USE OF NARROW LANES AND

NARROW SHOULDERS ON FREEWAYS:

A Primer on Experiences, Current Practice, and Implementation Considerations

PRACTICAL DESIGN FOR OPERATIONS
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Cover Photo Source: Kittelson & Associates, Inc.
Neudorff, L. (CH2M), Jenior, P., Dowling, R., Nevers, B. 

(Kittelson & Associates, Inc.)

CH2M

901 New York Ave NW 

Washington, DC 20001

Under contract to:

Kittelson & Associates Inc.

300 M Street SE, Suite 810

Washington, DC 20001

Technical Report

Informational Report

Sept 2014 to May 2016

FHWA

Congested freeways are often located in urban areas with constrained environments and/or rights-of-way where significant widening of the roadway 

is not practical due to adjacent developments and land use, physical constraints, along with limited availability of funding. Among the strategies for 

increasing freeway capacity in such constrained environments – and thereby reducing congestion and improving operations – is to add a travel lane 

within the existing roadway footprint by reducing the widths of the existing lanes and/or shoulders. The additional lane may be utilized by all traffic at 

all times, as a special use or managed lane that is open only to specific types of vehicles or movements (e.g., High Occupancy Toll (HOT) lane, exit 

only lane), or only during selected times of the day and/or when congestion warrants opening the lane (e.g., temporary shoulder use). Narrow lanes 

and shoulders may be applied to add capacity on the freeway mainline and in interchange areas including ramps.

This primer provides information to policy makers, transportation agency managers, designers and operators on the use of narrow lanes and 

narrow shoulders to improve capacity within an existing roadway footprint. Much of the information contained in the primer is presented in the 

broader context of both Performance Based Planning and Programming (PBPP) and Performance – Based Practical Design (PBPD). Primer 

contents include case studies on the use of narrow lanes, issues and approaches for analyzing the operational and safety impacts of narrow lanes 

and narrow shoulders, and the role of transportation systems management and operations (TSMO) in support of narrow lanes operations.

Publication

FHWA HOP-16-060

Use of Narrow Lanes and Narrow Shoulders on Freeways:

A Primer on Experiences, Current Practice, and Implementation 

Considerations

July 2016

Jim Hunt (jim.hunt@dot.gov) served as the Technical Manager for FHWA

Performance-Based Practical Design, Lane Width, 

Freeway Design, Shoulder Width

60

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha 

mi2 square miles 2.59 square kilometers km2

VOLUME

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3

yd3 cubic yards 0.765 cubic meters m3

NOTE: volumes greater than 1000 L shall be shown in m3

MASS oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius oC

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx 

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH

mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2 square millimeters 0.0016 square inches in2

m2 square meters 10.764 square feet ft2

m2 square meters 1.195 square yards yd2

ha hectares 2.47 acres ac 

km2 square kilometers 0.386 square miles mi2

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3

m3 cubic meters 1.307 cubic yards yd3

MASS

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit oF

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for th International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E e 380. 

(Revised March 2003)

Conversion Factors

7

Chapter 1 Overview 11

Chapter 2 Performance Based Practical Design

 and the Role of Operations 17

Performance Based Practical Design 17

Transportation Systems Management and Operations 23

Chapter 3 Case Studies of Narrow Lanes and

 Narrow Shoulders 29

Narrow Lanes as Part of Freeway Modernization along I-94 in Milwaukee, Wisconsin 29

Narrow Lanes to Accommodate Additional Managed Lane

along I-95 in Miami-Dade, Florida 31

Narrow Lanes to Accommodate Part Time Shoulder Use

on US 2 in Washington State 33

Junction Control in Los Angeles, California 34

Chapter 4 Analyzing the Effects of Narrow Lane

 and Shoulder Width 35

Operational Effects 35

Safety 39

Reliability 49

Chapter 5 Summary and Conclusions 51

Appendix 53

References 57

Table of Contents
8

List of Figures

Figure 1. Diagrams. Example Narrow Lanes and Shoulder Configurations. 14

Figure 2. Diagram. Framework for Performance-Based Planning and Programming. 18

Figure 3. Diagram. Concepts and Activities Associated with

 Performance-Based Practical Design. 19

Figure 4. Diagram. Potential Impact on Sight Distance From Moving the

 Left-Most Travel Lane To the Inside. 22

Figure 5. Flowchart. An Objectives-Driven, Performance-Based Approach. 23

Figure 6. Diagram. Example of Dynamic Speed Limits and Dynamic

 Lane Assignment in Support of Narrow Lanes and Shoulders. 26

Figure 7. Diagram. Overlapping Relationship between Performance-Based Practical

 Design, Performance-Based Planning and Programming,

 Transportation System Management and Operations, Context

 Sensitive Solutions, and Value Engineering. 27

Figure 8. Picture. Milwaukee, Wisconsin Case Study Project Area. 30

Figure 9. Picture. I-95 Express Lanes in Miami-Dade, Florida. 32

Figure 10. Picture. Shoulder Use Along US 2 in Washington State. 33

Figure 11. Graph. Change in Crash Rate On US-2 in Washington State. 33

Figure 12. Pictures. Ramp Configuration and Mainline Signage

 for Junction Control in Los Angeles, California. 34

Figure 13. Diagram. Example Conversion from 3 Lanes to 4 Lanes. 37

Figure 14. Graph. Nominal and Substantive Safety. 40

Figure 15. Graph. Crash Modification Factors for Lane Widths. 42

Figure 16. Graph. Crash Modification Factors for Inside Shoulder Widths. 42

Figure 17. Graph. Crash Modification Factors for Outside Shoulder Widths. 43

Figure 18. Graph. Predicted Crash Frequency With and Without Narrow Lanes 

 and Narrow Shoulders (Conversion from 4-lane Freeway to 6-lane Freeway). 44

Figure 19. Graph. Predicted Crash Frequency With and Without Narrow Lanes 

 and Narrow Shoulders (Conversion from 6-lane Freeway to 8-lane Freeway). 44

Figure 20. Graph. Predicted Crash Frequency With and Without Narrow Lanes 

 and Narrow Shoulders (Conversion from 8-lane Freeway to 10-lane Freeway). 45

Figure 21. Graph. Predicted Percent Change in Crash Frequency when Adding

 a Lane by Using the Right Shoulder and Narrowing Lanes. 46

List of Tables

Table 1. Examples: Narrowing Lanes to Add a General Purpose Lane. 13

Table 2. Examples: Narrowing Lanes to Add a Managed Lane. 13

Table 3. Examples: Narrowing Lanes to Add a Lane in the Vicinity of an Interchange. 13

Table 4. Recommended Shoulder Widths for Freeways. 22

Table 5. Transportation System Operations and Management Strategies Typically Used On 

Freeways and the Potential Relationships to Performance-Based Practical Design. 24

Table 6. Average Travel Speeds Before and After I-95 Conversion. 32

Table 7. Highway Capacity Manual Adjustment to Free-Flow Speed for Lane Width. 36

Table 8. Highway Capacity Manual Reductions in Free-Flow Speed for Right-Side

 Lateral Clearance on Freeways. 36

Table 9. Highway Capacity Manual Exhibit 11-17 (Maximum Service Flow

 Rates for Basic Freeway Segments). 37

Table 10. Inputs for Narrow Lane / Shoulder Alternative. 43

Table 11. Examples of Safety Benefits of Transportation System Management and Operations

Strategies. 48
List of Acronyms and Symbols

AADT Average Annual Daily Traffic

CMF Crash Modification Factors 

CMP Congestion Management Process (Federal guidelines) or

 Congestion Management Program (locally or regionally adopted)

DOT Department of Transportation

FFS Free Flow Speed

FHP Florida Highway Patrol

FHWA Federal Highway Administration

FI Fatal and Injury

GPS Global Positioning System

GIS Geographic Information Systems

HCM Highway Capacity Manual

HOT High Occupancy Toll

HOV High Occupancy Vehicle

ITS Intelligent Transportation Systems

LOS Level of Service

MPO Metropolitan Planning Organization

MTP Metropolitan Transportation Plan

NHS National Highway System

NPMRDS National Performance Management Research Data Set

PCE Performance Car Equivalent

PDO Property Damage Only Crashes

PeMS (California’s) Performance Measurement System

PBPD Performance-Based Practical Design 

PBPP Performance-Based Planning and Programming

PTI Planning Time Index

SHRP 2 The Second Strategic Highway Research Program

SOV Single Occupant Vehicle

SPF Safety Performance Functions

TIP Transportation Improvement Program

TMA Transportation Management Area

TMC Transportation Management Center

TSMO Transportation System Management and Operations

TTI Travel Time Index

V/C Volume to Capacity Ratio

VHD Vehicle Hours of Delay

VHT Vehicle Hours Traveled

VMT Vehicle Miles Traveled

9

Chapter 1 Overview

11

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Chapter 1 Overview

This primer is designed for policy makers, 

transportation agency managers, designers and 

operators working to find solutions to today’s 

transportation and mobility challenges. The purpose 

is to provide information on the use of narrow lanes 

and narrow shoulders to improve capacity within an 

existing roadway footprint, and to give the reader a 

starting point for exploring narrow lanes and shoulders 

as a potential solution.

Much of the information in this primer is presented 

in the broader context of Performance – Based 

Practical Design (PBPD). Per the Federal Highway 

Administration (FHWA) website (Reference 12):

PBPD is a decision making approach that helps 

agencies better manage transportation investments 

and serve system-level needs and performance 

priorities with limited resources. PBPD uses 

appropriate performance-analysis tools, considers both 

short and long term project and system goals while 

addressing project purpose and need.

The PBPD approach encourages designers and 

decision makers to exercise engineering judgment 

in identifying and analyzing alternatives - including 

narrow lanes - to deliver cost-effective operational 

improvements that meet both project and system 

objectives.

Topics covered in the primer include the following:

• Examples of narrow lane / narrow shoulder 

applications – Chapter 1

• Examining potential narrow lane / narrow shoulder 

solutions in the context of PBPD; and the role 

of Transportation Systems Management and 

Operations (TSMO) in supporting narrow lane 

applications – Chapter 2

• Case studies of the use of narrow lanes –

Chapter 3

• Issues and approaches for analyzing the 

operational and safety impacts of narrow lanes 

and narrow shoulders – Chapter 4

Congested freeways are often located in urban areas 

with constrained environments and/or rights-ofway where significant widening of the roadway is 

not practical due to the adjacent development and 

land use, physical constraints, along with limited 

available funding. Among the strategies for increasing 

freeway capacity in such constrained environments 

– and thereby reducing congestion and improving 

operations – is to add a travel lane within the existing 

roadway footprint by reducing the widths of the 

existing lanes and/or shoulder. The additional lane may 

be utilized by all traffic at all times, as a special use 

or managed lane that is open only to specific types 

of vehicles or movements (e.g., High Occupancy Toll 

(HOT) lane, exit only lane), or only during selected 

times of the day and/or when congestion warrants 

opening the lane (e.g., temporary shoulder use). 

Source: Kittelson & Associates, Inc.
U.S. Department of Transportation | Federal Highway Administration

12

Chapter 1 Overview

This concept of adding a travel lane within the 

existing roadway footprint (by narrowing existing 

lanes and/or shoulders) to increase capacity at a 

relative low cost is not a new concept. As noted in a 

1978 research document (1):

“When the congestion becomes so extensive and 

repetitive, measures to increase capacity or reduce 

demand should be undertaken. However, sufficient 

funds to make major changes to urban freeways may 

not be available, and in some instances, space may be 

so limited as to rule out normal expansions in roadway 

width. One approach that many transportation 

agencies are considering is the downscoping of 

design standards to achieve greater capacity at lower 

cost. The usual method to accomplish this is to reduce 

lane widths and to reduce or eliminate the roadway 

shoulders and create an additional lane for travel.”

Potential scenarios for implementing narrow 

lanes 1 include the following (with example lane 

configurations and widths resulting from the 

implementation of narrow lanes shown in Figure 1):

• Adding a general purpose lane to increase 

capacity and reduce recurring congestion. This 

can be for an extended section of roadway, or 

for a relatively short area as part of bottleneck 

reduction or to maintain lane continuity 2. 

Examples of this approach are listed in Table 1.

• Adding a managed lane, such a High Occupancy 

Vehicle (HOV) or HOT lane. Examples of this 

approach are listed in Table 2.

1 The additional lane and associated capacity may operate

 at all times or part time (e.g., peak periods).

2 Per the 3rd Edition of the AASHTO Highway Safety Design

 and Operations Guide, lane continuity allows a driver to

 remain in the through movement without changing lanes.

 Lack of lane continuity is usually the result of a change in

 the number of lanes, such as dropping a left lane at an exit

 without the use of an auxiliary lane. 

• Adding a lane in and/or within the vicinity of an 

interchange, to provide additional capacity on a 

ramp, an auxiliary lane between closely-spaced 

interchanges, or additional capacity beyond 

the interchange to prevent traffic from backing 

up into the interchange area. Examples of this 

approach are listed in Table 3.

A related application that can involve narrow lanes 

and narrow shoulders is to open either the left or 

right shoulder – as is, or perhaps widened (with a 

corresponding narrowing of general purpose lanes) 

– to traffic during selected times of the day or when 

congestion warrants. The shoulder may be open to 

all vehicles, only light-duty vehicles, or buses only. 

By definition, during times of shoulder use, there is 

no shoulder available for vehicle refuge. Additional 

information on part time shoulder use, including 

several locations where this strategy has been 

implemented, is provided in Reference 6 (“Use of 

Freeway Shoulders for Travel - Guide for Planning, 

Evaluating, and Designing Part-Time Shoulder Use 

as a Traffic Management Strategy; Publication No. 

FHWA-HOP-15-023).

Source: Kittelson & Associates, Inc.
Chapter 1 Overview

13

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Location Route(s) and Length Date Cross-Sections Reference(s)

Houston, Texas US 59 (Southwest 

Freeway)

Approx. 3.1 miles 

(both directions)

1976 4 lanes converted to 5 lanes

3 lanes converted to 4 lanes

(Refer to Figure 1)

1 

(Additional information 

provided in Appendix A) 

Honolulu, Hawaii H1

Approx. 3.5 miles 

(both directions)

2014 3 lanes converted to 4 lanes

(11.5 -12 ft. lanes re-striped 

to 10 ft. with reduced 

shoulder widths)

2

Northern Virginia I-395

(Approx. 1.5 miles NB; 

2.5 miles SB)

1989 3 lanes converted to 4 lanes

(12 foot lanes re-striped to 

11 ft., with reduced shoulder 

widths (2 ft. or less inside; 4 

to 10 ft. outside)

3

Milwaukee, Wisconsin I-94

Less than 1-mile

(both directions)

2015 

Planning

4 lanes to be narrowed from 

12 ft. to 11 ft., with narrow 

shoulders, due to R.O.W 

constraints, as part of a 

reconstruction project

6

(Refer to case study 

herein)

Table 1. Examples: Narrowing Lanes to Add a General Purpose Lane.

Location Route(s) and Length Date Cross-Sections Reference(s)

Los Angeles, California Multiple routes

Approximately 49 

miles

1993 5 lanes converted to 6 lanes

4 lanes converted to 5 lanes

Additional lane used as HOV 

in nearly all cases

(Refer to Figure 1)

1, 3, 4, 5, 16

(Additional information 

provided in Appendix A)

Miami – Dade, Florida I-95 and SR 826 Initial 

segment in 

2008

Ongoing

4 general purposed lanes 

+ HOV lane converted to 4 

lanes + 2HOT lanes

(Refer to Figure 1)

9

(Refer to case study 

herein)

Table 2. Examples: Narrowing Lanes to Add a Managed Lane.

Location Route(s) and Length Date Cross-Sections Reference(s)

Los Angeles, California NB SR 110 connector 

to NB I-5

2010 Connector ramp re-striped 

to provide two lanes, the 

second lane being the 

shoulder for part time use. 

Signage installed on SR 

110 to allow through and 

exit movements from a 

lane when connector ramp 

shoulder open to traffic.

7

(Refer to case study 

herein)

Everett, Washington US 2 from the I-5 / US 

2 Interchange EB to 

SR-204

(Approx. 1.6 miles) 

2009 2 general purpose lanes 

narrowed from 12 ft. to 11 ft., 

inside shoulder narrowed 

from 4 ft. to 2 ft., and 

outside shoulder widened 

from 10 ft. to 14 ft. (all via restriping). Shoulder opened 

to traffic during PM peak to 

prevent exiting traffic (from 

I-5 to US 2) from backing 

onto I-5, and to reduce 

crashes in the interchange.

(Refer to Figure 1)

10

(Refer to case study 

herein)

Table 3. Examples: Narrowing Lanes to Add a Lane in the Vicinity of an Interchange.
U.S. Department of Transportation | Federal Highway Administration

14

Chapter 1 Overview

Figure 1. Diagrams. Example Narrow Lanes and Shoulder Configurations.

Before After

Note: Three general purpose lanes converted to four general purpose lanes US 59 – Houston, Texas (Reference 1).

Before After

Note: Four general purpose lanes converted to five general purpose lanes US 59 – Houston, Texas (Reference 1).

Before After

Note: Four general purpose lanes and one High Occupancy Vehicle lane converted to four general purpose lanes and 

two High Occupancy Toll lanes Miami, Florida Typical (Reference 5).
Chapter 1 Overview

15

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Note: Four lanes total (two lanes in each direction) converted to five lanes total Inner Ring Expressway,

 Shanghai, China (Reference 11).

Figure 1. Diagrams. Example Narrow Lanes and Shoulder Configurations. (continuation)

Before After

Note: Four general purpose lanes converted to four general purpose lanes and an High Occupancy Vehicle lane

 Los Angeles, California Typical (Reference 5).

Before After

Note: Re-striping of general purpose lanes and inside shoulder to accommodate part time use of the outside shoulder 

Washington State US 2 (Reference 10).

Before After

Chapter 2 Performance Based Practical Design and the Role of Operations

17

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

PERFORMANCE-BASED 

PRACTICAL DESIGN

Performance-Based Practical Design (PBPD) modifies 

the traditional “top down, standards first” approach to 

a “design up” approach where designers and decision 

makers exercise engineering judgment to build up 

the roadway and operational improvements from 

existing conditions to meet both project and system 

objectives. PBPD uses appropriate analysis tools – 

such as those discussed in Chapter 4 – to evaluate the 

performance impacts of planning and design decisions 

in relation to the cost of providing various geometric 

elements and operational features.

PBPD should not be viewed as a stand-alone set of 

activities. Rather, it is an integral part of a broader 

process known as “Performance-Based Planning and 

Programming.” The Federal Highway Administration 

(FHWA) publication “Performance Based Planning and 

Programming Guidebook” (Reference 25) describes 

the application of performance management 

principles within the planning and programming 

processes of transportation agencies and regional 

entities (e.g., Metropolitan Planning Organization 

(MPOs)) to achieve desired performance outcomes for 

the multimodal transportation system. Figure 2 shows 

the Performance-Based Planning and Programming 

(PBPP) process, indicating where PBPD concepts and 

activities may be applied. As shown in Figure 2, PBPDrelated activities can be applied to the preliminary 

engineering and design activities, with any cost 

savings going to support additional projects as part of 

the regional programming process. 

PBPD concepts can also be used during planning 

activities to help identify strategies and analyze 

alternatives.

Figure 3 identifies and summarizes the various 

PBPD concepts and potential activities, starting with 

“baseline conditions” including design policies and 

guidelines, current and projected issues and needs, 

and stakeholder concerns; and then moving into 

analysis such as developing alternatives, analyzing 

these alternatives in terms of improved performance 

and costs, coupled with trade-offs and engineering 

judgment. The results of these PBPD-related activities 

and concepts (i.e., “Moving Forward”) is the selection 

of the optimal concepts and strategies for design, 

the identification of any design exceptions, and the 

documentation of the decisions. The optimal design 

concepts, along with any associated cost savings, are 

fed back into the PBPP framework.

 

As collectively shown in Figures 2 and 3, with PBPD 

designers apply a “design up” approach by using 

existing conditions as the baseline and engineer 

solutions that meet the project purpose based 

on explicitly defined transportation performance 

needs as derived from system and regional goals 

and objectives. This approach differs from a more 

conventional approach of setting project design 

criteria based solely on values listed in design 

specifications or standards for a set of given 

conditions. Designers then evaluate the solutions 

against the tradeoffs based on an objective analysis of 

performance data. Some of the tradeoffs considered 

include the estimated costs for each potential solution, 

coupled with due consideration of agency polices, 

legal requirements, stakeholder sensitivities, and any 

other potential constraints.

Chapter 2 Performance Based Practical Design and the Role of Operations

Source: Kittelson & Associates, Inc.
U.S. Department of Transportation | Federal Highway Administration

18

Chapter 2 Performance Based Practical Design and the Role of Operations

Figure 2. Diagram. Framework for Performance-Based Planning and Programming.

Note: Performance-Based Practical Design shown in orange.

(Source: Adapted from Federal Highway Administration document number HEP-13-041, Reference 25)
Chapter 2 Performance Based Practical Design and the Role of Operations

19

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Figure 3. Diagram. Concepts and Activities Associated with Performance-Based Practical Design.

(Figure is based on several Federal Highway Administration documents and

presentations on the subject of Performance-Based Practical Design)
U.S. Department of Transportation | Federal Highway Administration

20

Chapter 2 Performance Based Practical Design and the Role of Operations

A basic tenet of PBPD involves making project 

decisions that directly serve performance needs 

while considering whether the same investment of 

money would yield a greater return on investment 

if applied to other system needs and/or priorities. 

It is important to document the design decisions 

and present them to decision makers showing the 

benefits relative to the no-build option. These PBPD 

design and decision-making analyses can easily be 

transferred to design exception forms for review, 

approval and record-keeping.

By implementing a PBPD approach, agencies 

may reduce or eliminate project elements that are 

determined to be non-essential, resulting in lower 

cost and improved value by taking advantage of 

existing design flexibility. Agencies may also use the 

associated cost saving to deliver a greater number of 

projects that yield a greater performance return on 

investment than otherwise possible under existing 

project development and design approaches.

Relationship Between PBPD and Context 

Sensitive Solutions

Context-sensitive solutions (CSS) seek a 

transportation solution that addresses the needs of 

all road users and the functions of the facility within 

the context of its setting, considering land use, 

users, the environment, and other factors. CSS is a 

collaborative, interdisciplinary approach that includes 

the viewpoints of all stakeholders in the development 

of a shared vision of project goals, and uses a 

defined decision-making process. CSS and PBPD 

rely on flexibility to achieve results that meet the 

project purpose and need. PBPD compliments CSS 

by providing performance information that supports 

decision-making.

Design Criteria and Design Exceptions

As previously noted, PBPD moves away from the 

more conventional “top down, standards first” 

approach to more of a performance and valuebased “design up” approach. Designers that focus 

on the relationships between design dimensions and 

performance may become less obligated to meet 

one or more of the design guidelines, such as those 

found in the AASHTO Green Book (“A Policy on 

Geometric Design of Highways and Streets”). Design 

criteria and standards offer many benefits, including 

promoting consistency, establishing a design “norm, 

and promoting efficiency in design development. 

However, “standard” does not necessarily mean 

“best”, nor are standards intended to be a substitute 

for engineering judgment and context-specific 

considerations. PBPD provides planners and 

designers with the flexibility to make the optimum 

design decisions.

A design exception is a documented decision to 

design a highway element or a segment of highway 

to design criteria that do not meet minimum 

values or ranges established for that highway or 

project. Federal regulations (Reference 13) state 

that “Approval ...may be given on a project basis 

to designs which do not conform to the minimum 

criteria as set forth in the standards, policies, and 

standard specifications.” A design exception is NOT

an indication of failure or a “flawed” design; rather 

it is a necessary and legitimate process to allow 

professional and engineering judgment in the design 

process, providing a useful “tool” for employing 

practicality and flexibility in design decisions in a 

design-up approach such as PBPD.

As noted in the FHWA document “Mitigation 

Strategies for Design Exceptions” (Reference 14) 

there are a broad range of reasons why design 

exceptions may be considered and found to be 

necessary. Some of these include the following:

• Impacts to the natural environment

• Social or right-of-way impacts

• Preservation of historic or cultural resources

• Sensitivity to context and community values

• Construction or right-of-way costs

Widening the roadway footprint, including the 

possibility of additional right-of-way, will certainly 

impact the last bullet, and may impact one or more 

of the other bullets. Adding a lane by narrowing the 

existing lanes within the existing roadway footprint

can help reduce or even eliminate the concerns 

noted above in the bulleted list that so often apply to 

projects that end up widening the roadway.
Chapter 2 Performance Based Practical Design and the Role of Operations

21

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

A final notice published in the Federal Register on 

May 5, 2016 completed FHWA’s effort to update 

the policy regarding controlling criteria for design, 

applicable to projects on the National Highway 

System. FHWA reduced the number of controlling 

criteria from 13 to 10 for Interstate highways, other 

freeways, and roadways with design speed ≥ 50 mph, 

and now applies only 2 of those criteria to low speed 

roadways (non-freeways with design speed <50 

mph). FHWA also clarified when design exceptions 

are needed and the documentation that is expected 

to support such requests.

 

FHWA has adopted new policies to modify highway 

design standards that encourage greater flexibility in 

order to achieve a design that best suits the desires 

of the community, while satisfying the purpose for the 

project and needs of its users. As an example, FHWA 

published revisions to current federal policy that will 

help reduce cost and speed up the design of local 

roads and streets. In 1985, thirteen design criteria 

were prioritized because of their perceived impact 

on operations and safety. Under the new policy, ten 

criteria will be prioritized for high speed roadways, 

and only two criteria will be emphasized for lowerspeed roads such as rural roads that become main 

streets through smaller towns and cities. This will 

provide state and local engineers to develop flexible 

design solutions that meet local travel needs and 

goals.

Chapter 8 of the Green Book recommends shoulder 

widths for freeways shown in Table 4. Additionally, 

the AASHTO policy on design standards for the 

Interstate highway system requires a 10 ft. paved 

right (outside) shoulder. Shoulder widths less than 

the values shown in Table 4 will also require a design 

exception under the existing and proposed FHWA 

policy on Controlling Criteria.

Moving the inside travel lane closer to the roadway 

edge – including part time use of the shoulder as a 

travel lane – may also impact the horizontal alignment 

(to be renamed as “horizontal curve radius” under 

the proposed FHWA policy on Controlling Criteria) 

and sight distance as shown in Figure 4, and may 

therefore also require a design exception.

Even the Cover of the “Green Book” is Flexible

Controlling Criteria for Design 

Exceptions, 2016

1. Design Speed*

2. Lane Width

3. Shoulder Width

4. Horizontal Curve Radius

5. Superelevation

6. Maximum Grade

7. Stopping Sight Distance

8. Cross Slope

9. Vertical Clearance

10. Design Loading Structural Capacity*

* Design Speed and Design Loading 

Structural Capacity apply to all roads on the 

National Highway System
U.S. Department of Transportation | Federal Highway Administration

22

Chapter 2 Performance Based Practical Design and the Role of Operations

Side of Roadway DDHV for

truck traffic (veh/hr)

Total numbers of

freeway lanes

Recommended

shoulder width (ft)

Right Shoulder ≤ 250 All 10

Right Shoulder > 250 All 12

Left Shoulder ≤ 250 Less than 6 4

Left Shoulder ≤ 250 6 or more 10

Left Shoulder > 250 All 12

Table 4. Recommended Shoulder Widths for Freeways.

(Source: NCHRP Report 783: Evaluation of the 13 Controlling Criteria for Geometric Design; adapted from Chapter 8 of the 

AASHTO Green Book)

Figure 4. Diagram. Potential Impact on Sight Distance From Moving the Left-Most Travel Lane To the Inside.
Chapter 2 Performance Based Practical Design and the Role of Operations

23

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

TRANSPORTATION SYSTEMS 

MANAGEMENT AND OPERATIONS

Transportation Systems Management and 

Operations (TSMO)—also often referred to simply as 

“operations”—is defined as:

“Integrated strategies to optimize the performance 

of existing infrastructure through the implementation 

of multimodal and intermodal, cross-jurisdictional 

systems, services, and projects designed to preserve 

capacity and improve security, safety, and reliability of 

the transportation system.” 

Intelligent Transportation System (ITS) technologies 

– be they devices for monitoring traffic flow on the 

roadways, hardware and software at Transportation 

Management Centers (TMCs), and/or “Connected 

Vehicle” applications – are crucial to the success 

of these operations strategies. ITS represents the 

“enabling technology for operations.”

TSMO strategies—coupled with the supporting ITS 

technology—are a most important aspect of delivering 

transportation services to customers. Experience 

has shown that aggressive applications of these 

operations strategies can, in effect, “take back” much 

of the capacity lost due to congestion and disruptions. 

Operations strategies also enhance safety, promote 

reduced emissions, and increase system reliability. 

Perhaps most importantly, actively managing the 

transportation network can improve travelers’ 

experiences, providing them with real-time information 

and choices throughout the trip chain—from origin 

to destination—leading to network performance 

optimization and increased efficiency. TSMO 

strategies are relatively low cost (compared with 

adding capacity), much quicker to implement (two to 

three years), and offer substantial benefits (with very 

positive benefit-cost ratios).

FHWA recommends an “objectives-driven, 

performance-based approach” for including 

“operational and management strategies to improve 

the performance of existing transportation facilities” 

in the planning process. This objectives-driven, 

performance-based approach to planning for 

operations within a metropolitan area—conducted 

in collaboration among planners, transportation 

providers, operators, and other stakeholders—is shown 

in Figure 5.

The activities shown in Figure 5 parallel the PBPP and 

PBPD concepts identified in previous Figures 2 and 

3, including the development of potential strategies 

based on goals, objectives, and needs; and then 

evaluating and subsequently selecting strategies 

in terms of performance and cost. Moreover, lowcost, rapidly deployable, and flexible treatments – as 

provided by many TSMO strategies – all fall under 

the collective umbrella of PBPP and PBPD. Table 5 

provides a list of TSMO strategies that may be used 

in conjunction with narrow lanes and/or shoulders as 

part of the PBPD process. 

Figure 5. Flowchart. An Objectives-Driven, Performance-Based Approach.

(Adapted from Federal Highway Administration, Reference 25)
U.S. Department of Transportation | Federal Highway Administration

24

Chapter 2 Performance Based Practical Design and the Role of Operations

TSMO Strategy Description Potential Use and Value in Narrow 

Lanes/Shoulders

Incident Management

(Source: Florida DOT)

The systematic, planned, and 

coordinated use of human, 

institutional, electrical, mechanical, 

and technical resources to reduce 

the duration and impact of 

incidents, and improve the safety 

of motorists, crash victims, and 

incident responders.

Used to address safety and 

reliability concerns of narrow lanes 

or the loss of the shoulder as a 

vehicle refuge.

Ramp Management

(Source: Florida DOT)

The application of control 

devices, such as traffic signals, 

signing, and gates to regulate the 

number of vehicles entering or 

leaving the freeway, or to smooth 

out the rate at which vehicles enter 

and exit the freeway.

Metering the traffic that enters the 

freeway from on-ramps can help 

prevent flow breakdown on the 

mainline, and improve safety in 

merge and weaving areas that may 

be impacted by the use of narrow 

lanes and/or shoulders.

Managed Lanes (HOV,HOT)

(Source: Minnesota DOT)

Highway facilities or a set of lanes 

where operational strategies 

are proactively implemented and 

actively managed to optimize traffic 

flow and vehicular and person 

throughput. These strategies 

typically involve pricing, vehicle 

eligibility, and access control.

Used to improve travel time and 

reliability for vehicles carrying the 

most passengers or those willing 

to pay an additional fee for using 

the lane. The use of narrow lanes 

and shoulders may provide the 

opportunity to add or expand such 

lanes. 

Traveler Information (511, apps 

on Smartphones, DMS)

(Source: Minnesota DOT)

A combination of strategies for 

enabling better traveler decision 

making throughout the trip chain – 

before, during, and near the end of 

a trip. 

Allows drivers to adjust their 

route, time of travel, or mode, thus 

lessening demand on key facilities 

at peak times. DMS can also alert 

motorists that queues, significant 

slowdowns, or blocked lanes are 

ahead – as may result from narrow 

lanes or shoulders – thus reducing 

rear-end crashes and improving 

safety.

Table 5: Transportation System Management and Operations Strategies Typically Used On Freeways and the 

Potential Relationships to Performance-Based Practical Design.
Chapter 2 Performance Based Practical Design and the Role of Operations

25

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

TSMO Strategy Description Potential Use and Value in Narrow 

Lanes/Shoulders

Dynamic Speed Limits

(Source: Washington State DOT)

Adjusts speed limit (or advisory) 

displays based on real-time traffic, 

roadway, and/or weather conditions. 

They can be applied to an entire 

roadway segment or individual 

lanes. This “smoothing” process 

helps minimize the differences 

between the lowest and highest 

vehicle speeds.

Used to reduce speeds in advance of 

congestion, or perhaps in advance 

of segments with narrow lanes, 

limited shoulder widths, or reduced 

sight distance (e.g., requiring a 

reduced design speed as part of 

the design exception process). Is 

often used in conjunction with part 

time shoulder use, dynamic lane 

assignment. (Refer to Figure 6)

Dynamic Lane Assignment

(Source: Washington State DOT)

Dynamically closing or opening 

individual traffic lanes as warranted 

and providing advance warning of 

the closure(s), typically through lane 

control signs, to safely merge traffic 

into adjoining lanes.

Used to open and close a part time 

lane (e.g., part time shoulder use), 

or to close lane(s) upstream of a 

crash or disabled vehicle (e.g., with 

no shoulder for refuge). One or 

more lanes may also be closed to 

allow emergency vehicles to reach 

the crash scene quicker, particularly 

if there are narrow shoulders. This 

strategy is often used in conjunction 

with dynamic speed limits. 

Dynamic Junction Control

(Source: Caltrans)

Dynamically allocating lane access 

on mainline and ramp lanes in 

interchange areas. This may consist 

of assigning lanes dynamically either 

for through movements, shared 

through-exit movements, or exitonly. 

Used to better allocate available 

capacity at interchange areas and 

reduce the amount of weaving and 

merging. Is typically used with some 

sort of dynamic lane assignment. 

May be deployed to identify if the 

ramp shoulder is open to traffic and 

which mainline lanes can access 

the ramp; or may be deployed to 

promote safe merging operations 

during use of the mainline shoulder. 

Table 5: Transportation System Management and Operations Strategies Typically Used On Freeways and the 

Potential Relationships to Performance-Based Practical Design. (continuation)

Incorporating the consideration of TSMO strategies 

into the PBPD concepts and activities, designers and 

decision makers can expand the variety of options 

available to them, including perhaps the ability 

to postpone or reduce the need for conventional 

capacity improvements. Additionally, TSMO 

strategies may also help mitigate some of the safety 

and reliability impacts of PBPD solutions that result 

in less than full standard geometric design decisions, 

thereby providing solutions and support for any 

design exceptions. For example, dynamic speed limits 

and dynamic lane assignment strategies may be used 

in this context as shown in Figure 6.
U.S. Department of Transportation | Federal Highway Administration

26

Chapter 2 Performance Based Practical Design and the Role of Operations

Figure 6. Diagram. Example of Dynamic Speed Limits and Dynamic Lane Assessment in

Support of Narrow Lanes and Shoulders.

Before After

In summary, PBPD and TSMO – along with Context 

Sensitive Solutions and value engineering – are very 

complimentary as shown in Figure 7. Their respective 

approaches have much in common, and they all 

strive for the same goal – namely, providing a wellperforming transportation system using the most 

cost-effective improvements.

The FHWA website for PBPD (Reference 12) identifies 

notable attributes for PBPD as listed below. The 

phrase “and TSMO” can be added immediately after 

“PBPD” in this list and still ring very true.

• PBPD (and TSMO) focuses on performance 

improvements that benefit both project and 

system needs.

• Agencies make sound decisions based upon 

performance analysis.

• By scrutinizing each element of a project’s scope 

relative to value, need, and urgency, a PBPD 

(and TSMO) approach seeks a greater return on 

infrastructure investments.

• PBPD (and TSMO) strengthens the emphasis on 

planning-level corridor or system performance 

needs and objectives when planning, scoping and 

developing individual projects.

• PBPD (and TSMO) can be implemented within 

the Federal-aid Highway Program regulatory 

environment utilizing existing flexibility. PBPD 

does not eliminate, modify, or compromise 

existing design standards or regulatory 

requirements.
Chapter 2 Performance Based Practical Design and the Role of Operations

27

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Figure 7. Diagram. Overlapping Relationship between Performance-Based Practical Design, PerformanceBased Planning and Programming, Transportation System Management and Operations, Context Sensitive 

Solutions, and Value Engineering.

Chapter 3 Case Studies of Narrow Lanes and Narrow Shoulders

29

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Significant widening of a freeway in an urban area is 

often impracticable due to the adjacent development 

and land use and physical constraints, often coupled 

with funding constraints. An additional lane may 

nevertheless be provided within the existing roadway 

footprint by reducing the widths of the existing lanes 

and/or shoulder, or using the shoulder as a part 

time travel lane. The use of narrow lanes to provide 

additional capacity and improve operations within 

the existing roadway footprint is not new. Brief case 

studies of this approach are provided below.

NARROW LANES AS PART OF 

FREEWAY MODERNIZATION 

ALONG I-94 IN MILWAUKEE, 

WISCONSIN

This Case Study illustrates how a Performance-Based 

Practical Design (PBPD) approach can be used to 

analyze and make trade-offs during the design of 

major freeway reconstruction in a constrained urban 

area – specifically, the reconstruction of I-94, the 

East-West freeway, in Milwaukee, WI, and the ultimate 

recommendation to incorporate narrow lanes and 

shoulders along a segment of the project.

The I-94 East-West corridor study area is located in 

central Milwaukee County between the downtown 

Milwaukee Marquette Interchange and the Zoo 

Interchange. This portion of the I-94 East-West 

Freeway corridor is comprised of six lanes, three 

in each direction. It plays a key role in moving 

commuters, tourists, freight and other movements to 

major sites around and beyond the Southeast region 

of the state. As the corridor is nearing the end of its 

useful life, WisDOT initiated analyses to determine 

how the corridor should be rebuilt to ensure that it 

serves the economy of Southeastern Wisconsin for 

decades to come. The option of merely rebuilding the 

six-lane freeway was eliminated early in the process 

based on a DOT analysis showing the need for eight 

lanes to reduce congestion and accommodate future 

traffic growth.

One segment, less than a mile in length, passes 

between a number of cemeteries as shown in Figure 

8. As part of the screening process, two preferred 

alternatives were identified and scoped to avoid 

direct impacts to the adjacent cemeteries. 

Chapter 3 Case Studies of Narrow Lanes and Narrow Shoulders

Source: Kittelson & Associates, Inc.
U.S. Department of Transportation | Federal Highway Administration

30

Chapter 3 Case Studies of Narrow Lanes and Narrow Shoulders

• At-Grade Alternative: The At-grade alternative 

would reconstruct I-94 to eight travel lanes 

(four in each direction) at essentially the 

same elevation as the existing freeway. To 

avoid encroachment on the cemeteries, the 

reconstructed freeway mainline would include 

narrowing the lanes and converting the outside 

shoulder to a permanent fourth lane in each 

direction. As a result, the freeway would have less 

than 12-foot driving lanes between the adjacent 

cemeteries. Eastbound and westbound traffic 

would travel in 11-foot lanes for roughly 30 feet in 

each direction, with the lanes transitioning from 

12 feet to 11 feet (and back to 12 feet) for several 

hundred feet east and west of the 11-foot-lane 

segment. The shoulder widths would vary in this 

segment as the available right-of-way varies (with 

the shoulders being as narrow as two feet). East 

and west of the cemeteries, the freeway would 

have standard 12-foot lanes and full shoulders. 

Active Traffic Management (ATM) tools – such 

as advance warning signs alerting drivers to the 

narrow lanes and narrow shoulders, and dynamic 

lane assignment and dynamic speed limits to 

warn drivers of closed lanes and reduced speeds 

in the narrow segment – are being considered to 

make the narrow lane/narrow shoulder segment 

operate as safe as possible. 

• Double Deck Alternative: The Double Deck 

alternative would reconstruct I-94 to eight travel 

lanes (four in each direction). A Double Deck 

(with one set of freeway lanes elevated over the 

other) would be constructed in the area between 

the cemeteries to avoid any direct impacts. 

All I-94 lanes would be 12-feet-wide under this 

alternative. The shoulder widths would vary 

slightly in this segment because insufficient 

right-of-way is available near the cemeteries to 

provide full shoulder width. East and west of the 

cemeteries, the freeway would have full shoulders 

in both directions.

Figure 8. Picture. Milwaukee, Wisconsin Case Study Project Area.

(Source: Wisconsin Department of Transportation)
Chapter 3 Case Studies of Narrow Lanes and Narrow Shoulders

31

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Analyses were conducted for the various scenarios 

for the design year 2040 conditions using the 

proposed geometric alternatives and projected 

traffic data. The results of the analyses relative to the 

system needs and objectives are summarized below.

• Safety: Based on a crash prediction analysis 

to measure the number of annual crashes, and 

addressing the freeway and nearby arterial 

streets, the double deck alternative would have 

fewer total crashes than the at-grade alternative 

(with narrow lanes and shoulders) over the 20-

year analysis period. Nevertheless, the at-grade 

alternative would have 23 percent fewer crashes 

than the replace in kind alternative over the 20-

year period, thereby addressing improved safety 

and reduced crashes on I-94.

• Traffic Volumes and Operations: Another goal 

of the I-94 project is to accommodate existing 

and future traffic demand at an acceptable Level 

of Service (LOS). The at-grade and double deck 

alternatives would provide LOS C or D in the A.M. 

and P.M. peak periods in 2040, thereby satisfying 

this operational goal. 

• Cost: The total costs for the at-grade alternative 

are significantly less compared to the double 

deck alternative (i.e., $125 million as compared 

to $295 to $345 million, in 2014 dollars), and the 

at-grade alternative satisfies the project need and 

purpose.

WisDOT has identified the at-grade alternative, with 

narrow lanes, as the preferred alternative 31. As noted 

in the WisDOT press release:

“The alternative selected provides the community 

with the best balance when all critical factors are 

evaluated together. We are recommending an 

approach which addresses the problems of crumbling 

infrastructure, congestion, and integration with the 

local street network. The at-grade alternative is the 

least expensive to construct and have lower potential 

for community and cultural resources impacts.”

3 As of the writing of this Primer, the preferred alternative is still

 going through the final environmental impact statement process.

The 11-foot travel lanes and narrow shoulders through 

the cemetery area (with the At-Grade alternative) 

do not meet WisDOT and AASHTO criteria for the 

approximately 2,000-foot distance between the 

cemeteries and require design exceptions. 

The At-Grade Alternative also requires a design 

exception for inadequate sight distance in the 

cemetery area (that is, the slight curve on I-94 

through the cemeteries, combined with the 2 foot 

shoulders, would cause the concrete median barrier 

to reduce sight distance). These design exceptions 

do not reach a level that makes proceeding with 

the project in light of the stated purpose and need 

unreasonable, nor do they result in unacceptable 

safety or operational problems. Moreover, potential 

safety concerns with the designs could be mitigated 

to some extent by the inclusion of Transportation 

System Management and Operations (TSMO) 

strategies such as ATM.

In summary, this example illustrates how the use of 

narrow lanes and shoulders – identified and analyzed 

using a PBPD approach – can result in a feasible and 

cost-effective solution for upgrading an older urban 

freeway.

NARROW LANES TO 

ACCOMMODATE ADDITIONAL 

MANAGED LANE ALONG I-95 IN 

MIAMI - DADE, FLORIDA

I-95 from I-395 in Downtown Miami to I-595 in 

Broward County was comprised of four general 

purpose lanes and a single High Occupancy Vehicle 

(HOV) lane, with the separation of the HOV and 

general purpose lanes via striping only. The HOV 

lanes had an occupancy requirement of 2+ during 

peak hours (7-9 AM and 4-6 PM). The HOV and 

general purpose lanes on I-95 were operating at LOS 

F during peak periods. Additionally, the violation 

rates on the HOV lane were very high, exceeding 30 

percent in many segments. 
U.S. Department of Transportation | Federal Highway Administration

32

Chapter 3 Case Studies of Narrow Lanes and Narrow Shoulders

Florida DOT implemented a conversion of the facility 

– shown in Figure 9 – as follows:

• Existing HOV lane converted to a High 

Occupancy Toll (HOT) lane

• A second HOT lane was added by re-striping the 

entire facility with narrower lanes and shoulders. 

No new pavement was added. (Refer to diagrams 

in previous Figure 1) 

• The separation of the HOT lanes and the general 

purpose lanes is via delineators spaced 10 feet 

apart within a one-two foot painted buffer.

• The occupancy limit was raised to three+

A carpool registration component of I-95 Express 

was developed due to the right of way constraints 

that prohibited a dedicated declaration lane (for 

occupancy verification).

The ability to leverage Florida Turnpike Enterprises 

electronic violation enforcement system also 

provided the Department with the opportunity to 

reduce the scope of Florida Highway Patrol (FHP) 

enforcement in the lanes.

Current users of the HOT lane are three+ registered 

carpools and buses for free; with SunPass users 

allowed to use the lanes and pay a toll rate which is 

variable in relationship to the lane usage.

The converted I-95 Express lanes have improved 

overall traffic conditions along the project corridor 

since its inception as shown in Table 6.

In summary, this example illustrates how the use 

of narrow lanes and shoulders within an existing 

roadway footprint can help increase the capacity of 

managed lanes and improve the operation of both the 

managed lanes and general purpose lanes.

Figure 9. Photo. I-95 Express Lanes in Miami-Dade, Florida.

(Source: Google Maps / Street View)

Am Peak - Southbound PM Peak - Northbound

Before

(2008 HOV Study)

After

(12 months – FY 2009-2010)

Before

(2008 HOV Study)

After

(12 months – FY 2009-2010)

General Purpose Lanes 15 mph 51 mph 15 mph 41 mph

HOV lane (Before) / 

Express Lanes (After)

20 mph 64 mph 18 mph 56 mph

Table 6. Average Travel Speeds Before and After I-95 Conversion.
Chapter 3 Case Studies of Narrow Lanes and Narrow Shoulders

33

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

NARROW LANES TO 

ACCOMMODATE PART TIME 

SHOULDER USE ON US 2 IN 

WASHINGTON STATE

This site is located on the US-2 eastbound Trestle 

between the I-5 and SR-204 interchanges (MP 0.66 

to MP 2.22). This segment was re-striped from a 

configuration consisting of 4’ left shoulder, two 12’ 

general purpose lanes and a 10’ right shoulder; to a 

2’ left shoulder, two 11’ general purpose lanes, and a 

14 foot auxiliary lane/shoulder on the right, as shown 

in previous Figure 1. A photograph of the current 

configuration is shown in Figure 10.

The primary purposes for WSDOT to develop this 

static shoulder use were to reduce collisions in the 

I-5 / US-2 interchange and to keep traffic on the 

ramp from I-95 NB to US-2 from backing up onto the 

mainline of I-5 NB. The shoulder was first opened to 

traffic on April 6, 2009. The shoulder use operates in 

the PM peak period from 3:00 PM to 7:00 PM Monday 

through Friday. The shoulder is not open to traffic 

during all other time periods.

The average speeds on the I-5 NB connector to US-2 

EB have improved from 10 MPH to 37 MPH during the 

PM period of heaviest traffic.

The capacity in the narrower general purpose lanes 

along US 2 is approximately 2,000 vphpl and the 

corresponding speed is approximately 50 mph. The 

capacity on the shoulder is approximately 1,400 

vphpl and the corresponding speed is approximately 

40 mph. When considered in terms of crash rates, 

since the shoulder was opened to traffic in the 

eastbound direction of US-2, the rate of crashes in 

terms of crashes per million vehicle miles traveled 

(crashes/MVMT) has decreased overall as shown in 

Figure 11.

Figure 10. Photo. Shoulder Use Along US 2 in 

Washington State.

(Source: Washington State Department of Transportation)

Figure 11. Graph. Change in Crash Rate On US-2 in Washington State.

(Source: Washington State Department of Transportation)
U.S. Department of Transportation | Federal Highway Administration

34

Chapter 3 Case Studies of Narrow Lanes and Narrow Shoulders

A significant finding from the analysis of the new 

lane configuration is that incident clearance times 

on US-2 during the PM peak (when shoulder use is 

allowed) are significantly longer than the PM peak 

clearance times before shoulder use was allowed – 

from approximately 20 minutes to 28 minutes for 

all incident types combined. This is an important 

consideration whenever shoulders are narrowed 

or when a full width shoulder is used by traffic. 

As previously noted, (TSMO) strategies such as 

enhanced incident management and dynamic lane 

assignment may help mitigate such concerns. 

In summary, this example illustrates how by narrowing 

travel lanes and one shoulder can help convert the 

other shoulder into a part-time travel lane, thereby 

improving operation of the facility, including reducing 

back-ups at an interchange. The reduction in 

congestion can also improve safety.

JUNCTION CONTROL IN LOS 

ANGELES, CALIFORNIA

Junction control is an Active Traffic Management 

(ATM) strategy that dynamically allocates lane 

access on mainline and ramp lanes in interchange 

areas where high traffic volumes are present, and the 

relative demand on the mainline and ramps change 

throughout the day. For off-ramp locations, this may 

consist of assigning lanes dynamically either for 

through movements, shared through-exit movements, 

or exit-only. For on-ramp locations, this may involve 

a dynamic lane reduction on the mainline upstream 

of a high-volume entrance ramp and/or providing 

an additional lane for the on ramp. Volumes on the 

mainline lanes and ramps are continuously monitored, 

and lane access is dynamically changed based on the 

real-time and anticipated conditions. Implementing 

junction control may involve narrowing lanes, and/or 

part time use of the ramp and/or mainline shoulder 

thereby resulting in a narrow or no shoulder during 

junction control operation.

A junction control system was installed in Los 

Angeles at the northbound State Route 110 connector 

to northbound I-5. The system required re-striping 

of the SR-110 mainline and the off ramp to provide 

a second ramp lane and an optional turn from the 

second mainline lane. The mainline and ramp lanes 

were not narrowed; but during junction control 

operation, there is minimal shoulder on the ramp. 

The project also included the installation of blank 

out signs allowing the lane adjacent to the exit-only 

lane to also be used as an exit lane (in addition to 

remaining a through lane) during peak periods. The 

new ramp configuration and signage are shown in 

Figure 12.

Following implementation of this junction control 

system, the average ramp delay reduced from greater 

than 20 minutes to under five minutes, and the 

number of crashes decreased 30 percent from the 

previous year prior to installation as queued vehicles 

now are stopped towards the end of the slower 

moving ramp versus being stopped on the mainline of 

SR-110.

In summary, this example illustrates how converting 

an off-ramp shoulder into a part time travel lane, 

and using Intelligent Transportation Systems (ITS) 

technologies to designate the appropriate freeway 

lane for both through and exit movements, can 

increase the capacity of an interchange during peak 

periods, thereby reducing ramp delay and improving 

safety.

Figure 12. Photo. Ramp Configuration and Mainline 

Signage for Junction Control in Los Angeles, California.

(Source: Caltrans)
Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

35

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

As previously discussed, Performance-Based 

Practical Design (PBPD) and Performance-Based 

Planning and Programming (PBPP), plus the 

“objectives-driven, performance- based approach” 

to planning for operations, are all very compatible 

and consistent with one another. They also contain 

the term “performance-based;” and one of the 

key activities in all three approaches is to analyze 

and evaluate the resulting performance of various 

alternatives and strategies and how well they will 

meet the purpose and need of the project. From 

the perspective of narrowing lanes and/or shoulder 

widths to provide additional capacity within the 

existing footprint of an urban freeway, the operational 

and safety effects are perhaps the most critical 

aspect of this evaluation. This chapter provides an 

overview of some of the tools and analysis methods 

that can be used to estimate these effects.

OPERATIONAL EFFECTS

By definition, adding a travel lane – whether 

permanently or part time (as is often the case with 

shoulder use) – will increase overall roadway capacity, 

thereby reducing recurring congestion and improving 

operations. However, with narrower lanes, vehicles 

are traveling in closer proximity to each other, 

increasing the likelihood of lower speeds. Evaluations 

of narrow lane operations from the literature – as 

summarized in Table A-1 in the Appendix and as 

described in the case studies from the previous 

chapter – bear this out. The Level of Service (LOS) 

is generally improved, but with a slight reduction in 

average speed.

Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

Adding a lane by narrowing the existing 

lanes and shoulders generally improves 

operations and level of service, but with 

a decrease in average speeds.

Source: Kittelson & Associates, Inc.
U.S. Department of Transportation | Federal Highway Administration

36

Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

Analyzing Operational Impacts

The PBPD approach includes the evaluation 

of alternative solutions and making tradeoffs 

and decisions based on an objective analysis of 

performance data. The most recent version of the 

Highway Capacity Manual (HCM – Reference 18) 

includes information that can be used to estimate 

the operational impacts of additional, but narrow, 

lanes. For example, Table 7 shows the relationship 

between freeway lane widths, lateral clearances, 

and the resulting capacity and Free Flow Speed 

(FFS). As shown in this Table, a reduction in lane 

widths to less than 12’ will result in a reduction in 

free-flow speed. This reduction in free-flow speed 

does reduce throughput of an individual lane; but the 

additional lane more than offsets this loss in “per lane” 

throughput and capacity.

Adjustments should also be made to estimated FFS 

and lane capacity for lateral clearances if the width of 

the right side shoulder is reduced to accommodate 

an additional general purpose lane, or if the shoulder 

itself is used as a travel lane during parts of the day. 

Lateral clearance is measured from the right edge of 

the travel lane to the edge of the paved shoulder. As 

shown in Table 8, if the right-side lateral clearance 

is greater than or equal to 6 ft., no reduction in freeflow speed is made. The amount of free-flow speed 

reduction increases as the right-side lateral clearance 

decreases. The HCM assumes that left-side lateral 

clearance is greater than or equal to 2 ft. for all cases.

Average Lane Width (ft.) Reduction in FFS, fLW (mi/h)

≥12 0.0

≥11–12 1.9

≥10–11 6.6

Table 7. Highway Capacity Manual Adjustment to Free-Flow Speed for Lane Width.

(2010 Highway Capacity manual; Exhibit 11-8)

(Free-Flow Speed values are based on freeway speeds ranging from 55 mph to 75 mph)

Right-Side Lateral 

Clearance (ft)

Lanes in One Direction

2 3 4 ≥5

≥6 0.0 0.0 0.0 0.0

5 0.6 0.4 0.2 0.1

4 1.2 0.8 0.4 0.2

3 1.8 1.2 0.6 0.3

2 2.4 1.6 0.8 0.4

1 3.0 2.0 1.0 0.5

0 3.6 2.4 1.2 0.6

Table 8. Highway Capacity Manual Reductions in Free-Flow Speed for Right-Side Lateral Clearance on 

Freeways. (2010 Highway Capacity Manual Exhibit 11-9)
Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

37

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Adding a lane within an existing roadway footprint will typically involve both the narrowing of the travel lanes 

and a reduction in shoulder width (and the associated right side lateral clearance). For example, consider a 

scenario where a directional roadway is converted from three 12 ft. lanes and a 10 ft. right shoulder to four 10.5 

ft. lanes with a 4’ shoulder as shown in Figure 13. 

Figure 13. Diagram. Example Conversion from three Lanes to four Lanes.

Before After

Per the information in Tables 7 and 8, this example narrow lane conversion will reduce the free flow speed by an 

estimated 7.0 mph (6.6 mph from Table 7 plus 0.4 mph from Table 8). Using the maximum service flow rates 

from HCM Exhibit 11-17 (recreated in Table 9 below), and assuming a 10 mph reduction in FFS from 65 to 55 MPH 

(somewhat larger than the 7.4 mph value from Tables 7 and 8) and a target LOS D, the per-lane capacity will be 

reduced 6.4 percent from 2030 pc/hr to 1900 pc/hr (as highlighted in blue). However, converting this directional 

roadway from three lanes to four narrower lanes increases the total estimated directional throughput from 6090 

pc/hr (2,030 X three lanes) to 7600 pc/hr (1,900 X four lanes) – nearly a 25 percent increase. 

FFS

(mph)

Target Level of Service

A B C D E

75 820 1310 1750 2110 2400

70 770 1250 1690 2080 2400

65 710 1170 1630 2030 2350

60 660 1808 1560 2010 2300

55 600 990 1430 1900 2250

Note: All values rounded to nearest 10 pc/hr/ln

Table 9. Highway Capacity Manual Exhibit 11-17 (Maximum Service Flow Rates for Basic Freeway Segments).
U.S. Department of Transportation | Federal Highway Administration

38

Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

It is critical to remember that when using tools such 

as the HCM, the baseline values and the adjustment 

factors – such as those shown in Tables 8 and 9 – are 

statistical estimates or expected values. Moreover, it 

is important to remember that while the HCM can give 

close approximations to changes in FFS, the speedflow curves have not been calibrated with real data 

for different (i.e., narrower) lane widths, and their true 

impact on driver behavior and the resulting capacity 

cannot be known for sure. As such, the HCM and other 

analytical tools should not be viewed as 100 percent 

accurate prognosticators of future conditions. The 

actual impact of narrow lanes may be expected to vary 

around the estimated values, dependent on a variety 

of factors. Decisions supported by the results of these 

analytical tools involve some degree of uncertainty, 

and engineering judgment is required as part of the 

overall PBPD process.

Impact of Trucks on Narrow Lanes Operations

One such area of uncertainty involves the operational 

impacts of trucks on narrow lanes operations. As 

discussed in NCFRP Report 31 - Incorporating Truck 

Analysis into the Highway Capacity Manual (Reference 

22), most HCM chapters – including freeway – 

convert heavy vehicles 41to equivalent Passenger Car 

Equivalent (PCE) units 52(passenger cars per hour per 

lane) and add them to the passenger car volumes to 

obtain the total equivalent passenger car volume that 

is used in the HCM methodologies.

4 A heavy vehicle is defined in the HCM as “A vehicle with more 

than four wheels touching the pavement during normal operation.” 

Three heavy vehicle types are defined: transit buses, recreational 

vehicles (RVs), and trucks. These three types are grouped in the 

HCM under the broader category of heavy vehicles.

5 These adjustment factors vary with the percent of grade, length 

of grade, and the proportion of heavy vehicles in the traffic stream 

– from the lowest value are 1.5 PCE per heavy vehicle for grades 

less than 2 percent for all listed percentages of trucks and buses 

(1 to 25 percent), to the highest value of 7.0 PCE per heavy vehicle 

for an upgrade greater than 6 percent stretching for over a mile in 

length and 2 percent trucks. As the number of trucks increases for 

this steep grade, the PCE value decreases to a value of 4.0 for 25 

percent trucks (The equivalents decrease as the number of heavy 

vehicles increases, because these vehicles tend to form platoons 

and have operating characteristics that are more uniform as a group 

than those of passenger cars)

The HCM analysis then estimates the capacity, density, 

speed, delay, and LOS for the equivalent passenger car 

stream. Truck speeds and delays are not isolated from 

the values predicted using the equivalent passenger 

car stream performance.

The NCFRP Report evaluates the 2010 HCM from 

two perspectives: its ability to predict the specific 

performance of trucks, and its ability to model the 

effects of trucks on the traffic stream. Regarding the 

ability of the HCM to model the effects of trucks on the 

traffic stream NCFRP report states the following: 

• The HCM truck classification scheme is extremely 

simplistic, not reflecting the spectrum of truck 

performance capabilities in the U.S. fleet.

• The HCM PCEs are too simplistic since they do 

not reflect the variation in the truck fleet or the 

influence of truck proportion or grades on urban 

street PCEs.

• The HCM PCE look-up tables stop at 25 percent 

trucks (as a percentage of total traffic flow) even 

though there are many facilities in the United 

States where trucks routinely exceed 25 percent 

and can exceed 50 percent of the average daily 

traffic flow.

• The HCM approach is independent of significant 

variables like the truck type and weight-tohorsepower ratio.

Thus, while narrow lanes are addressed in the HCM, 

their impact on truck operations is not addressed 

other than incorporating an increase in PCE’s, which 

in turn may result in a further decrease in LOS and 

speeds that may not be reflected in the narrow lanes 

analyses.
Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

39

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

SAFETY

The consideration of safety is arguably the primary 

issue involved in a PBPD-based analysis and 

subsequent decision to include narrow lanes and/or 

shoulders in a roadway design (and to subsequently 

approve a design exception). Evaluations of the safety 

of narrow lanes from the literature – as summarized 

in Table A-2 in the Appendix and as described in 

the case studies from the previous chapter – show 

mixed results; although there appears to be a general 

tendency for the frequency (or number) of crashes 

to increase with a narrowing of lanes and shoulders 

(although not always); while the crash rate (e.g., 

number of crashes per million vehicle-miles) often 

decreases (again, with exceptions). It may be that 

even with an increase in the number of crashes, the 

additional throughput provided by the extra lane 

results in an even greater increase in the denominator 

of vehicle-miles of travel, resulting in a decreased crash 

rate 61. There are undoubtedly several other factors 

that can impact crash frequency and rates associated 

with narrow lanes – such as volumes, speeds, the 

resulting decrease in congestion and improved traffic 

flow, the length of the narrow lane segment, horizontal 

and vertical curves, percentage of heavy vehicles in 

the traffic stream – which may explain the variations in 

results between different studies of the safety impacts 

of narrow lanes. Additionally, comparing studies 

and findings in terms of their statistical significance 

is difficult due to the different approaches used for 

statistical analysis.

As was the case with the operational analyses, the 

designer should address the entire network and 

system as part of the PBPD process, making sure that 

implementing narrow lanes to improve throughput 

in one segment doesn’t increase congestion in a 

downstream segment potentially resulting in an 

increased number of crashes in that segment.

6 The additional throughput may be the result of freeing up a 

bottleneck or other constraint to flow, induced demand, or some 

combination.

Analyzing Safety Impacts

In performing a safety analysis of alternative lane 

and shoulder configurations and widths (as part of 

the PBPD process), it is important to understand the 

relationship of safety to design criteria and standards, 

along with the concepts of nominal and substantive 

safety as shown in Figure 14 and discussed below.

Nominal Safety

The concept of nominal safety is a consideration 

of whether a roadway, design alternative, or design 

element meets minimum design criteria. According 

to this concept, a highway or proposed design is 

considered to have nominal safety if its design features 

(such as lane width, shoulder width, lateral clearance, 

etc.) meet the minimum values or ranges. The measure 

of nominal safety is simply a comparison of design 

element dimensions to the adopted design criteria; 

an “either – or” scenario where a design feature either 

meets minimum criteria or it does not. Thus, narrowing 

one or more lanes of an urban freeway to less than the 

standard 12 ft. width (per the Green Book) would not 

meet the concept of nominal safety. 

When reviewing and comparing safety 

studies of narrow lanes, it is important 

to note which specific measures were 

used in the study:

• Crash frequency (i.e., the number of 

crashes during a specific period of 

time).

• Crash rate (i.e., number of crashes 

per some amount of vehicle-miles of 

travel.)
U.S. Department of Transportation | Federal Highway Administration

40

Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

In actuality, the safety effects of incremental 

differences in a given design dimension can be 

expected to produce an incremental, not absolute, 

change in safety. The nominal safety concept is limited 

in that it does not examine or express the actual or 

expected safety performance of a highway. This 

second dimension of safety is critical to making good 

decisions regarding design exceptions.

Substantive Safety

Substantive safety is defined as the expected, or 

estimated long-term average, safety performance 

of a roadway. The concept of substantive safety 

encompasses methods for estimating the following 

expected quantitative measures:

• Crash frequency (number of crashes per mile or 

location over a specified time period).

• Crash type (run-off-road, intersection, pedestrian, 

etc.).

• Crash severity (fatality, injury, property damage).

Understanding a location’s substantive safety 

and making judgments about whether it meets 

expectations may involve formal comparisons of its 

crash profile with aggregate data for facilities with 

similar characteristics (e.g., traffic volumes, number 

of current and proposed lanes and widths, location 

(urban, rural, suburban), inclusion of Transportation 

System Management and Operations (TSMO) 

strategies, and terrain); predictive methods such 

as those presented in the AASHTO Highway Safety 

Manual (HSM); or some combination.

In evaluating project alternatives from a substantive 

safety perspective, the practitioner is interested in the 

future safety performance of a facility and comparing 

that future performance for alternative geometrics, 

lane and shoulder widths, operational strategies, etc. 

Crash history is used to identify and diagnose safety 

concerns on an existing facility; but it may not be the 

most accurate approach for estimating long-term 

average safety performance. The HSM argues for 

the value of using predictive methods in addition to 

crash history, to improve accuracy and precision of 

estimates. 

Figure 14. Graph. Nominal and Substantive Safety.
Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

41

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Highway Safety Manual

The HSM (Reference 20) complements design 

guidelines, such as the AASHTO Green Book, by 

allowing a more scientifically rigorous methodology to 

quantify the “safety effects” of various design choices 

as part of the PBPD process. The 2014 Supplement 

to the HSM, 1st Edition, provides a structured 

methodology and specialized procedures to estimate 

the expected average crash frequency for various 

freeway facilities.

The HSM freeway chapters provide Safety 

Performance Functions (SPF) for 4- to 10-lane freeway 

facilities that account for several variables that need 

to be considered when looking at cross-section 

alternatives, including the following:

• Average Annual Daily Traffic (AADT), 

• Proportion of AADT during high-volume hours, 

• Number of through lanes, 

• Distance to median barrier,

• Lane width, and 

• Shoulder width (left and right).

The HSM also provides Crash Modification Factors 

(CMS’s) for various roadway treatments. These are 

used to estimate the expected number of crashes 

after implementing a given treatment. A CMF less than 

1.0 – the value that corresponds to a 12-ft. lane width 

for freeways, a 6-ft. width for inside shoulders, and 

a 10-ft. width of outside shoulders – indicates that a 

treatment has the potential to reduce the number of 

crashes. Figures 15 -17 show the CMF’s for lane widths, 

inside shoulder widths, and outside shoulder widths, 

respectively. Looking at the CMF alone, narrowing 

a freeway lane to under 12 ft. and/or narrowing the 

outside shoulder to less than 10 ft. will result in an 

increase in the number of crashes (i.e., the associated 

CMF values are greater than 1.0.) 

 
U.S. Department of Transportation | Federal Highway Administration

42

Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

Figure 15. Graph. Crash Modification Factors for Lane Widths.

Figure 16. Graph. Crash Modification Factors for Inside Shoulder Widths.
Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

43

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Figure 17. Graph. Crash Modification Factors for Outside Shoulder Widths.

HSM Evaluation of Narrower Lanes and Narrower Shoulders

The HSM freeway crash prediction model can be used to assess the change in crash frequency and severity 

associated with increasing the number of freeway lanes by reducing lane and shoulder widths. The before 

and after alternatives listed in Table 10 were analyzed for before conditions of a 4-lane freeway, 6-lane 

freeway, and 8-lane freeway (bi-directional in each case) with an increase in the number of lanes (one per 

direction) by narrowing the current lane widths from 12 ft. to 11 ft. and narrowing the shoulders so as to 

add the lane within the existing roadway footprint (i.e., no widening). Figures 18, 19, and 20 illustrate the 

predicted frequency of Fatal and Injury (FI) Crashes and Property Damage Only Crashes (PDO) for each 

freeway alternative shown in the Table.

Variable

Narrow Lane / Shoulder Alternatives

4-Lane Freeway 6-Lane Freeway 8-Lane Freeway

Before After Before After Before After

Number of Lanes (bidirectional)4 6 6 8 8 10

Lane Width 12 11 12 11 12 11

Right Shoulder Width 10 4* 10 4* 10 4*

Left Shoulder Width 6 3* 6 4* 6 5*

Table 10. Inputs for Narrow Lane / Shoulder Alternative.

* These are the narrowest shoulder widths for which the HSM has modification factors
U.S. Department of Transportation | Federal Highway Administration

44

Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

Figure 18. Graph. Predicted Crash Frequency With and Without Narrow Lanes and Narrow Shoulders 

(Conversion from 4-lane Freeway to 6-lane Freeway).

Figure 19. Graph. Predicted Crash Frequency With and Without Narrow Lanes and Narrow Shoulders 

(Conversion from 6-lane Freeway to 8-lane Freeway).
Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

45

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Figure 20. Graph. Predicted Crash Frequency With and Without Narrow Lanes and Narrow Shoulders 

(Conversion from 8-lane Freeway to 10-lane Freeway).

The HSM freeway crash prediction models estimate 

that narrowing lanes and shoulder widths to create 

an additional lane in each direction might have the 

following influence on crash frequency and severity:

• Reduce the frequency of PDO crashes 

• Increase the frequency of FI crashes when 

converting existing 4- or 6-lane freeways (i.e., 

two lanes and three lanes in each direction)

• Have little to no effect on the frequency of FI 

crashes when converting existing 8-lane freeways 

(i.e., four lanes in each direction)

While Figures 15 and 17 indicate an increase in the 

number of crashes with a narrowing of travel lanes 

to less than 12 ft. or a narrowing of the outside 

shoulder to less than 10 ft., and Figures 18-20 show 

a slight decrease in PDO crashes (but an increase in 

injury crashes); there are other considerations in any 

safety analysis of narrow lanes – specifically, that 

the increase in the number of lanes (and available 

capacity) may also reduce congestion, which in turn 

may improve overall safety. 

Figure 21 summarizes the scenarios shown in 

previous Table 12 and shows the safety effects of 

adding a lane by narrowing the general purpose 

lanes to 11 feet and reducing shoulder widths. The 

point at which each line crosses the 0 percent mark 

on the y-axis indicates the AADT above which the 

implementation of narrow lanes and shoulders – 

with an increase in the number of lanes – would 

be expected to decrease crash frequency 71. In 

general, the greater the average daily traffic – and 

presumably the greater level of congestion during the 

“before” condition – the more likely that the safety 

benefits from reduced congestion (resulting from 

an additional lane) will outweigh the potential safety 

issues associated with narrower lanes and shoulders. 

7 In one respect, Figure 21 does not appear intuitive; that the AADT 

threshold between an increase to a decreases in crash frequency is 

greater for the conversion from six to eight lanes (three to four lanes 

in each direction) than for the eight to ten lane conversion. The 

HSM is all field-data based, and sometimes the data yields slightly 

unpredictable results. Another way of looking at this is it reflects the 

model prediction that an eight- to ten-lane conversion is expected 

to have a greater reduction in crashes than a six- to eight-lane 

conversion.
U.S. Department of Transportation | Federal Highway Administration

46

Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

Figure 21. Graph. Predicted Percent Change in Crash Frequency when Adding a Lane by

Using the Right Shoulder and Narrowing Lanes.

For the example identified in previous Figure 13, 

adding a fourth lane to a the 3-lane directional 

roadway (as shown in the line with “diamonds” in 

Figure 21 – “6 to 8 Lane Freeway Conversion”), would 

be predicted to result in a net reduction in the crash 

frequency if the two-way average annual daily traffic 

was greater than approximately 95,000 vehicles per 

day.

Other considerations in performing a safety analysis 

of narrow lanes include:

• As addressed in an August 2011 article in the 

Journal of Economic Literature (Reference 23), 

even if narrower lanes and shoulders increase 

accident rates by 10 percent, applying standard 

parameters for costs of accidents, these extra 

costs may not reverse the advantage of a 

“narrow” design (in terms of travel time savings 

and the additional costs associated with widening 

to maintain 12-foot lanes).

• The predictive methods in the HSM do not include 

the effect of traffic volume variations throughout 

the day (other than a factor for the proportion of 

AADT during peak periods) or the percentages 

of different vehicle types. Per the HSM, these 

variables were not necessarily excluded because 

they have no effect in crash frequency; it may 

merely mean that the effect is not fully known or 

has not been quantified at this time.

Impact of Trucks on Narrow Lanes Safety

Trucks are wider than cars. Federal size regulations 

for commercial motor vehicles stipulate a maximum 

width of 2.6 meters (102.36 inches, or 8.53 feet), 

excluding mirrors and other safety devices. Thus, 

with a 12 foot lane, a truck driving in the center will 

have approximately 21 inches on either side between 

the truck and the adjacent lanes (not counting 

mirrors). If the lanes are narrowed to 11 feet, this 

clear distance to the adjacent lanes is reduced to 

15 inches (and even less between mirrors of trucks 

in adjacent lanes). Thus, narrower lanes may make 

it more difficult for the drivers of heavy vehicles 

(including buses) to position their vehicle completely 

within their lane. A truck encroaching into an 

adjacent lane can cause a sideswipe crash. Moreover 

this is probably a greater concern on tight horizontal 

curves.
Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

47

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

The literature on the subject of narrow lanes, trucks, 

and safety is somewhat sparse (e.g., as noted above, 

the HSM does not address the proportion of trucks.) 

Some highlights from the available literature are 

noted below:

• The 1995 NCHRP Report 369 (“Use of Shoulders 

and Narrow Lanes to Increase Freeway 

Capacity” – Reference 3) studied several altered 

and unaltered 81corridor segments, concluding 

that “truck accident rates are almost always 

higher on altered sections compared with 

unaltered”. At the same time, looking at the 

results available in the report, there does not 

seem to be any correlation between the change 

in truck accident rate and the percentage of 

trucks (up to 10 percent trucks, the greatest 

percentage of truck traffic for the study sites 

with crash data for trucks).

• As reported Ng and Small (Reference 23), on 

the New Jersey Turnpike, which has two parallel 

roadways (both with standard lane widths) of 

which one is for cars only, accident rates are 

higher in the lanes that allow trucks. 

• A Florida study of the influence of arterial lane 

width on bus safety (Reference 24) suggests 

a strong relationship between lane width and 

bus vehicle safety, noting that the narrower 

the lane width, the higher the likelihood of 

having bus sideswipe and mirror crashes. 

The results indicate that narrow lane widths, 

especially lane widths of 10 feet and narrower, 

are overrepresented in the occurrences of bus 

sideswipe crashes. 

Based on this admittedly limited documentation, 

coupled with the notion discussed above that narrow 

lanes reduce the margin of error for a heavy vehicle 

operator in terms of keeping the truck in the lane, 

an analysis of roadway design alternatives involving 

narrow lanes should consider the safety impacts of 

trucks, particularly when the percent of trucks in the 

traffic flow is greater than five to ten percent. 

8 Unaltered refers to standard 12-foot lane; altered refers to 

narrowed lanes and shoulders.

Considerations in this regard include the length of the 

segment and the horizontal and vertical alignments 

throughout the segments. Potential mitigation 

measures include the use of dynamic speed limits 

wherein the speed limits are lowered depending 

on the percentage of truck traffic in the flow, the 

horizontal and vertical curvature, and weather and 

visibility conditions. Consideration should also be 

given to keeping one or two of the lanes – including 

possibly a shoulder lane, assuming that the shoulder 

was constructed at full depth and can accommodate 

trucks – at 12-feet or greater, and restricting trucks to 

those lanes.

Transportation Systems Management and 

Operations and Safety

Implementing TSMO strategies (refer to previous 

Table 5) may provide additional safety benefits 

beyond the changes in crash frequency predicted 

by the HSM tools, and should be considered as part 

of the PBPD activities and the associated trade-off 

analyses. Table 11 shows the safety benefits resulting 

from several TSMO applications. 
U.S. Department of Transportation | Federal Highway Administration

48

Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

TSMO Strategy Location Safety Benefits

Traffic Incident Management (TIM)

(Reference 21)

General • Incident duration reduced 30–50 

percent (For example, average 

total incident duration in New 

Jersey has declined 48 percent, 

from 2.75 hours (1995) to 1.44 

hours (2008)

• Effective TIM reduces the 

occurrence of secondary crashes. 

The likelihood of a secondary 

crash increases by 2.8 percent for 

each minute the primary incident 

continues to be a hazard.

• Faster detection of and response 

to highway incidents saves lives. 

Ramp Metering

(Reference 7)

Portland, Oregon • 43 percent reduction in peak 

period crashes

Seattle, Washington • 39 percent reduction in crash rate

Minneapolis, Minnesota • 24 percent reduction in peak 

period crashes

Dynamic Speed Limits and Dynamic 

Lane Assignment 

(Reference 7)

Seattle, Washington (Seven mile 

segment of I-5)

This corridor was already actively 

managed via ramp metering and a 

robust incident management program.

• A before-and-after study showed 

total crashes decreased 4.1 

percent along the ATM segment. 

(During the same period, the 

southbound segment of I-5 – 

without ATM – experienced a 4.4 

percent increase in the number of 

crashes.)

London, England (M-25 Orbital) • Injury crashes decreased by 10 

percent.

• Damage-only crashes decreased 

by 30 percent. 

Dynamic Shoulder Lanes with Dynamic 

Speed Limits and Dynamic Lane 

Assignment

(Reference 7)

Minneapolis, Minnesota I-35W (with 

speed advisories (not legal limits) and 

shoulder used as part time HOT)

Crash reductions in the 6-month postdeployment period were as follows: 

• 9 percent for fatal plus injury 

crashes

• Greater than 20 percent for 

property damage only, and for 

total crashes (when the change 

in vehicle miles traveled was 

accounted for)

General – Results of 2006 FHWA 

Scanning Tour of Europe 

• A decrease in primary incidents of 

3 to 30 percent.

• A decrease in secondary incidents 

of 40 to 50 percent.

Table 11. Examples: Safety Benefits of Transportation System Management and Operations Strategies.
Chapter 4 Analyzing the Effects of Narrow Lane and Shoulder Width

49

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

RELIABILITY

The concept of travel time reliability has been 

receiving significant attention of late, particularly 

as part of the Strategic Highway Research Program 

(SHRP 2). The overall goal of the SHRP 2 Reliability 

program is to reduce congestion through incident 

reduction, management, response, and mitigation, 

thereby significantly improving travel time reliability 

for many types of person and freight trips on the 

nation’s highways. Per SHRP documentation: “travel 

time reliability refers to how travel time varies over 

time and the impacts of this variance on highway 

users. In other words, for repeated travel or vehicles 

making similar trips, there is an underlying distribution 

of travel time for a particular type of trip within a 

specific time period between two points. Individual 

travelers respond differently to the factors and 

uncertainties associated with the travel time.

Considering the example in previous Figure 13 (three 

lanes to four narrower lanes), and assuming that 

this conversion covers a five-mile segment with a 

peak hour volume of 5500 pce (compared to the 

aforementioned before capacity of 6090 pce/hr 

at LOS D), and an average travel speed of 40 mph 

during the peak hour; using equations identified in 

Reference 22 91, the Travel Time Index (TTI) for this 

before condition is estimated as 1.4 (indicating that the 

average peak hour travel time is 6.4 minutes, nearly 

two minutes greater than the free flow travel time of 

4.6 minutes).

9 The average annual mean travel time index (TTIm) = 

 1 + FFS * (RDR + IDR), where:

FFS = free flow speed (mi. / hr.)

RDR = recurring delay rate (hr. / min.)

IDR = incident delay rate (hr. / min.)

RDR is calculated using: RDR = 1/S – 1/FFS 

IDR is calculated using: IDR = [0.020 – (N – 2) * 0.003] * X12

Where:

S = peak hour average speed

N = number of lanes (between 2 and 4)

X = peak hour volume to capacity ratio

Free flow travel time (in minutes) is calculated as [Length of 

Segment (mi.) / FFS] * 60 minutes / hr. 

Following the conversion to four lanes, it is assumed 

that the peak hour volume increases to 6200 pce 

(i.e., the result of some induced demand brought 

about by the additional capacity – now at 7600 pce/

hr. – provided by the fourth lane), with the directional 

roadway now operating at a reduced FFS of 55 MPH 

(due to the narrower lanes) during the peak period, 

resulting in a travel time of 5.45 minutes over the 

5-mile segment. The new average annual mean travel 

time for the after condition is calculated as 5.8 minutes 

(with a TTIm of 1.067), an improvement over the before 

condition. 

These equations and the associated reliability 

measures do not take into consideration the possibility 

of an increase in crashes, thereby causing nonrecurring congestion and increased travel times 

(and less reliability. Moreover, a large reduction 

in shoulder width – such as occurs in the example 

(i.e., where the shoulder is reduced from 8 ft. to 2. 

ft.) – may negatively impact reliability. For example, 

there may no longer be a safe refuge for emergency 

stops and broken-down vehicles outside the traveled 

way, nor space for drivers of errant vehicles to make 

steering corrections before leaving the roadway. 

Moreover, without a wide shoulder, response times 

for emergency service vehicles – which often use 

the shoulder to bypass slow traffic when responding 

to a crash scene – may increase, thereby increasing 

incident-related congestion and reducing reliability. 

Accordingly, enhanced incident management 

strategies (e.g., frequent service patrols), dynamic lane 

assignment allowing the closure of a lane upstream of 

a crash site, and/or emergency refuge areas should be 

considered when analyzing the possibility of adding a 

lane via narrow lane and shoulder widths. 

Travel Time Index

The ratio of the travel time during the 

peak period to the time required to 

make the same trip at free-flow speeds

51

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Chapter 5 Analyzing the Effects of Narrow Lane and Shoulder Width

Implementing narrow lanes and shoulders to add 

a lane within an existing roadway footprint can 

be a viable and cost effective approach to reduce 

congestion. In addition to improving mobility, narrow 

lanes and shoulders may also be implemented with 

minimal negative impacts on safety and reliability. 

Potential scenarios for implementing narrow lanes 

include the following:

• Adding a general purpose lane to increase 

capacity and reduce recurring congestion. This 

can be for an extended section of roadway, or 

for a relatively short area as part of bottleneck 

reduction.

• Adding a managed lane, such a High Occupancy 

Vehicle (HOV) or High Occupancy Toll (HOT) 

lane. 

• Adding a lane in and/or within the vicinity of an 

interchange, to provide additional capacity on a 

ramp, an auxiliary lane between closely-spaced 

interchanges, or additional capacity beyond the 

interchange to prevent traffic from backing up 

into the interchange area.

Narrow lanes and shoulders should not be 

implemented without first conducting a thorough 

evaluation and analysis of their ability to meet the 

project purpose and needs, such as provided by 

a Performance-Based Practical Design (PBPD) 

approach. Each site and segment must be evaluated 

individually. Analysis considerations should include:

• Existing conditions (e.g., number of lanes and 

widths, shoulder widths, horizontal and vertical 

site distance, lateral clearance, locations and 

causes of congestion, traffic volumes and mix, 

lane and balance, current safety issues / types 

and causes of crashes, pavement joints and 

seem patterns, distances between upstream and 

downstream ramps.

• Alternative configurations of narrow lanes and 

shoulders. In this regard, NCHRP Report 369 

(Reference 3) recommends the following priority 

order for narrowing: narrow to 11 foot lanes, 

reduction in width of the left shoulder, reduction 

in width of the right shoulder.

• Length of treatment (to ensure that a bottleneck 

is not merely relocated to some other location)

• Estimated changes in level of service, capacity 

and crash frequencies / rates (and other 

performance measures) using appropriate 

analysis tools

• Potential Transportation System management 

and Operations (TSMO) strategies and Intelligent 

Transportation Systems (ITS) technologies – in 

advance of and through the implementation area 

– to improve the operations, reliability, and safety 

of the narrowed lane / shoulder configuration

• Estimated costs

This information will help in developing and getting 

approval of any design exceptions.

It is also important to involve emergency response 

personnel, enforcement personnel, and operations 

and maintenance staff in the analysis and evaluation. 

Public information and outreach to educate and alert 

the driving public of the changes should also be 

addressed.

Chapter 5 Summary and Conclusions

Source: Kittelson & Associates, Inc.

53

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Appendix

Appendix Summary of Impacts Resulting from Narrow Lanes and Shoulders

Location Configuration Results and Lessons Learned

Houston, Texas

US 59

(Reference 1)

3- 12’ lanes to 4-10.5’ lanes

4- 12’ lanes to 5-10.5’ lanes

Narrowed shoulders

(Refer to Figure 1)

• The level of service generally 

improved, with the major 

operational benefits realized in 

the peak periods.

• Total delay in one section was not 

reduced significantly, with the 

bottleneck merely being shifted. 

(Thus the need to address the 

entire “system” as part of the 

PBPD process, and not just an 

isolated problem location) 

• One potential operational 

problem was that the altered 

outside lane was composed partly 

of the right shoulder and partly of 

the mainline pavement, and each 

of these pavement materials were 

a different texture and contract. 

At one of the entrance ramps, the 

contrast appeared to guide the 

entrance ramp traffic out of the 

rightmost lane.

Texas - Multiple locations in Dallas, 

Houston, and San Antonio, Texas 

(Reference 5)

Not a before and after analysis of 

conversions from wider to narrower 

lanes; but a comparison of different 

roadway segments – some with 12’ 

lanes, others with 11’ lanes.

• Reduction in speed of about 2.2 

mph for 11-ft lanes as compared 

with 12-ft lanes. 

• A reduced lane and right-side 

lateral clearance combination 

directly corresponds to a 

reduction in the capacity of the 

travel lanes.

Multiple locations 

(Reference 3)

NCHRP study of freeway segments 

throughout the US where lanes were 

narrowed to 11’ to increase capacity.

• Altered sites (i.e., narrower lanes) 

exhibited slightly lower speeds 

for a given volume range and a 

slightly greater tendency to fall 

into LOS F conditions.

• Field observations indicate that 

operational impacts of reduced 

shoulder or lane widths are most 

notable in the transition area 

(i.e., the beginning of the altered 

segment).

Table A-1. Summary of Operational Impacts Resulting From Narrow Lanes and Shoulders.
U.S. Department of Transportation | Federal Highway Administration

54

Appendix

Location Configuration Results and Lessons Learned

General

(Reference 17)

FHWA evaluation of the operational 

and safety characteristics of shoulders 

used as part time travel lanes.

• The functional capacity of most 

shoulder lanes is approximately 

one-half to two-thirds that 

of a normal general-purpose 

lane, depending on the lane’s 

geometric characteristics. 

• Speeds in the shoulder lane also 

tend to be 5 to 10 mi/h (8 to 16 

km/h) slower than the adjacent 

general-purpose lanes.

• The limited functional capacity 

may be due in part to the 

geometric deficiencies, such as 

narrow width, close proximity of 

fixed objects, or lack of continuity 

associated with the lane, and to 

the fact that motorists may feel 

uncomfortable using the shoulder 

as a temporary travel lane. 

Table A-1. Summary of Operational Impacts Resulting From Narrow Lanes and Shoulders. (continuation)

Location Configuration Results and Lessons Learned

Multiple locations 

(Reference 3)

NCHRP study of freeway segments 

throughout the US where lanes were 

narrowed (i.e., “altered”) to 11’ to 

increase capacity. 

• Truck crash rates are almost 

always higher on altered sections 

(i.e., narrower lanes) compared 

with unaltered (12 ft. lanes).

• Crash rates for altered sites were 

higher in three out of the five 

corridors studies, two of which 

used shoulders and narrow lanes 

on a continuous basis for an 

extended length (more than a 

mile). 

• In two locations where narrow 

lanes and shoulders were used to 

relieve specific bottlenecks and 

improve lane continuity, the result 

was a “smoothing of traffic with 

a better balance between supply 

and demand, and the crash rate 

decreased slightly.

• A difference in lane width (12 to 

11 feet) by itself had no significant 

impact.

• Crash rates for altered sites 

tend to be somewhat higher 

than unaltered sites. However, if 

strategies are carefully applied 

in concert with lane balance and 

lane continuity concepts, rates 

for altered sections may be lower 

than for unaltered. 

Table A-2. Summary of Safety Impacts Resulting From Narrow Lanes and Shoulders.
55

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

Appendix

Location Configuration Results and Lessons Learned

Houston, Texas

US 59

(Reference)

3- 12’ lanes to 4-10.5’ lanes

4- 12’ lanes to 5-10.5’ lanes

Narrowed shoulders

(Refer to Figure 1)

• The number of crashes and the 

crash rates declined in the altered 

sections during the two years 

following modification for each of 

the four time periods studied (24 

hour, peak, daytime, nighttime).

• The larger reductions in crash 

frequencies occurred during the 

peak periods (the same period as 

the greatest operational benefits). 

• No significant change in the 

number or rate of severe 

accidents.

• The upstream segment entering 

the modified section (with 

narrow lanes and shoulders) 

also experienced a reduction in 

crashes and crash rate – likely 

attributed to the better operations 

in the downstream segments 

where capacity had been 

increased by the additional lane.

• The crash rate in the section 

downstream from the modified 

segments experienced a 

significant increase in the crash 

rate, with the greatest increase 

occurring the two peak hours – 

likely attributed to an increase 

in demand and flow (from the 

modified segments) but with no 

increase in capacity.

Los Angeles, California – Multiple 

segments

(Reference 4, 16)

5 lanes converted to 6 lanes

4 lanes converted to 5 lanes

12’ lanes to 11’ lanes

Additional lane used as HOV in nearly 

all cases.

(Refer to Figure 1)

• The projects converting four lanes 

to five lanes, on average, resulted 

in increases of 10 to 11 percent in 

crash frequency, which was found 

to be statistically significant. 

• The five- to six-lane conversion 

projects resulted in an increase in 

crash frequency of 3 to 7 percent, 

not statistically significant.

• The use of the added lanes as 

HOV lanes --- and the associated 

increase in speed differential 

between the HOV and general 

purpose lanes – may be an 

explanation for the increase crash 

frequency.

Table A-2. Summary of Safety Impacts Resulting From Narrow Lanes and Shoulders. (continuation)
U.S. Department of Transportation | Federal Highway Administration

56

Appendix

Location Configuration Results and Lessons Learned

Various locations in California, Texas, 

and Arizona

(Reference 19) 

Lane widths reduced from 12’ 

to 11’ (and sometimes 10.5’) to 

accommodate an additional lane.

• Analyses focused on crash rates 

(i.e., crashes per million vehicle 

miles).

• Left shoulder removals (all but 

2 feet) appear to be safe and 

effective capacity improvements. 

On severely congested freeways 

(ADT greater than 20,000 

vehicles per lane per day) left 

shoulder removals appear to aid 

safety if congestion levels are 

removed. 

• The use of 11-foot lanes as a 

remedial measure to reduce 

congestion appears to operate 

safely.

Texas - Multiple locations in Dallas, 

Houston, and San Antonio, Texas 

(Reference 5)

Not a before and after analysis of a 

conversion to narrower lanes; but 

a comparison of different roadway 

segments – some with 12’ lanes, others 

with 11’ lanes. 

• When comparing freeways with 

12 ft. and 11 ft. lanes, there is an 

increase in the number of fatal 

and serious injury crashes in 

the segments with 11ft. lanes, all 

other roadway characteristics 

being equal. The increase in crash 

frequency ranges from 5 percent 

for 2-lane freeways, up to 12 

percent for 5-lane freeways.

Table A-2. Summary of Safety Impacts Resulting From Narrow Lanes and Shoulders. (continuation)
57

Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer

References

References 

1 - “The Use of Freeway Shoulders to Increase Capacity;” McCasland, William R; FHWATX-78-210-2; September 

1978

2 - http://h1rehab.com/

3 - NCHRP Report 369; “Use of Shoulders and Narrow Lanes to Increase Freeway Capacity;” 1995

4 – “Safety Effects of Using Narrow Lanes and Shoulder-Use Lanes to Increase the Capacity of Urban Freeways” 

(http://www.fhwa.dot.gov/publications/research/safety/05001/)

5 - Urbanik, T., Bonilla, C. R., “California Experience with Inside Shoulder Removals,” Transportation Research 

Record 1122, Transportation Research Board, Washington, DC, 1987, pp. 37-36.

6 – Demonstrating Performance-Based Practical Design through Urban Freeway Reconstruction FHWAHOP-16-034;2016

7 – “Active Traffic Management (ATM) Feasibility and Screening Guide”; FHWA-HOP-14-019; May 2015,

http://www.ops.fhwa.dot.gov/publications/fhwahop14019/index.htm 

8 – “Use of Freeway Shoulders for Travel – Guide for Planning, Evaluating, and Designing Part-Time Shoulder Use 

as a Traffic Management Strategy, FHWA-HOP-023, February 2016,

http://www.ops.fhwa.dot.gov/publications/fhwahop15023/index.htm

9 – Materials provided by Florida DOT

10 - http://www.wsdot.wa.gov/Projects/US2/TrafficManagement/default.htm

11 – “Relationship to Lane Width to Capacity for Urban Expressways;” Transportation Research Record 2483 – 

Highway Capacity and Quality of Service; 2015

12 - FHWA PBPD website; https://www.fhwa.dot.gov/design/pbpd/

13 - 23CFR PART 625—DESIGN STANDARDS FOR HIGHWAYS; 625.3 Application; (f) Exceptions

14 - “Mitigation Strategies for Design Exceptions;”

http://safety.fhwa.dot.gov/geometric/pubs/mitigationstrategies/; July 2007

15 – “Reducing Lane and Shoulder Width to Permit an Additional Lane On a Freeway – Technical Report;” 

FHWA/TX-15/0-6811-1; March 2015

16 – “Safety Effects of Narrow Lanes and Shoulder-Use Lanes to Increase Capacity of Urban Freeways;” 

Transportation Research Record No. 1897; pages 71-80; 2004

17 – Evaluation of Operational and Safety Characteristics of Shoulders Used for Part-Time Travel Lanes” (FHWAHOP-12-008)

18 – Highway Capacity Manual, TRB, 2010

19 - Safety and Operational Evaluation of Shoulders on Urban Freeways; FHWA/TX-87/32+395-1

20 – Highway Safety Manual, AASHTO, 2010

21 – Federal Highway Administration Focus States Initiative: Traffic Incident Management Performance Measures 

Final Report; http://ops.fhwa.dot.gov/publications/fhwahop10010/presentation.htm

22 – NCFRP Report 31 - Incorporating Truck Analysis into the Highway Capacity Manual 

23 – Ng, Chen Feng and Small, Kenneth: “Tradeoffs among Free-flow Speed, Capacity, Cost, and Environmental 

Footprint in Highway Design”; Journal of Economic Literature; August 30, 2011

24 – Florida Department of Transportation, Transit Office; “Integrating Transit Into Traditional Neighborhood 

Design Polices – The Influence of Lane Width on Bus Safety “, June 2010

25 – Performance Based Planning and Programming Guidebook, FHWA HEP 13-041, September 2013,

http://www.fhwa.dot.gov/planning/performance_based_planning/v/


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