

Longitudinal Wood Decks on Low-Volume Roads. 1998, USDA Forest Products 

Laboratory. 

23. Mak, K.K. and D.L. Sicking, Evaluation of Performance Level Selection Criteria for 

Bridge Railings. 1994, National Cooperative Highway Research Program: Washington, 

D.C.

24. AASHTO, Roadside Design Guide. 4th ed, ed. AASHTO. 2011, Washington, D.C: 

American Association of State Highway and Transportation Officials.

25. AASHTO, Roadside Design Guide. 1st ed. 1989, Washington, D.C: American 

Association of State Highway and Transportation Officials.

26. AASHTO, Guidelines for geometric design of very low-volume local roads (ADT < 400). 

Guidelines for Geometric Design of Very Low-Volume Local Roads. 2001: AASHTO.

27. AASHTO, Roadside Design Guide. 3rd ed. 2002, Washington D.C.: American 

Association of State Highway and Transportation Officials.

28. Reclamation, B.o., Guidelines for Determining the Adequacy of Existing Bridge Traffic 

Safety Features at Reclamation's Bridges (6/06). 2006, U.S. Department of the Interior. 

29. IDOT, Bureau of Local Roads and Streets Manual, in Chapter 35, Roadside Safety. 2007. 

30. IowaDOT. Instructional Memorandum 3.213 Traffic Barriers (Guardrail and Bridge 

Rail). 2013 07/18/2013 [cited 2020 09/21/2020]; Available from: 

https://iowadot.gov/local_systems/publications/im/rev_2013-02.pdf. 

31. IowaDOT. Instructional Memorandum 3.230 Traffic Barriers (Guardrail and Bridge 

Barrier Rail). 2018 04/24/2018 [cited 2020 09/21/2020]; Available from: 

https://www.iowadot.gov/local_systems/publications/im/3230.pdf. 

32. Seitz, R.J., et al., Guardrail and Bridge Rail Recommendations for Very Low-Volume 

Local Roads in Kansas. 2014. p. 166. 

33. Seitz, R.J., T. Salfrank, and T. Kansas Department of, Guardrail and Bridge Rail 

Recommendations for Very Low-Volume Local Roads in Kansas: Addendum to Include 

Bridges Up to 100 Feet Long and Traffic Volume Up to 100 Vehicles Per Day. 2017. p. 

158. 
37 

34. MoDOT and E.P.G. contributors. 606.1 Guardrail. 2019 07/22/2019; Available from: 

https://epg.modot.org/index.php?title=606.1_Guardrail&oldid=45956. 

35. SDDOT, Local Roads Plan. 2011. 

36. Bigelow, J., et al., Bridge Rail and Approach Railing for Low-Volume Roads in Iowa. 

2010. p. 223. 

37. Gates, T.J., D.A. Noyce, and P.H. Stine, Safety and Cost-Effectiveness of Approach 

Guardrail for Bridges on Low-Volume Roads. Transportation Research Record: Journal 

of the Transportation Research Board, 2006(Issue 1967): p. 46-57. 

38. Mak, K.K. and D.L. Sicking, Roadside Safety Analysis Program (RSAP) -- Engineer's 

Manual. 2003, Transportation Research Board, Texas Transportation Institute: 

Washington, D.C. p. 74. 

39. Ray, M.H., C.E. Carrigan, and C.A. Plaxico, Roadside Safety Analysis Program (RSAP) 

Update: Engineer's Manual. Vol. (pending). 2012, Washington, D.C.: National 

Cooperative Highway Research Program, Transportation Research Board. 178. 

40. Carrigan, C.E., Recommended Guidelines for the Selection and Placement of Test Levels 

2 through 5 Median Barriers. 2020, National Cooperative Highway Research Program, 

Transportation Research Board: Washington, D.C. 

41. Ray, M.H., Development of Safety Performance Based Guidelines for the Roadside 

Design Guide [Project], in National Cooperative Highway Research Program. 2021 

Expected, Transportation Research Board, National Cooperative Highway Research 

Program: Washington, D.C. 

42. Gabler, H.C., Long-Term Roadside Crash Data Collection Program [Project]. 2022 

Expected, Virginia Polytechnic Institute and State University.

43. Irwig, L., et al., Smart Health Choices: Making Sense of Health Advice. Smart Health 

Choices: Making Sense of Health Advice. 2008, London: Hammersmith Press. 

44. Bonita, R., et al., Basic Epidemiology. 2 ed. 2006: World Health Organization. 

45. Higgins, J.P.T. and S.e. Green, Cochrane Handbook for Systematic Reviews of 

Interventions Version 5.1.0 [Updated March 2011], ed. J.P.T. Higgins and S. Green. 

2011: John Wiley & Sons.

46. FSH, Forest Service Handbook, F. Service, Editor. 2014, U. S. Forest Service: 

Washington, D.C. 

47. FSH, FSM 7700 Chapter 7720 - Transportation System Development, F. Service, Editor. 

2014, U. S. Forest Service: Washington, D.C. 

48. AASHTO, AASHTO LRFD Bridge Design Specifications. Eighth Edition ed. 2018, 

Washington, D.C.: American Association of State Highway and Transportation Officials. 

49. Ray, M.H., C.E. Carrigan, and C.A. Plaxico, Guidelines for Shielding Bridge Piers. Vol. 

NCHRP Report 892. 2018, Washington, D.C.: Report 892, National Cooperative 

Highway Research Program, Transportation Research Board. 

50. FHWA, Manual on Uniform Traffic Control Devices for Streets and Highways, 2009 

Edition Including Revision1 and Revision 2. 2012. p. 862. 
38 

51. Ray, M.H., C.E. Carrigan, and E.M. Ray, Development of Safety Performance Based 

Guidelines for the Roadside Design Guide in National Cooperative Highway Research 

Program. 2022 [expected], Transportation Research Board, National Cooperative 

Highway Research Program: Washington, D.C. 

52. Ray, M.H., C.E. Carrigan, and C.A. Plaxico, Method for Modeling Crash Severity with 

Observable Crash Data. Transportation Research Record: Journal of the Transportation 

Research Board, 2014(2437): p. pp 1–9. 

53. Carrigan, C.E., Recommended Guidelines for the Selection and Placement of Test Levels 

2 through 5 Median Barriers. 2022, National Cooperative Highway Research Program, 

Transportation Research Board: Washington, D.C. 

54. Weckbach, S., et al., Survival following a vertical free fall from 300 feet: the crucial role 

of body position to impact surface. Scandinavian journal of trauma, resuscitation and 

emergency medicine, 2011. 19(1): p. 1-8. 

55. Turgut, K., et al., Falls from height: a retrospective analysis. World journal of 

emergency medicine, 2018. 9(1): p. 46. 

56. Lapostolle, F., et al., Prognostic factors in victims of falls from height. Critical care 

medicine, 2005. 33(6): p. 1239-1242. 

57. Ray, M.H. and C.E. Carrigan, Meta-Analysis of the Risk of Fatal and Incapacitating 

Injury in Tangent W-Beam Guardrail Terminal Collisions, in International Conference 

on Transportation and Development 2018. 2018, American Society of Civil Engineers. p. 

141-151. 

58. Carrigan, C.E., M.H. Ray, and A.M. Ray, Evaluating the Performance of Roadside 

Hardware, in 96th Annual Meeting of the Transportation Research Board. 2017, 

Transportation Research Board: Compendium of Papers: Washington, D.C. p. 16p. 

59. NHTSA. Test Procedures. 2021 [cited 2021 7 April 2021]; Available from: 

https://www.nhtsa.gov/vehicle-manufacturers/test-procedures. 

60. Ray, M.H. and C.E. Carrigan, Roadside Hardware Replacement Analysis: User Guide. 

2021, National Cooperative Highway Research Program: Washington, D.C. 

61. Rosenbaugh, S.K., et al., Development of a TL-1 Timber, Curb-Type, Bridge Railing for 

use on Transverse, Nail-Laminated, Timber Bridges. 2009.