Mitigation Strategies 

for Design Exceptions

U.S. Department 

of Transportation

Federal Highway 

Administration

July 2007

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NOTICE 

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

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

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

report does not constitute a standard, specification, or regulation. 

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

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

essential to the objective of this document.

QUALITY ASSURANCE STATEMENT 

The Federal Highway Administration (FHWA) provides high-quality 

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

promotes public understanding. Standards and policies are used to ensure and 

maximize the quality, objectivity, utility, and integrity of its information. FHWA 

periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement. 

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Technical Report Documentation Page 

1. Report No. 

FHWA-SA-07-011 

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

5. Report Date 

July 2007 

4. Title and Subtitle 

Mitigation Strategies for Design Exceptions 

6. Performing Organization Code 

7. Author(s)* 

William J. Stein, P.E., and Timothy R. Neuman, P.E. 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

CH2M HILL, Inc. 

1295 Northland Drive 

Mendota Heights, MN 55120 

11. Contract or Grant No. 

DTFH61-05-D-00125 

13. Type of Report and Period Covered 

September 2005 to May 2007 

12. Sponsoring Agency Name and Address 

U.S. Department of Transportation 

Federal Highway Administration, Office of Safety 

1200 New Jersey Avenue , SE, HSSD 

Washington, D.C. 20590 14. Sponsoring Agency Code 

FHWA/HSSD 

15. Supplementary Notes: 

Contracting Office’s Technical Representative: Guan Xu, P.E. 

Technical Panel: Clayton Chen, Mark Doctor, Keith Harrison, Ray Krammes, Bill Prosser, Robert Schlicht 

FHWA Editor: Bonny Falk 

Document Preparation: Mary Pickens (editing), Jennifer Comer (graphics design), Amanda Stanley (formatting) – 

CH2M HILL 

16. Abstract 

Design criteria, established through years of practice and research, form the basis by which highway designers strive to balance 

cost, safety, mobility, social and environmental impacts, and the needs of a wide variety of roadway users. For many situations, 

there is sufficient flexibility within the design criteria to achieve a balanced design and still meet minimum values. On occasion, 

designers encounter situations in which the appropriate solution may suggest that using a design value or dimension outside the

normal range of practice is necessary. In these cases, a design exception may be considered. A design exception is a documented

decision to design a highway element or a segment of highway to design criteria that do no meet minimum values or ranges 

established for that highway or project. 

This publication provides detailed information on design exceptions and mitigating the potential adverse impacts to highway 

safety and traffic operations. 

• Chapter 1 provides basic information on design exceptions. Also discussed are the concepts of nominal and substantive safety, 

which are fundamental to the topic of design exceptions, their mitigation, and decision making. 

• Chapter 2 discusses the steps of an effective design exception process. 

• Chapter 3 clarifies the 13 controlling criteria, including when design exceptions are required, how safety and operations are 

affected by the 13 controlling criteria, and what the potential adverse impacts are if design criteria are not met. Information

on substantive safety is provided where available. 

• Chapter 4 presents and illustrates potential mitigation strategies. 

• Chapters 5 through 8 are case studies that illustrate how several States have effectively approached projects with difficult site 

constraints and design exceptions, including implementation of mitigation strategies. 

17. Key Words 

Design exceptions, highway safety, design exception case 

studies, design exception mitigation, controlling criteria, 

nominal safety, substantive safety, highway safety 

strategies 

18. Distribution Statement 

No restrictions. 

19. Security Classif. (of this report) 

 Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

171 

22. Price 

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

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III 

Table of Contents 

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

Chapter 1 

Design Exceptions .............................................................................................................................. 3

Chapter 2 

The Design Exception Process ....................................................................................................... 15 

Chapter 3 

The 13 Controlling Criteria............................................................................................................. 23 

Chapter 4 

Mitigation Strategies ....................................................................................................................... 67 

Chapter 5 

Case Study 1 – Interstate 235 Reconstruction............................................................................ 105 

Chapter 6 

Case Study 2 – Tensleep-Buffalo Highway (U.S. 16)............................................................... 115 

Chapter 7 

Case Study 3 – State Route 99 Reconstruction.......................................................................... 127 

Chapter 8 

Case Study 4 – State Route 110 (The Arroyo Seco Parkway).................................................. 145 

Bibliography.................................................................................................................................... 157

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V 

List of Figures 

Figure 1 Comparison of nominal and substantive concepts of safety............................. 10 

Figure 2 NCHRP Report 500 series ...................................................................................... 13 

Figure 3 Steps in the design exception process .................................................................. 15 

Figure 4 Advanced technology is making the collection of in-service data more 

effective and more efficient.................................................................................... 22 

Figure 5 Because it is a design control, design speed affects the curvature, stopping 

sight distance, superelevation, and other features of 

this horizontal curve ............................................................................................... 25 

Figure 6 Accident Modification Factors for lane width on rural 

two-lane highways .................................................................................................. 29 

Figure 7 Shoulders on this urban freeway provide enough width for crash avoidance, 

storage of disabled vehicles, maintenance activities, and enforcement .......... 33 

Figure 8 Partially-paved shoulders on this rural arterial improve bicycle 

accommodation and reduce risky passing maneuvers...................................... 34 

Figure 9 Pavement edge drop-off......................................................................................... 34 

Figure 10 Comparison of how shoulder width affects stopping sight distance past 

concrete bridge rail along horizontal curves ....................................................... 35 

Figure 11 Accident Modification Factors for shoulder width on rural two-lane 

highways................................................................................................................... 37 

Figure 12 Bridge width............................................................................................................. 41 

Figure 13 Vehicle shying towards the centerline on a narrow bridge............................... 41 

Figure 14 Horizontal alignment.............................................................................................. 44 

Figure 15 Superelevation ......................................................................................................... 47 

Figure 16 Horizontal curve at the base of a steep grade ..................................................... 50 

Figure 17 Vertical stopping sight distance at a crest vertical curve................................... 53 

Figure 18 Headlight sight distance at a sag vertical curve.................................................. 54 

Figure 19 Sight distance at an undercrossing ....................................................................... 54 

Figure 20 Horizontal stopping sight distance....................................................................... 55 

Figure 21 Not all locations with limited stopping sight distance are the same 

in terms of safety risk.............................................................................................. 57 

Figure 22 Stopping sight distance profile.............................................................................. 58 

Figure 23 Cross-slope break on the high side of a superelevated curve........................... 60 

Figure 24 Interstate closure after an impact with a bridge ................................................. 63 

Figure 25 Lateral offset to obstruction is an operational offset and is not the same as 

clear zone .................................................................................................................. 64 

Figure 26 Signs can be used to warn drivers in advance of a change in lane width....... 72 

Figure 27 Wide pavement markings ...................................................................................... 73 

Figure 28 Recessed pavement markings................................................................................ 73 

Figure 29 Raised pavement markings.................................................................................... 74 

Figure 30 Post-mounted delineators ...................................................................................... 74 

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VI 

List of Figures, continued 

Figure 31 Lighting .....................................................................................................................75 

Figure 32 Shoulder rumble strips............................................................................................76 

Figure 33 Centerline rumble strips .........................................................................................76 

Figure 34 Painted edgeline rumble strips ..............................................................................77 

Figure 35 Partially paved shoulders .......................................................................................78 

Figure 36 Safety edge (top) and after the shoulder has been graded over the edge 

(bottom).....................................................................................................................79

Figure 37 Fixed object removal. Separate box culverts were extended, connected, and 

covered at this interchange.....................................................................................80 

Figure 38 Breakaway light poles .............................................................................................80 

Figure 39 Shielding fixed objects with barrier.......................................................................81 

Figure 40 Pull-off area on the inside of a horizontal curve .................................................81 

Figure 41 Signs can be used to warn drivers in advance of a narrow bridge...................82 

Figure 42 Reflector tabs on guardrail .....................................................................................82 

Figure 43 Object markers and post-mounted delineators at a narrow bridge..................83 

Figure 44 White concrete bridge rail.......................................................................................83 

Figure 45 Anti-icing system on a bridge ................................................................................84 

Figure 46 Bridge rail and guardrail transition in compliance with NCHRP 

Report 350 .................................................................................................................85 

Figure 47 Turn warning sign with flashing beacon..............................................................87 

Figure 48 Curve warning sign. ................................................................................................88

Figure 49 Dynamic curve warning system ............................................................................88 

Figure 50 Delineation with large chevrons............................................................................89 

Figure 51 Delineation with post-mounted delineators ........................................................90 

Figure 52 Delineations with reflectors on barrier .................................................................90 

Figure 53 Advance warning of a steep grade........................................................................92 

Figure 54 Climbing lane ...........................................................................................................93 

Figure 55 Truck escape ramp...................................................................................................94 

Figure 56 Continuous drainage system..................................................................................94 

Figure 57 Sign for crest vertical curve with inadequate stopping sight distance.............95 

Figure 58 Intersection warning sign with flashers activated by vehicles 

entering from the side road ....................................................................................96 

Figure 59 A STOP sign with a flashing beacon improves visibility of the sign at this 

intersection with limited vertical sight distance..................................................97 

Figure 60 Intersection lighting.................................................................................................98 

Figure 61 SLIPPERY WHEN WET sign..................................................................................98 

Figure 62 Longitudinal texture applied to fresh pavement to improve 

surface friction..........................................................................................................99 

Figure 63 Transverse grooving to improve surface drainage and friction......................100 

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VII 

List of Figures, continued 

Figure 64 An example of transitioning the cross slope of the shoulder 

to mitigate a cross-slope break greater than 8%................................................ 100 

Figure 65 Vertical clearance signing..................................................................................... 101 

Figure 66 Vertical clearance signing..................................................................................... 102 

Figure 67 Warning sign with hanging chimes installed at the same height as 

the vertical clearance of the structure................................................................. 102 

Figure 68 Reflective sheeting on utility poles ..................................................................... 103 

Figure 69 Reflective sheeting on utility poles ..................................................................... 104 

Figure 70 The project runs through Des Moines, Iowa ..................................................... 105 

Figure 71 Approximate project limits of design exception............................................... 105 

Figure 72 Interstate 235 before reconstruction.................................................................... 106 

Figure 73 Interstate 235 after reconstruction....................................................................... 107 

Figure 74 Cross section within the unconstrained locations ............................................ 107 

Figure 75 The unconstrained cross section.......................................................................... 108 

Figure 76 Cross section within the area of restricted width ............................................. 108 

Figure 77 Narrower lane widths would place the lane lines in a different transverse 

position than the underlying longitudinal joints.............................................. 110 

Figure 78 Inside lane and shoulder widths within the constrained areas 

were narrowed....................................................................................................... 111 

Figure 79 Lighting was placed on the outside of the freeway through the design 

exception area ........................................................................................................ 112 

Figure 80 Lighting was placed down the center of the median where more crosssectional width was available .............................................................................. 112 

Figure 81 The project is located in north-central Wyoming ............................................. 115 

Figure 82 Approximate project limits .................................................................................. 115 

Figure 83 The steep cut on the north side of the highway and deep canyon 

on the south illustrate the difficult terrain faced by designers ....................... 116 

Figure 84 The Tensleep-Buffalo Highway leads into the Big Horn National 

Forest in the Rocky Mountains............................................................................ 117 

Figure 85 A massive retaining wall illustrates the difficult site constraints that were 

encountered............................................................................................................ 118 

Figure 86 Signing for the steep grades is provided throughout the project................... 119 

Figure 87 Electronic signing provides enhanced warning for the non-standard 

horizontal curves ................................................................................................... 120 

Figure 88 Advance signing for the truck escape ramp...................................................... 120 

Figure 89 Advance signing for the brake-check turnout................................................... 121 

Figure 90 A pull-off area provides tourist information and spectacular views............. 122 

Figure 91 Runaway truck ramp with arrestor system....................................................... 122 

Figure 92 Runaway truck ramp ............................................................................................ 123 

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VIII 

List of Figures, continued 

Figure 93 Electronic signing provides advance warning when the 

truck ramp is closed...............................................................................................123 

Figure 94 Climbing lane .........................................................................................................124 

Figure 95 Guardrail on the outside of a horizontal curve .................................................125 

Figure 96 State Route 99 is an urban arterial in Seattle, Washington ..............................127 

Figure 97 State Route 99 parallels Interstate 5 through the Seattle metro area ..............127 

Figure 98 SR 99 before and after reconstruction in Des Moines .......................................129 

Figure 99 SR 99 before and after reconstruction in SeaTac (top) and 

Shoreline (bottom) .................................................................................................130 

Figure 100 Drawing of proposed improvements to SR 99 in Shoreline ............................131 

Figure 101 Left-turn lane and U-turn areas after reconstruction in Federal Way............132 

Figure 102 New transit stop in Des Moines...........................................................................133 

Figure 103 SR 99 before and after reconstruction in Des Moines .......................................134 

Figure 104 Median tree plantings along SR 99 in SeaTac ....................................................135 

Figure 105 Impact with tree in median (SeaTac)...................................................................136 

Figure 106 Information from City maintenance personnel provided data 

on tree impacts that were not reported...............................................................137 

Figure 107 When the in-service evaluation showed that many tree hits were 

occurring at the narrow-median locations adjacent to turn lanes, 

trees were no longer planted in these areas in subsequent phases of the 

project (SeaTac) ......................................................................................................139 

Figure 108 A low-profile barrier was used along the median of SR 99 in Des Moines. 

Figure 109 Sloped-down end section adjacent to turn lane.................................................141 

Figure 110 Two low-profile barriers have passed NCHRP Report 350 test-level 2 (45 

mph) crash testing..................................................................................................142 

Figure 111 Los Angeles, California .........................................................................................145 

Figure 112 State Route 110. The Arroyo Seco Parkway .......................................................145 

Figure 113 SR 110 shortly after construction in 1940............................................................147 

Figure 114 Historic structures along the Arroyo Seco Parkway.........................................148 

Figure 115 The San Gabriel Mountains on the horizon illustrate the 

Parkway’s scenic context ......................................................................................149 

Figure 116 The Arroyo Seco Parkway has a narrow cross section, a curvilinear 

alignment, and non-standard interchange geometry .......................................149 

Figure 117 The Arroyo Seco Channel runs adjacent to the Parkway, constraining 

its width on one side..............................................................................................150 

Figure 118 Single-slope concrete median barrier at a horizontal curve ............................151 

Figure 119 Pull-off areas provided periodically along the outside lanes..........................152 

Figure 120 Enhanced delineation with raised pavement markers and pavement 

markings with high retroreflectivity...................................................................153 

Figure 121 Warning signs for curvature, slowing traffic, and vertical clearance.............154 

Figure 122 Geometry at the State Street Exit Ramp..............................................................155 

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IX 

List of Tables 

Table 1 Ranges for Design Speed........................................................................................ 26 

Table 2 Relative Risk of Differential Speed Caused by Changes in Roadway 

Geometry .................................................................................................................. 27 

Table 3 Ranges for Lane Width ........................................................................................... 28 

Table 4 Operational Effects of Freeway Lane Widths...................................................... 30 

Table 5 Operational Effects of Lane and Shoulder Width on 

Two-Lane Highways............................................................................................... 30 

Table 6 Lane Width: Potential Adverse Impacts to Safety and Operations................. 31 

Table 7 Ranges for Minimum Shoulder Width................................................................. 35 

Table 8 Operational Effects of Freeway Shoulder Widths .............................................. 38 

Table 9 Shoulder Width: Potential Adverse Impacts to Safety and Operations ......... 39 

Table 10 Bridge Width: Potential Adverse Impacts to Safety and Operations.............. 42 

Table 11 Horizontal Alignment: Potential Adverse Impacts to 

Safety and Operations............................................................................................. 45 

Table 12 Accident Modification Factors for Superelevation on Rural Two-Lane 

Highways.................................................................................................................. 48 

Table 13 Superelevation: Potential Adverse Impacts to Safety and Operations........... 48 

Table 14 Accident Modification Factors for Grade on Rural Two-Lane Highways ...... 51 

Table 15 Grade: Potential Adverse Impacts to Safety and Operations........................... 51 

Table 16 Relative Safety Risk of Various Conditions in Combination with NonStandard Stopping Sight Distance ........................................................................ 56 

Table 17 Stopping Sight Distance: Potential Adverse Impacts to Safety and 

Operations ................................................................................................................ 59 

Table 18 Cross Slope: Potential Adverse Impacts to Safety and Operations................. 61 

Table 19 Ranges for Minimum Vertical Clearance............................................................. 62 

Table 20 Vertical Clearance: Potential Adverse Impacts to Safety and Operations ..... 63 

Table 21 Lateral Offset to Obstruction: Potential Adverse Impacts to Safety and 

Operations ................................................................................................................ 65 

Table 22 Potential Mitigation Strategies .............................................................................. 67 

Table 23 Combinations of Lane and Shoulder Width Considered ................................ 109 

Table 24 Types of Before-and-After Data Collected......................................................... 138 

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XI 

Acronyms and Abbreviations 

3R resurfacing, restoration, and rehabilitation 

AADT Annual Average Daily Traffic 

AASHTO American Association of State Highway and Transportation Officials 

AMF accident modification factor 

Caltrans California Department of Transportation 

DDHV design hourly volume for one direction 

DOT Department of Transportation 

FHWA Federal Highway Administration 

HMA hot mix asphalt 

HMVMT hundred million vehicle miles traveled 

HOV high occupancy vehicle 

HSM Highway Safety Manual

IHSDM Interactive Highway Safety Design Model 

ITS intelligent transportation systems 

km/h kilometers per hour 

mi/h miles per hour 

MUTCD Manual of Uniform Traffic Control Devices

NCHRP National Cooperative Highway Research Program 

NHS National Highway System 

PCC portland cement concrete 

SPF safety performance function 

SR State Route 

TWLTL two-way left-turn lane 

vpd vehicles per day 

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1 

Introduction 

Mitigation Strategies for Design Exceptions was developed to provide designers with practical 

information on design exceptions and strategies that can be implemented to mitigate their 

potential adverse impacts to highway safety and traffic operations. 

Design criteria, established through years of practice and research, form the basis by which 

highway designers strive to balance cost, safety, mobility, social and environmental impacts, 

and the needs of a wide variety of roadway users. For many situations, there is sufficient 

flexibility within the design criteria to achieve a balanced design and still meet minimum 

values. On occasion, designers encounter situations with especially difficult site constraints 

and an appropriate solution may suggest the use of design values or dimensions outside the 

normal range of practice. In such cases, a design exception may be considered. 

Designers should recognize, however, that design exceptions have the potential to 

negatively affect highway safety and traffic operations. For this reason, consideration of a 

design exception should be deliberative and thorough and a clear understanding of the 

potential negative impacts should be developed. 

If the decision is made to go forward with a design exception, it is especially important that 

measures to reduce or eliminate the potential impacts be evaluated and, where appropriate, 

implemented. This guide presents and illustrates a variety of mitigation strategies, 

including real-world case studies from several States. 

Mitigation Strategies for Design Exceptions is organized as follows: 

Chapter 1 provides basic information on design exceptions. Also discussed are the concepts 

of nominal and substantive safety, which are fundamental to the topic of design exceptions, 

their mitigation, and decision making. 

Chapter 2 discusses the steps of an effective design exception process. A standard 

procedure is not prescribed; rather the activities that are fundamental to an effective design 

exception process are discussed. Guidance on design exception documentation is included. 

Chapter 3 clarifies the 13 controlling criteria, including when design exceptions are 

required, how safety and traffic operations are affected by the 13 controlling criteria, and 

what the potential adverse impacts are if design criteria are not met. Information on 

substantive safety is provided where available to help designers quantitatively evaluate the 

expected safety performance of design exceptions under consideration. 

Chapter 4 presents and illustrates potential mitigation strategies for the 13 controlling 

criteria. The strategies are summarized, by criterion, in Table 22 beginning on page 67. 

Chapters 5 through 8 are case studies that illustrate how several States have effectively 

approached projects with difficult site constraints and design exceptions, including 

implementation of mitigation strategies. 

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3 

CHAPTER 1 

Design Exceptions 

Designers and engineers are faced with many complex tradeoffs when designing highways 

and streets. A good design balances cost, safety, mobility, social and environmental impacts, 

and the needs of a wide variety of roadway users. Good design is also context-sensitive—

resulting in streets and highways that are in harmony with the natural and social 

environments through which they pass. 

Highway design criteria that have been established through years of practice and research 

form the basis by which roadway designers achieve this balance. These criteria are 

expressed as minimum dimensional values or ranges of values for various elements of the 

three-dimensional design features of the highway. The criteria are intended to deliver an 

acceptable, generally cost-effective level of performance (traffic operations, safety, 

maintainability, and constructability). The criteria are updated and refined as research and 

experience increase knowledge in the field of highway engineering, traffic operations, and 

safety. 

Designers are trained to use accepted design criteria throughout the project development 

process. Striving to meet design criteria is important because it is the primary means by 

which a resultant high-quality roadway will be produced. A highway or roadway that 

reflects full compliance with accepted design criteria decreases the probability that safety or 

traffic operational problems will develop. Using design values that lie within typical ranges 

thus provides a high degree of quality control and reduced risk. 

It must be recognized, however, that to achieve the balance described above, it is not always 

possible to meet design criteria. There is a wide variety of site-specific conditions and 

constraints that designers encounter. Roadways have a multitude of contexts. Establishing 

design criteria that cover every possible situation, each with a unique set of constraints and 

objectives, is not possible. On occasion, designers encounter situations in which the 

appropriate solution may suggest that using a design value or dimension outside the 

normal range of practice is necessary. Arriving at this conclusion requires the designer to 

understand how design criteria affect safety and operations. For many situations, there is 

sufficient flexibility within the design criteria to achieve a balanced design and still meet 

minimum values. However, when this is not possible, that is when a design exception may be 

considered. 

What is a Design Exception? 

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. 

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Mitigation Strategies For Design Exceptions 

4 

Why are Design Exceptions Needed? 

There is 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 

• Sensitivity to community values 

• Construction or right-of-way costs 

The reason for a design exception may be a combination of several factors. For example, in a 

mountainous area, flattening the grades and lengthening vertical curves to achieve a vertical 

alignment that meets design criteria may have both severe environmental impacts and an 

exorbitant economic cost. 

Even though there may be valid reasons for design exceptions, designers should be 

reluctant to design outside of accepted values. Understanding that the basis for the criteria 

is related to important performance as discussed above, it is reasonable to assume that any 

given design exception may also have the potential to adversely affect safety and traffic 

operations. A location where a design exception is being considered should therefore be 

thoroughly analyzed and the potential impacts understood before committing to the design 

exception. 

When the decision is made to go forward with a design exception, mitigation measures 

should be evaluated and, where appropriate, implemented to minimize the potential 

adverse impacts to the safety and operation of the highway. 

Where are Design Exceptions Required? 

Design decision making and approval authority varies based on ownership of the highway 

in question and its functional role or classification within the nation’s highway system. 

Broadly, roads can be considered as part of the National Highway System (NHS) or other 

(non-NHS). Evaluating mitigation techniques and implementing them where appropriate 

can improve safety and traffic operations on any highway. 

The National Highway System 

The NHS is a network of approximately 160,000 miles (256,000 km) of highways that are 

important to the nation’s economy, defense, and mobility. The NHS includes the Interstate 

system. Other NHS routes are principal arterials serving major travel destinations, 

highways that provide an important function for national defense, and highways that 

provide connections to other intermodal transportation facilities, such as airports and 

seaports. Additional information and State maps of the NHS are available on the Federal 

Highway Administration (FHWA) website: http://www.fhwa.dot.gov/hep10/nhs/. 

By federal regulation, FHWA is responsible for establishing standards on the NHS (23 CFR 

625). Through the federal rule-making process, FHWA has adopted several American 

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Chapter 1—Design Exceptions 

5 

Association of State Highway and Transportation Officials (AASHTO) publications as the 

minimum design criteria for the NHS (see the following section on “Sources of Design 

Criteria”). 

Design exceptions are required on any project on the NHS when design values are used that 

do not meet minimum criteria. FHWA is responsible for design decisions on NHS projects, 

specifically including approval of design exceptions. This authority exists regardless of the 

funding source for the project. 

FHWA has developed specific guidance on what constitutes the need for a design exception, 

and how design exceptions are to be studied, documented, and approved. This Guide 

addresses FHWA requirements for design exceptions. For additional information on 

FHWA’s requirements for design exceptions, see the Federal Aid Policy Guide:

http://www.fhwa.dot.gov/legsregs/directives/fapg/0625sup.htm. 

Non-NHS Highways 

Non-NHS projects are designed, constructed, operated, and maintained in accordance with 

State laws, regulations, directives, and safety, design, and construction standards. Therefore, 

there is no federal requirement for design exceptions on highways and streets that are not 

part of the NHS, regardless of funding source. However, States are encouraged to analyze 

situations and document exceptions on non-NHS routes in a similar fashion when design 

values are used that do not meet their adopted criteria. 

Sources of Highway Geometric Design Criteria 

AASHTO’s A Policy on Geometric Design of Highways and Streets (the Green Book) is the 

principal source for design values and ranges for highway and roadway design criteria and 

other geometric elements. For projects on the Interstate system, AASHTO’s A Policy on 

Design Standards, Interstate System should be consulted for design values and ranges. These 

two publications, through the federal rule-making process, establish the minimum design 

criteria to be used on the NHS, including the Interstate system. 

Other publications that offer complementary guidance to these two resources include the 

following: 

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

• A Guide for Achieving Flexibility in Highway Design, AASHTO. 

• Flexibility in Highway Design, FHWA. 

• Designing Safer Roads (Practices for Resurfacing, Restoration, and Rehabilitation), Special 

Report 214, Transportation Research Board.

• 23 CFR 625, for additional guides and references. 

Many States also publish their own design manuals. Sometimes these manuals specify 

design criteria that are more stringent than criteria cited in the Green Book. FHWA only 

requires that the minimum values cited in the Green Book are met. If the chosen design value 

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Mitigation Strategies For Design Exceptions 

6 

should not meet minimum State criteria, that State may choose to formally analyze, 

approve, and document a design exception, independent of FHWA.

The 13 Controlling Criteria 

The Green Book covers a wide range of geometric elements and design dimensions. In the 

interest of focusing the attention of the design profession on the most important or critical 

elements, FHWA performed a technical review of the adopted minimum criteria in the 

Green Book, with the understanding that requiring a design exception evaluation for every 

design element was impractical. Thirteen criteria, commonly referred to as the 13 controlling 

criteria, have been identified by FHWA as having substantial importance to the operational 

and safety performance of any highway such that special attention should be paid to them 

in design decisions. FHWA requires a formal written design exception if design criteria on 

the NHS are not met for any of these 13 criteria, listed below. 

1. Design speed 

2. Lane width 

3. Shoulder width 

4. Bridge width 

5. Horizontal alignment 

6. Superelevation 

7. Vertical alignment 

8. Grade 

9. Stopping sight distance 

10. Cross slope 

11. Vertical clearance 

12. Lateral offset to obstruction 

13. Structural capacity 

States or other agencies may add additional design elements to this list, but the 13 

controlling criteria reflect FHWA decision making and form the basis for formal written 

design exceptions on the NHS. See Chapter 3 for additional information. 

Types of Construction 

The FHWA design exception process can also vary based on the type of project. To 

understand design exceptions and the design exception process, three types of roadway 

construction are defined below. 

New construction is defined as roadways that are built on new alignment. During the route 

location process, designers should be identifying corridors with sufficient width to enable 

full criteria to be met. It should therefore generally be easier to meet design criteria with 

new construction because alignments can be chosen and refined to reduce site constraints 

and minimize impacts. As a result, there are usually fewer design exceptions on new 

construction projects. 

Reconstruction is defined as roadways that are rebuilt primarily along existing alignment. 

Reconstruction normally involves full-depth pavement replacement. Other work that would 

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Chapter 1—Design Exceptions 

7 

fall into the category of reconstruction would be adding lanes adjacent to an existing 

alignment, changing the fundamental character of the roadway (e.g., converting a two-lane 

highway to a multi-lane divided arterial) or reconfiguring intersections and interchanges. 

According to FHWA, design exceptions are required for projects involving an existing 

alignment or corridor for which reconstruction is proposed. Design exceptions may be more 

common on reconstruction projects because of additional site constraints and, in some areas, 

years of development and land use changes. 

The term 3R stands for resurfacing, restoration, and rehabilitation projects. 3R projects 

typically involve pavement improvement work (short of full-depth replacement) and 

targeted safety improvements. 3R projects generally involve retention of the existing threedimensional alignment. States may request approval of 3R-specific criteria for non-freeway 

3R projects on the NHS, or they may use the same minimum criteria used for new 

construction. If 3R criteria are approved by FHWA, any of the 13 controlling criteria not 

meeting these values would require a design exception for a 3R project on the NHS. 

For reconstruction and 3R projects, highways are often modified temporarily during 

construction to provide space for construction work and equipment. Because the adopted 

criteria are based on assumptions for vehicles traveling on finished highways and there is 

such a wide variety of site-specific issues within construction zones, formal design 

exceptions are not required for the design of work zones. 

Nominal and Substantive Safety 

The consideration of safety is arguably the central issue involved in a decision to accept or 

approve a design exception. Understanding the relationship of safety to the criteria, the 

design process, and a desired or expected outcome of the design is important. The concepts 

of nominal and substantive safety are fundamental to the topic of design exceptions and 

their mitigation. 

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, alignment, sight distance, 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. 

As an example, the criterion for Interstate lane width is 12 feet. A design alternative that 

proposes 12-foot lane widths suggests a nominally safe design, whereas an alternative that 

proposes 11-foot lane widths would not. 

Nominal safety is an “either-or”—a design feature or roadway either meets minimum 

criteria or it does not. Highways built to satisfy at least the minimum design criteria may be 

referred to as ‘nominally safe.’ By definition, a design exception is the acceptance of a 

condition that does not meet nominal safety. 

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Mitigation Strategies For Design Exceptions 

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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 actual long term or expected safety performance of a 

roadway. This would be determined by its crash experience measured over a long enough 

time period to provide a high level of confidence that the observed crash experience is a true 

representation of the expected safety characteristics of that location or highway. 

Quantitative measures of substantive safety include: 

• 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). 

Expected safety performance will vary based on inherent differences among highway types 

and contexts. For example, the frequency and other characteristics of crashes differ for a 

two-lane road in rolling rural terrain versus a multi-lane urban arterial versus a freeway 

interchange. 

Understanding a location’s substantive safety and making judgments about whether it 

meets expectations should involve formal comparison of its crash profile with aggregate 

data for facilities with similar characteristics—traffic volume, location (urban, rural, 

suburban), functional classification, facility type (two-lane, multi-lane divided, etc.), and 

terrain. There are well-established methods for characterizing a location’s substantive safety. 

This generally includes applying statistical models of crash experience from broader data 

bases (safety performance functions and accident modification factor analysis). It should be 

based on models and data from the same jurisdiction of the site being studied. See 

“Resources to Support Substantive Safety Analysis and Decision Making” on page 10 for 

more information. 

Comparing Nominal and Substantive Safety 

What is important to understand is that the substantive or long term safety performance of a 

roadway does not always directly correspond to its level of nominal safety. It is not 

uncommon for a roadway to be nominally safe (i.e., all design elements meet design criteria) 

but at the same time substantively unsafe (i.e., it demonstrates or reflects a high crash 

problem relative to expectations). Similarly, some roadways that are nominally unsafe (one 

or more design elements do not meet design criteria) can and do function at a high level of 

substantive safety. There are many reasons for this—primary among them is the fact that 

the criteria are based on many factors (safety being just one) and are derived from 

simplifying models and assumptions that are broadly applied. 

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Chapter 1—Design Exceptions 

9 

In the context of design exceptions and design decision making, the concept of 

understanding both nominal and substantive safety is critical. When applying design 

standards and criteria to their full extent, the presumption by the designer is that the 

resulting highway will perform in a safe (acceptable) manner. In other words, by meeting 

criteria the road is nominally safe, and as such the designer expects it to be substantively 

safe in the long term. In actual experience, the level of performance will vary based on the 

context and type of highway as described above. 

When faced with decisions to incorporate one or more design exceptions, the designer 

should reflect on whether the design exception will influence substantive safety, and if so to 

what extent. In other words, if a design exception is to be used, the designer should seek 

the best information available that characterizes the long term substantive safety risk of that 

exception (frequency, type, and severity of crashes). 

The following are basic questions designers should ask when contemplating a design 

exception: 

• If this is an existing location and a design exception is being studied, how good (or poor) 

is the existing substantive safety performance? 

• If this is new construction or reconstruction and a design exception is being studied, 

what should the long term safety performance of the roadway be? 

• Given the specifics of the design exception (geometric element, degree/magnitude of the 

variance, length of highway over which it is applied, traffic volume, etc.) what is the 

difference in expected substantive safety if the exception is implemented? 

By definition, locations with design exceptions are nominally unsafe, in that one or more 

design elements do not meet minimum criteria. That does not mean, however, that the 

highway cannot function at an acceptable level of substantive safety. The objective should 

be to understand the quantifiable (substantive) safety effects expected with a nominally 

unsafe design decision. 

Figure 1 is an illustrative comparison of the concepts of nominal and substantive safety with 

respect to their crash risk models. Current understanding of the relationships among 

roadway elements, traffic, drivers, and other factors suggests that the true safety risk is 

better represented by the red line (substantive safety). That is, incremental changes in 

design dimensions (typical of design exception decisions) may result in incremental (not 

order of magnitude) changes in substantive safety. What designers should seek is 

knowledge and data that enable them to establish the substantive safety of a contemplated 

design decision. This will allow for good judgments about what is acceptable and what is 

not and also will lead to investigation of mitigation measures to address the potential 

adverse safety impacts of a design exception. 

The preceding discussion is not meant to imply that meeting design criteria is unimportant. 

Safety or traffic operational problems are less likely to develop if design criteria are met. 

Throughout the design process, designers should strive to meet criteria and look first at 

using the flexibility inherent in the adopted criteria to achieve a balanced, safe, and contextsensitive design. In some situations, design exceptions will be necessary and the goal is to 

achieve a high level of substantive safety and efficient traffic operations. At all times the 

designer should retain the basic understanding that their goal in design is to assemble the 

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Mitigation Strategies For Design Exceptions 

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geometric elements and implement measures that will deliver as high a level of long term 

substantive safety as practical. 

FIGURE 1 

Comparison of nominal and substantive concepts of safety. A primary goal of design exception mitigation is to increase 

substantive safety. (Source: NCHRP Report 480, Transportation Research Board, 2002) 

Resources to Support Substantive Safety Analysis and 

Decision Making 

Resources are available and under development that support good decision making by 

helping designers consider both nominal and substantive safety, evaluate design 

alternatives (including potential design exceptions), and quantify impacts to safety and 

traffic operations. 

The Interactive Highway Safety Design Model 

The Interactive Highway Safety Design Model (IHSDM) is a software tool developed by 

FHWA that can assist designers with evaluating design alternatives for two-lane rural 

highways. The software is used to generate quantitative information on the safety and 

operational effects of geometric design alternatives. The current version of IHSDM consists 

of five evaluation modules: 

1. The Policy Review Module checks design elements for compliance with geometric 

design criteria (in effect, it produces a ‘nominal safety’ analysis). For projects on 

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Chapter 1—Design Exceptions 

11 

existing roadways, it can provide an initial assessment of how the existing geometric 

design compares to current design criteria. The module can be used throughout the 

design process to check compliance with design criteria. 

2. The Crash Prediction Module estimates the frequency and severity of crashes that can 

be expected on a highway based upon its geometric design and traffic characteristics. 

This module can help identify potential improvement projects on existing roadways, 

compare the relative safety performance of design alternatives, and assess the safety 

cost-effectiveness of design decisions. 

3. The Design Consistency Module helps diagnose safety concerns at horizontal curves by 

providing estimates of the magnitude of potential speed differential. Design 

consistency evaluations provide valuable information for diagnosing potential safety 

issues on existing highways. These evaluations also provide quality-assurance 

checks of both preliminary and final alignment designs. 

4. The Intersection Review Module evaluates an existing or proposed intersection 

geometric design to identify potential safety concerns and suggest possible 

treatments to mitigate those concerns. 

5. The Traffic Analysis Module estimates traffic quality–of-service measures for an 

existing or proposed design under current or projected traffic. This module is 

particularly useful during project scoping and preliminary engineering to evaluate 

the operational performance of alternatives to two-lane cross sections, including 

passing lanes, climbing lanes, and short four-lane sections. 

IHSDM software may be downloaded free of charge through the IHSDM public software 

Web site: http://www.ihsdm.org/. 

The Highway Safety Manual 

The Highway Safety Manual (HSM) is a resource currently under development to provide a 

comprehensive manual for highway safety. The HSM will be a synthesis of validated 

highway research, as well as practical information and tools to more quantitatively 

incorporate safety into the decision-making process. This will include analytical methods 

for predicting the impact of proposed alternatives on safety. For more information on the 

HSM, see http://www.highwaysafetymanual.org/. 

A Guide for Achieving Flexibility in Highway Design 

This AASHTO Guide provides information on the background, assumptions, and methods 

for how current design criteria have been developed. The Guide also provides information 

on how traffic volume, traffic composition, speed, location, other design elements, and other 

variables influence the level of risk associated with deviations from design criteria. A better 

understanding of these two issues can improve decision making. 

NCHRP Report 500-Series Safety Guides 

FHWA and AASHTO are leading a national effort to reduce the nation’s fatality rate to 1.0 

per hundred million vehicle miles traveled (HMVMT)—from a current nationwide rate of 

1.5 per HMVMT. This will result in approximately 9,000 fewer fatalities per year. The 

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Mitigation Strategies For Design Exceptions 

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AASHTO Strategic Highway Safety Plan, developed to guide this national effort in a 

coordinated, comprehensive manner, lists 22 safety emphasis areas. See the Web site: 

http://safety.transportation.org/plan.aspx. 

A series of safety guides to support implementation of the Safety Plan is being developed 

through the National Cooperative Highway Research Program (NCHRP). The guides focus 

on each emphasis area in the Safety Plan by providing technical discussions on potential 

strategies and programs for reducing highway fatalities and injuries. A comprehensive 

approach is presented, with detailed discussion on each emphasis area from the perspective 

of programs related to the "4 Es": Engineering, Education, Enforcement, and Emergency 

Medical Services. 

In terms of design exceptions, the guides can be a useful resource for identifying and 

evaluating mitigation strategies. After identifying overrepresented crash types, designers 

can refer to the appropriate Report 500 volume for potential countermeasures (Figure 2). 

The guides are available in printed form from the Transportation Research Board bookstore 

(http://gulliver.trb.org/bookstore/), and PDF versions are available at no cost from the 

AASHTO Strategic Highway Safety Plan Web site 

(http://safety.transportation.org/plan.aspx). 

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Chapter 1—Design Exceptions 

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FIGURE 2 

NCHRP Report 500 Series. 

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15 

CHAPTER 2 

The Design Exception Process 

The process to evaluate and document a decision to deviate from the adopted design criteria 

must be deliberative and thorough. Design exception procedures vary to some extent from 

State to State, but the activities described in this chapter are fundamental to a good design 

exception process. 

FIGURE 3 

Steps in the design exception process.

1. Determine the Costs and Impacts of Meeting Design Criteria 

The design process should begin with the presumption that the selected geometric design 

elements will meet or exceed the design criteria. Before considering a design exception, the 

following questions should be asked and evaluated: 

What would it take to fully meet design criteria? What would the implications be to fully 

meet design criteria? 

Issues to consider when making this evaluation include: 

• How well does a design that meets full criteria fit in with its surroundings? 

• What are the impacts to the natural environment? 

• What are the social impacts—impacts to neighborhoods, communities, historic and 

cultural resources? 

• What are the construction and right-of-way costs and impacts of fully meeting design 

criteria? 

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Mitigation Strategies For Design Exceptions 

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• What is the expected safety and operational performance of the design that meets full 

criteria? 

Some costs and impacts, such as construction and right-of-way, are relatively easy to 

quantify. Impacts to communities or the natural environment may be more difficult to 

quantify but are still very important. These impacts should at least be identified and an 

understanding of their level of magnitude should be developed. A full understanding of 

impacts can best be obtained through stakeholder involvement that is early, ongoing, and 

an integral part of the project development process. Following the principles of contextsensitive solutions is important. See the following Web sites for more information: 

http://www.fhwa.dot.gov/csd/index.cfm and http://www.contextsensitivesolutions.org/

In summary, the first step should be investigating what it takes to fully meet design criteria 

and developing a clear understanding of the costs and impacts. 

2. Develop and Evaluate Multiple Alternatives 

If it appears that meeting design criteria may not be feasible at a particular location, 

multiple alternatives should be developed, evaluated, and compared, including the 

alternative that meets full criteria. As discussed in Chapter 1, good design involves making 

tradeoffs and achieving a balance between cost, safety, mobility, and impacts. Examining 

multiple alternatives provides a way to understand and evaluate these tradeoffs. 

From the standpoint of risk management and minimizing tort liability, evaluating multiple 

alternatives demonstrates the complex, discretionary choices involved in highway design. 

Case Study 1 (presented in Chapter 5) illustrates how one State considered multiple 

combinations of lane and shoulder widths on an urban freeway reconstruction project with 

constrained cross-sectional width. This process allowed the design team to compare the 

various combinations, examine and weigh the tradeoffs, and come to a consensus on the 

combination that would best maintain a high level of substantive safety and efficient traffic 

movement while preserving resources important to the community. 

3. Evaluate Risk 

Agencies are confronted with two fundamental types of risk when dealing with design 

exceptions. The first involves the risk of the solution not performing as expected. The 

second involves the risk concerning the agency’s ability to defend itself against potential 

legal actions as a result of its decisions. Most States incur some risk of tort lawsuits arising 

from crashes alleged to be associated with a design or other problem created by the agency. 

Design exceptions in particular may represent a potential future risk to the agency if not 

handled properly. 

When designing highways in areas with difficult site constraints, designers should first 

acknowledge that the inability to meet design criteria may increase the risk of safety and/or 

operational problems. The degree of risk of these problems should be evaluated before 

moving forward with a design exception. This is primarily a technical process involving 

knowledge and tools (such as the IHSDM) that help designers understand operational and 

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Chapter 2—The Design Exception Process 

17 

safety implications of varying design conditions. The questions below are fundamental to 

this evaluation and should be looked at in combination because one can have an effect on 

others and the level of risk as a whole. 

What are the Traffic Volumes, the Composition of Traffic, and Speeds? 

Exposure to traffic is one of the most critical factors in measuring the safety risk of any 

highway element or feature. The more traffic to which the location is exposed, the greater 

the risk of a crash and/or measurable traffic operational problems. A designer may 

reasonably accept a design exception for curvature on a two-lane rural highway with low 

traffic, but be less inclined to do so in a geometrically or physically comparable context with 

significantly higher volumes. The composition of traffic is also a consideration. For example, 

there will be a higher level of risk for narrowed lane widths on a highway with a high 

percentage of large trucks than a highway that carries predominantly passenger vehicles. 

The speed or anticipated speed (for proposed designs) is another factor that influences risk. 

Particularly in terms of substantive safety, the probability of severe crashes will increase as 

speeds increase. 

What is the Degree/Severity of the Design Exception? 

How much a proposed design exception deviates from the design criteria is one measure for 

evaluating risk. The probability of safety or operational problems developing may increase 

as the deviation from design criteria increases. For example, the ability to provide 450 feet of 

stopping sight distance when 500 feet is specified may be acceptable, but providing only 250 

feet may not be. Designers should be able to translate variable dimensions to meaningful 

operational or substantive safety measures to help make these judgments. 

Are there Multiple Design Exceptions at the Same Location? 

Another factor that influences risk is the presence of two or more design exceptions at a 

particular location interacting with each other. There is research to support the view that the 

presence of multiple geometric problems represents particular risk to drivers. For example, 

one might expect that the risk associated with a horizontal curve that does not meet criteria 

for curvature and superelevation will increase if horizontal stopping sight distance is also 

less than the minimum value. Other combinations of design exceptions may function 

independently and have no effect on each other—for example, vertical clearance and 

horizontal alignment. The nature of the design elements involved influences whether there 

is an interaction or cumulative effect that might increase risk. 

What is the Length of the Design Exception? 

The length of highway affected by the design exception influences the degree of risk. Length 

is another fundamental measure of exposure. The extent of this influence depends on many 

factors, including the magnitude of variance of the design exception. 

Design exceptions may occur at just a point location or for a very limited length—for 

example, a short bridge that does not meet bridge width criteria. Another example would be 

stopping sight distance at a curve. In these instances, the section of roadway affected by the 

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Mitigation Strategies For Design Exceptions 

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design exception is relatively limited and so the designer may expect the operational or 

safety risk to be somewhat limited. 

In other cases, a design exception may extend for several miles. An example would be an 

area with constrained cross-sectional width where narrower lane and/or shoulder widths 

are used over an extended segment of the highway. Designers should recognize that the 

presence of a significant design exception over an extended length of highway greatly 

increases the risk of operational or safety problems to drivers exposed to it. 

What is the Expected Duration of the Design Exception? 

Is the design exception expected to be permanent? Or is there a reasonable expectation that 

other planned improvements in the near future may remove or lessen the non-standard 

condition? 

Where is the Location of the Design Exception Relative to other Risk Factors? 

Another important consideration is other highway elements (not necessarily design 

exceptions) that may have an interaction with the design element being evaluated. A good 

example of this is a crest vertical curve where there are intersections within the curve or just 

beyond the crest. The safety risk of non-standard stopping sight distance is greater at such a 

location compared to a curve where there are no intersections present. 

What is the Substantive Safety at the Design Exception Location? 

Knowledge of the past safety performance at the location is essential for evaluating risk. 

Both the crash history and the types of crashes will be needed for this evaluation because 

the crash types of primary interest would be those with a possible relationship to the design 

element that does not meet criteria. In addition, the designer needs full knowledge of the 

expected substantive safety performance of this location. Designers should not expect or 

promise zero crashes. What they need to understand is how well (or poorly) a location 

appears to perform compared with others similar to it. 

There are tools, methods and published studies that enable formal evaluation of the 

substantive safety of a condition or location. Designers need to incorporate the use of this 

knowledge base in their risk evaluations. A location exhibiting acceptable, long-term safety 

performance relative to expectation, despite having design features that do not meet current 

criteria, may indicate a lower level of risk. Conversely, designers should resist employing a 

design exception at a location that is fully in compliance with design criteria but known to 

be a high crash location. 

Care should be taken in relying on historical crash data for locations where significant 

changes are expected. For example, significant changes in land use and traffic or nearby 

geometric changes to intersections and interchanges may change how the location functions 

in the future. So, a high level of substantive safety based on crash data alone does not 

necessarily mean that the design element should be maintained in its existing condition. The 

safety performance of the existing roadway may change, particularly if other conditions 

change. For these situations, the models for predicting expected safety performance are 

particularly valuable. 

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Chapter 2—The Design Exception Process 

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4. Evaluate Mitigation Measures 

For alternatives that incorporate one or more design elements that do not meet criteria, the 

designer will have an understanding of the potential adverse impacts to safety and 

operations. 

Equipped with this understanding, measures should be evaluated that are targeted at 

mitigating those impacts. Mitigation measures may include providing advance notice to the 

driver of the condition, enhancing the design of another geometric element to compensate 

for a potentially adverse action, implementing features designed to lessen the severity of an 

incident or action, or some combination of these. Chapter 4 provides information on 

mitigation techniques for the 13 controlling criteria. The goal, as discussed in Chapter 1, is to 

implement mitigation measures that will maximize the probability of a nominally unsafe 

design operating at a high level of substantive safety and operational efficiency. 

5. Document, Review, and Approve 

Effective documentation of design exceptions is important for several reasons. 

First, agency staff typically complete many projects simultaneously across a jurisdiction. 

Important decisions such as design exceptions require review, oversight, and approval, 

usually from multiple levels of management. Requiring complete documentation using 

prescribed formats and technical references is an effective means of maintaining quality 

control over decisions and outcomes. 

Second, documentation offers an historical benefit for future designers. If a safety or 

operational problem arises or if the location is being reconstructed, understanding the 

thought process and reasons for the decisions that were made in earlier projects can be 

valuable information for designers, particularly where design exceptions were used. For this 

to be useful, an archive system is needed that allows designers to quickly and easily find 

historical documentation for decisions made at their project locations. 

Third, if a design decision is questioned in a lawsuit and design negligence is alleged, 

design exception documentation provides proof that the decision was made in a 

deliberative, thorough manner after fully evaluating the impacts and the alternatives. In 

most states, designers are afforded some level of discretionary immunity for their design 

decisions. Regardless of the level of immunity, documentation and retention of such 

documentation for later reference is essential to limiting an agency’s liability should a 

lawsuit over design negligence be filed. Crashes and resultant legal action may occur many 

years after the highway was constructed. 

Fundamentals for Effective Design Exception Documentation 

The person who prepares the design exception document is normally very familiar with and 

knowledgeable about the project and the design. The goal should be to prepare a clear and 

concise explanation of the design recommendation—one that will provide the person(s) in 

charge of review and approval, who usually has much less detailed knowledge of the 

project, enough information to understand the decision and make an informed judgment on 

whether it should move forward. Length of documentation is not important. The key is to 

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Mitigation Strategies For Design Exceptions 

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provide clarity and completeness to someone not familiar with the project or the design 

exception. 

Another audience to consider is future designers. They should be able to clearly understand 

the design team’s reasons for the design exception, even many years after construction. 

Documentation should demonstrate the designer’s clear understanding of the design criteria 

and their functional relationships, the unique context, careful consideration of alternative 

solutions, and a reasonable weighing of impacts and effects in support of a recommendation 

to deviate from the adopted criteria. Critical to this documentation and the ultimate 

recommendation is a record of the consideration and application of strategies and features 

to mitigate the potential risk of the design exception. 

Although the content of the design exception document will vary based on the situation, the 

following is a list of items and issues to include: 

Identify the location of the design exception, including the length or beginning and ending points, if 

applicable. A map or graphic may be appropriate. 

State the design speed. 

Basic 

Information 

State the traffic volumes and the composition of traffic. 

State the design element(s) to which the design exception applies. 

State the minimum value or range. 

State the resource that was used to obtain the design value and its year of publication (for example, 

the 2004 edition of AASHTO’s Policy on Geometric Design of Highway’s and Streets). 

Design Element(s) 

 and Criteria

State the value being proposed. 

Describe the reasons for the design exception. 

Describe the site constraints. 

Describe and, if possible, quantify the costs and impacts involved with fully meeting design criteria. 

Some costs, such as construction and right-of-way costs, are relatively easy to quantify. Social 

costs, such as impacts to communities or the natural environment, are more difficult to quantify but 

are still very important. Use tables, charts, and drawings as appropriate to illustrate and clarify the 

impacts. 

Describe the other alternatives that were considered. 

Explanation 

Discuss the potential impacts to safety and traffic operations. 

Describe the mitigation measures that were considered. 

Mitigation 

Describe the mitigation measures that will be implemented. Include drawings if appropriate. 

For locations where an existing feature that does not meet criteria is being maintained and current 

crash data are available, quantify the substantive safety of the location and how it compares to 

similar facilities. 

Supporting 

Information 

If any research or other technical resources were consulted as part of the evaluation process, 

identify them.

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Chapter 2—The Design Exception Process 

21 

Non-Controlling Criteria 

Many design elements not included in the list of 13 controlling criteria are important for the 

safety and operation of a highway. Providing a clear zone, turn lanes, acceleration and 

deceleration length, and barriers that meet current crash test standards are a few examples. 

Exceptions to non-controlling criteria should be identified, justified, and documented, 

taking into consideration the effect of any deviation from design criteria on safety. The 

project files should include this information. The design exception information should be 

organized to assist in periodic program analysis and archived in a way that it can be easily 

retrieved in the future. 

Review and Approval 

Because of the different organizational structures at State Departments of Transportation 

(DOTs) and effective processes already in place for review and approval of design 

exceptions, a standard national process is not appropriate. The key is to have the design 

exception document reviewed and approved by an individual or small group that is not 

part of the design team proposing the design exception (for some agencies, final approval 

rests with someone with a high level of authority, such as the State Design Engineer). This 

process allows the design exception to be looked at from a fresh perspective and evaluated 

objectively. The review step provides a level of quality control and consistency. An 

independent review also demonstrates a complete process, which can reduce tort liability. 

FHWA has review and approval authority for any design exception on the interstate system. 

For design exceptions on other NHS routes, the role of FHWA Divisions should be defined 

by written agreement between the Division Office and the State DOT. 

6. Monitor and Evaluate In-Service Performance 

Monitoring the performance of design exception locations after construction is the final step 

in the design exception process. Because of limited financial and human resources, the 

extent of in-service evaluation will and should vary, but monitoring the safety and 

operational performance at design exception locations has several benefits. First, if problems 

do develop, design changes or modified mitigation techniques are warranted to improve 

performance. Second, the lessons learned from in-service evaluation increase the body of 

knowledge about the safety and operational effects of design exceptions and mitigation 

measures. This knowledge will lead to better decisionmaking, both in terms of evaluating 

design exceptions and in mitigating their potential adverse impacts. 

The rare and random nature of crashes means that several years of crash data may be 

needed before any conclusions can be drawn as to whether a crash problem is statistically 

significant and whether it is related to the design exception. In addition to reviewing crash 

data, in-service evaluation techniques can be implemented to obtain information over much 

shorter time periods. Predictions can be developed from this information on how well the 

location will perform, and additional or modified mitigation measures can be implemented. 

For example, speeds can be monitored at a curve that does not meet criteria for curvature or 

stopping sight distance. 

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Mitigation Strategies For Design Exceptions 

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Advanced technologies (Figure 4) can be useful tools for collecting this type of immediate 

data. These technologies can also provide much more information on what is contributing to 

a crash problem than a written crash report, based on the limited information available at 

crash locations. 

Case Study 3 (presented in Chapter 7) illustrates how one State is collecting and analyzing 

in-service data for a design that incorporated trees in a raised median. If the crash data 

indicate poor substantive safety, the agency has committed to removing the trees or 

implementing other mitigation measures. 

FIGURE 4

Advanced technology is making the collection of in-service data more effective and more efficient.

Summary 

Establishing and then maintaining a formal design exception process is essential to an 

agency making effective design decisions, maintaining quality control, and managing risk. 

Central to a good design exception process are both the development and management 

(storage, retrieval, and use) of documentation of design exceptions.

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23 

CHAPTER 3 

The 13 Controlling Criteria 

As discussed in Chapter 1, FHWA has identified 13 design criteria as having substantial 

importance for the safe and efficient operation of highways. A formal design exception is 

required if these controlling criteria are not met on the NHS: 

1. Design speed 

2. Lane width 

3. Shoulder width 

4. Bridge width 

5. Horizontal alignment 

6. Superelevation 

7. Vertical alignment 

8. Grade 

9. Stopping sight distance 

10. Cross slope 

11. Vertical clearance 

12. Lateral offset to obstruction 

13. Structural capacity 

Exceptions to non-controlling criteria should also be identified, justified, and documented, 

taking into consideration the effect of any deviation from design criteria on safety. The 

project files should include this information. 

Traffic Operational and Safety Effects 

This chapter provides additional technical information on the 13 controlling criteria, 

including clarifications on when formal design exceptions are required and the potential 

impacts to traffic operations or substantive safety that a designer should consider when 

evaluating design exceptions and mitigation strategies. 

Traffic operational effects may include the influence of a change in a design dimension on 

the facility’s capacity, on speed, or on changes in speed or other operating behavior for 

either the overall traffic stream or certain critical vehicle types. Substantive safety effects 

may include expected or predicted changes in the crash frequency, severity, or both, 

associated with an incremental change in a design dimension. For both traffic operational 

and substantive safety effects, the information provided in this chapter represents a 

synthesis of research and technical literature. 

With respect to substantive safety effects, effects will be described in two ways. Safety 

performance functions (SPFs) describe the expected crash frequency for a condition or element 

as a function of traffic volume and other fundamental values. SPFs are usually expressed as 

an equation or mathematical function. Accident modification factors (AMFs) describe the 

expected change in crash frequency (total or particular crash types) associated with an 

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Mitigation Strategies For Design Exceptions 

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incremental change in a design dimension. AMFs may be shown in tabular form or in some 

cases as a simple function. They are expressed as a decimal, with an AMF less than 1.0 

meaning the crash frequency would be lower and an AMF greater than 1.0 meaning the 

crash frequency would increase. So, for example, an AMF of 0.95 means a reduction in 

expected crash frequency of 1.0 – 0.95, or 5 percent. 

Designers should be aware that traffic operational and substantive safety effects associated 

with incremental design dimensions will vary by facility type and context. For example, the 

change in capacity associated with a 1-foot change in lane width is different for a two-lane 

rural highway versus urban freeway versus signalized intersection approach. So, 

considering a design exception in each case will mean a different operational effect should 

be expected. 

Designers should also be mindful of the fundamental concept of exposure. As discussed in 

Chapter 2, exposure to traffic volume, length of highway, and duration of the design 

exception are of primary importance. A 5 percent reduction in capacity or expected increase 

in crash frequency will in many cases be negligible when converted to an annualized value; 

but in other contexts (say, a high-volume urban freeway) a 5 percent reduction in 

performance may translate to significant annual impacts. 

The information presented in each section is intended to provide the reader with a basic 

awareness and understanding of expected effects of design exceptions. At the end of the 

discussion of each criterion, a list of resources is provided for further consultation. 

Design Speed 

AASHTO defines design speed as follows: 

Design speed is a selected speed used to determine the various geometric features of the 

roadway. The assumed design speed should be a logical one with respect to the topography, 

anticipated operating speed, the adjacent land use, and the functional classification of the 

highway. 

Design speed is different from the other controlling criteria in that it is a design control, 

rather than a specific design element. In other words, the selected design speed establishes 

the range of design values for many of the other geometric elements of the highway 

(Figure 5). Because of its effect on so much of a highway’s design, the design speed is a 

fundamental and very important choice that a designer makes. The selected design speed 

should be high enough so that an appropriate regulatory speed limit will be less than or 

equal to it. Desirably, the speed at which drivers are operating comfortably will be close to 

the posted speed limit. 

In recognition of the wide range of site-specific conditions, constraints, and contexts that 

designers face, the adopted criteria allow a great deal of design flexibility by providing 

ranges of values for design speed (see Table 1) on page 26. For most cases, the ranges 

provide adequate flexibility for designers to choose an appropriate design speed without 

the need for a design exception. A Guide for Achieving Flexibility in Highway Design

(AASHTO) provides additional information on how to apply this flexibility for selecting 

appropriate design speeds for various roadway types and contexts. 

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Chapter 3—The 13 Controlling Criteria 

25 25 

For projects on extended alignments, design exceptions will be rare primarily because, as 

shown in Table 1, the range for acceptable design speeds is broad. If a limited portion of an 

alignment must be designed to a lower speed, it may be more appropriate to evaluate 

specific geometric element(s) and treat those as design exceptions (instead of the design 

control). 

In the rare instances where a design exception for design speed appears necessary over an 

extended alignment, it is best to evaluate the expected performance of the continuous 

alignment to refine the design, and highlight specific locations for mitigation. 

FIGURE 5 

Because it is a 

design control, 

design speed affects 

the curvature 

(radius), stopping 

sight distance, 

superelevation, and 

other features of this 

horizontal curve. 

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Mitigation Strategies For Design Exceptions 

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TABLE 1 

Ranges for Design Speed 

Type of Rural Urban 

Roadway Terrain 

US (mi/h) Metric (km/h) US (mi/h) Metric (km/h) 

Level 70 110 50 min 80 min 

Rolling 70 110 50 min 80 min 

Freeway 

Mountainous 50–60 80–100 50 min 80 min 

Level 60–75 100–120 30–60 50–100 

Rolling 50–60 80–100 30–60 50–100 

Arterial 

Mountainous 40–50 60–80 30–60 50–100 

Level 40–60 60–100 30+ 50+ 

Rolling 30–50 50–80 30+ 50+ 

Collector 

Mountainous 20–40 30–60 30+ 50+ 

Level 30–50 50–80 20–30 30–50 

Rolling 20–40 30–60 20–30 30–50 

Local 

Mountainous 20–30 30–50 20–30 30–50 

Source: A Policy on Geometric Design of Highways and Streets, AASHTO 

Clarification: Ramp Design Speeds for Freeways and 

Interchanges 

Exhibit 10-56 in the Green Book provides “guide values” for selection of ramp design speeds 

as a function of the highway design speed. According to the Policy, ramp design speeds 

should not be less than the low range presented in Exhibit 10-56, with other specific 

guidance offered for particular types of ramps (loops, direct and semi-direct connections). 

Some States have adopted design policies requiring the use of middle or higher range 

values for certain cases, such as system interchanges. 

Designers are occasionally confronted with situations in which the appropriate ramp design 

speed per Exhibit 10-56 may not be achievable. Such cases are almost always associated with 

the inability to achieve minimum radius for the controlling curvature of the exit or entrance 

ramp. Not meeting the lower (50 percent) range per Exhibit 10-56 requires a design 

exception per FHWA policy. Where the design issue involves curvature, a design exception 

should be prepared for the non-standard horizontal curve rather than for the use of a lower 

design speed for the ramp. 

Evaluating Reduced Design Speed 

Research confirms that lower speeds are safer and lowering speed limits can decrease both 

crash frequency and severity. However, speeds cannot be reduced simply by changing the 

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Chapter 3—The 13 Controlling Criteria 

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posted speed limit. Geometric and cross-sectional elements, in combination with the 

context, establish a driving environment where drivers choose speeds that feel reasonable 

and comfortable. 

One tool that designers can use to determine where operating speeds may exceed the design 

speed on rural two-lane highways is the Design Consistency Module of the IHSDM (see 

Chapter 1). This module can identify speed discrepancies, both in terms of level of 

magnitude and length of highway affected. Mitigation strategies can then be targeted to the 

locations where speed discrepancies are expected. 

Research suggests that crash risk increases with increasing differentials in speed (Table 2). 

Such differentials can be between adjoining highway sections (change in 85th percentile 

speeds due to changes in roadway geometry) or between speeds of vehicles in the same 

traffic stream (such as trucks and passenger vehicles). Exhibit 3-58 in the Green Book

provides information on the crash rate of trucks as a function of the speed differential of 

trucks to the average running speed of all traffic. 

TABLE 2

Relative Risk of Differential Speed Caused by Changes in Roadway Geometry 

Speed Differential (∆V) Safety Risk 

∆V < 5 mi/hr Low 

5 mi/hr < ∆V < 15 mi/hr Medium 

∆V > 15 mi/hr High 

Design Speed Resources 

• A Policy on Design Standards Interstate System, AASHTO, 2005. 

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

• A Guide for Achieving Flexibility in Highway Design, AASHTO, 2004. 

• Design Speed, Operating Speed, and Posted Speed Practices, NCHRP Report 504, 

Transportation Research Board, 2003. 

• A Guide to Best Practices for Achieving Context Sensitive Solutions, NCHRP Report 480, 

Transportation Research Board, 2002. 

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

2001.

• Highway Safety Design and Operations Guide, AASHTO, 1997. 

Lane Width 

The adopted criteria describe design values for through travel lanes, auxiliary lanes, ramps, 

and turning roadways. There are also recommended widths for special-purpose lanes such 

as continuous two-way left-turn lanes. AASHTO also provides guidance for widening lanes 

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Mitigation Strategies For Design Exceptions 

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through horizontal curves to provide for the off-tracking requirements of large trucks. Lane 

width does not include shoulders, curbs, and on-street parking areas. Table 3 summarizes 

the range of lane widths for travel lanes and ramps. 

TABLE 3 

Ranges for Lane Width 

Type of Roadway Rural Urban 

US (feet) Metric (meters) US (feet) Metric (meters) 

Freeway 12 3.6 12 3.6 

Ramps (1-lane) 12–30 3.6–9.2 12–30 3.6–9.2 

Arterial 11–12 3.3–3.6 10–12 3.0–3.6 

Collector 10–12 3.0–3.6 10–12 3.0–3.6 

Local 9–12 2.7–3.6 9–12 2.7–3.6 

Source: A Policy on Geometric Design of Highways and Streets, AASHTO

It is FHWA policy that the requirement of a formal design exception for lane width is 

applicable for all travel lanes, including auxiliary lanes and ramps. With respect to the 

practice of widening lanes through horizontal curves, a formal design exception is not 

necessary for cases not providing additional lane width, but the decision should be 

documented in project records. Exhibit 7-3 in the Green Book describes minimum lane widths 

for two-lane rural highways for a range of design speeds and design-year traffic. The table 

entries show a 24-foot traveled way (12-foot lanes) for most conditions. Careful inspection of 

this table (see subnote [a]) shows that 11-foot lanes are acceptable and within policy for 

reconstruction projects in which an existing 22-foot dimension is operating in a satisfactory 

manner. For such cases, the designer should document this is the case, but retention of the 

11-foot width would not require a design exception. 

Safety 

Speed is a primary consideration when evaluating potential adverse impacts of lane width 

on safety. On high-speed, rural two-lane highways, an increased risk of cross-centerline 

head-on or cross-centerline sideswipe crashes is a concern because drivers may have more 

difficulty staying within the travel lane. On any high-speed roadway, the primary safety 

concerns with reductions in lane width are crash types related to lane departure, including 

run-off-road crashes. The mitigation strategies for lane width presented in Chapter 4 focus 

on reducing the probability of these crashes. 

In a reduced-speed urban environment, the effects of reduced lane width are different. On 

such facilities, the risk of lane-departure crashes is less. The design objective is often how to 

best distribute limited cross-sectional width to maximize safety for a wide variety of 

roadway users. Narrower lane widths may be chosen to manage or reduce speed and 

shorten crossing distances for pedestrians. Lane widths may be adjusted to incorporate 

other cross-sectional elements, such as medians for access control, bike lanes, on-street 

parking, transit stops, and landscaping. The adopted ranges for lane width in the urban, 

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Chapter 3—The 13 Controlling Criteria 

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low-speed environment normally provide adequate flexibility to achieve a desirable urban 

cross section without a design exception. 

Designers should understand the interrelationships among lane width and other design 

elements. On high-speed roadways with narrow lanes that also have narrow shoulders, the 

risk of severe lane-departure crashes increases. Drivers on rural two-lane highways may 

shift even closer to the centerline as they become less comfortable next to a narrow shoulder. 

At other times, they may shift closer to the shoulder edge and are at greater risk of driving 

off the paved portion of the roadway (and over potential edge drop-offs) as they meet 

oncoming traffic. 

Horizontal alignment is another factor that can influence the safety of lane width reductions. 

Curvilinear horizontal alignments increase the risk of lane departure crashes in general, and 

when combined with narrow lane widths, the risk will further increase for most high-speed 

roadways. In addition, trucks and other large vehicles can affect safety and operations by 

off-tracking into adjacent lanes or the shoulder. This affects the safety of other drivers, as 

well as non-motorized users such as bicyclists who may be using the adjacent lane or 

shoulder. It is important to understand this interaction of design elements when a design 

exception for lane with is being evaluated. 

Substantive Safety 

Figure 6 shows accident modification factors for variations in lane width on rural two-lane 

highways. Note that there is little difference between 11- and 12-foot lanes. 

FIGURE 6 

Accident Modification Factors for Lane Width on Rural Two-Lane Highways. 

(Source: Prediction of the Expected Safety Performance of Rural Two-Lane Highways, FHWA) 

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Mitigation Strategies For Design Exceptions 

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For multilane urban arterials and multilane rural arterials, the expected difference in 

substantive safety for variations in lane width is much less—on the order of a few 

percentage points when comparing lane widths of 10 to 12 feet. 

Traffic Operations 

Lane width has an effect on traffic operations and highway capacity, particularly for highspeed roadways. The interaction of lane width with other geometric elements, primarily 

shoulder width, also affects operations. 

When determining highway capacity, adjustments are made to reflect the effect of lane 

width on free-flow speeds. Lane widths of less than 12 feet (3.6 meters) reduce travel speeds 

on high-speed roadways, as summarized in Tables 4 and 5. 

TABLE 4 

Operational Effects of Freeway Lane Widths 

Lane width (ft) Reduction in Free-Flow Speed (mi/h) 

12 0.0 

11 1.9 

10 6.6 

Lane width (m) Reduction in Free-Flow Speed (km/h) 

3.6 0.0 

3.5 1.0 

3.4 2.1 

3.3 3.1 

3.2 5.6 

3.1 8.1 

3.0 10.6 

Source: Highway Capacity Manual 

TABLE 5 

Operational Effects of Lane and Shoulder Width on Two-Lane Highways 

Reduction in Free-Flow Speed (mi/h) 

Shoulder Width (ft) 

Lane width (ft) ≥0<2 ≥2<4 ≥4<6 ≥6 

9<10 6.4 4.8 3.5 2.2 

≥10<11 5.3 3.7 2.4 1.1 

≥11<12 4.7 3.0 1.7 0.4 

≥12 4.2 2.6 1.3 0.0 

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Chapter 3—The 13 Controlling Criteria 

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TABLE 5 (CONTINUED) 

Operational Effects of Lane and Shoulder Width on Two-Lane Highways 

Reduction in Free-Flow Speed (km/h) 

Shoulder Width (m) 

Lane width (m) ≥0.0<0.6 ≥0.6<1.2 ≥1.2<8 ≥1.8 

2.7<3.0 10.3 7.7 5.6 3.5 

≥3.0<3.3 8.5 5.9 3.8 1.7 

≥3.3<3.6 7.5 4.9 2.8 0.7 

≥3.6 6.8 4.2 2.1 0.0 

Source: Highway Capacity Manual 

Summary 

Table 6 summarizes the potential adverse impacts to safety and operations for a design 

exception for lane width. 

TABLE 6 

Lane Width: Potential Adverse Impacts to Safety and Operations

Safety & Operational Issues Freeway Expressway Rural 

Two-Lane 

Urban 

Arterial 

Run-off-road crashes X X X 

Cross-median crashes X X 

Cross-centerline crashes X 

Sideswipe (same direction) crashes X X X 

Rear-end crashes if operations 

deteriorate (abrupt speed reduction) X X X 

Reduced free-flow speeds X X X X 

Large vehicles off-tracking into adjacent 

lane or shoulder X X X X 

Freeway: high-speed, multi-lane divided highway with interchange access only (rural or urban). 

Expressway: high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban). 

Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, or local). 

Urban Arterial: urban arterials with speeds 45 mi/h (70 km/h) or less. 

Lane Width Resources 

• A Policy on Design Standards Interstate System, AASHTO, 2005. 

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

• Guide for the Planning, Design, and Operation of Pedestrian Facilities, AASHTO, 2004. 

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Mitigation Strategies For Design Exceptions 

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• A Guide for Reducing Collisions on Horizontal Curves, NCHRP Report 500, Volume 7, 

Transportation Research Board, 2004. 

• A Guide for Reducing Collisions Involving Pedestrians, NCHRP Report 500, Volume 10, 

Transportation Research Board, 2004. 

• A Guide for Reducing Collisions Involving Heavy Trucks, NCHRP Report 500, Volume 13, 

Transportation Research Board, 2004. 

• A Guide for Addressing Head-On Collisions, NCHRP Report 500, Volume 4, Transportation 

Research Board, 2003. 

• A Guide for Addressing Run-Off-Road Collisions, NCHRP Report 500, Volume 6, 

Transportation Research Board, 2003. 

• Roadside Design Guide, AASHTO, 2002. 

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

2001.

• Highway Capacity Manual, Transportation Research Board, 2000. 

• Guide for the Development of Bicycle Facilities, AASHTO, 1999. 

• Highway Safety Design and Operations Guide, AASHTO, 1997. 

• Use of Shoulders and Narrow Lanes to Increase Freeway Capacity, NCHRP Report 369, 

Transportation Research Board, 1995. 

• Roadway Widths for Low-Traffic Volume Roads, NCHRP Report 362, Transportation 

Research Board, 1994. 

• Effective Utilization of Street Width on Urban Arterials, NCHRP Report 330, Transportation 

Research Board, 1990. 

• FHWA Roadside Hardware Web site 

http://safety.fhwa.dot.gov/roadway_dept/road_hardware/index.htm

Shoulder Width 

Shoulders provide a number of important functions. Safety and efficient traffic operations 

can be adversely affected if any of the following functions are compromised: 

• Shoulders provide space for emergency storage of disabled vehicles (Figure 7). 

Particularly on high-speed, high-volume highways such as urban freeways, the ability to 

move a disabled vehicle off the travel lanes reduces the risk of rear-end crashes and can 

prevent a lane from being closed, which can cause severe congestion and safety 

problems on these facilities. 

• Shoulders provide space for enforcement activities (Figure 7). This is particularly 

important for the outside (right) shoulder because law enforcement personnel prefer to 

conduct enforcement activities in this location. Shoulder widths of approximately 8 feet 

or greater are normally required for this function. 

• Shoulders provide space for maintenance activities (Figure 7). If routine maintenance 

work can be conducted without closing a travel lane, both safety and operations will be 

improved. Shoulder widths of approximately 8 feet or greater are normally required for 

this function. In northern regions, shoulders also provide space for storing snow that 

has been cleared from the travel lanes. 

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Chapter 3—The 13 Controlling Criteria 

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• Shoulders provide an area for drivers to maneuver to avoid crashes (Figure 7). This is 

particularly important on high-speed, high-volume highways or at locations where there 

is limited stopping sight distance. Shoulder widths of approximately 8 feet or greater 

are normally required for this function. 

• Shoulders improve bicycle accommodation (Figure 8). For most highways, cyclists are 

legally allowed to ride on the travel lanes. A paved or partially paved shoulder offers 

cyclists an alternative to ride with some separation from vehicular traffic. This type of 

shoulder can also reduce risky passing maneuvers by drivers. 

• Shoulders increase safety by providing a stable, clear recovery area for drivers who have 

left the travel lane. If a driver inadvertently leaves the lane or is attempting to avoid a 

crash or an object in the lane ahead, a firm, stable shoulder greatly increases the chance 

of safe recovery. However, areas with pavement edge drop-offs can be a significant 

safety risk. Edge drop-offs (Figure 9) occur where gravel or earth material is adjacent to 

the paved lane or shoulder. This material can settle or erode at the pavement edge, 

creating a drop-off that can make it difficult for a driver to safely recover after driving 

off the paved portion of the roadway. The drop-off can contribute to a loss of control as 

the driver tries to bring the vehicle back onto the roadway, especially if the driver does 

not reduce speed before attempting to recover. 

• Shoulders improve stopping sight distance at horizontal curves by providing an offset to 

objects such as barrier and bridge piers (Figure 10). 

• On highways with curb and enclosed drainage systems, shoulders store and carry water 

during storms, preventing water from spreading onto the travel lanes. 

• On high-speed roadways, shoulders improve capacity by increasing driver comfort. 

FIGURE 7 

Shoulders on 

this urban 

freeway provide 

enough width for 

crash 

avoidance, 

storage of 

disabled 

vehicles, 

maintenance 

activities, and 

enforcement. 

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Mitigation Strategies For Design Exceptions 

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FIGURE 8 

Partially-paved 

shoulders on this 

rural arterial 

improve bicycle 

accommodation 

and reduce risky 

passing 

maneuvers. 

FIGURE 9 

Pavement edge 

drop-off. 

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Chapter 3—The 13 Controlling Criteria 

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FIGURE 10 

Comparison of how 

shoulder width affects 

stopping sight 

distance past 

concrete bridge rail 

along horizontal 

curves. 

Table 7 summarizes the range of minimum shoulder widths for travel lanes and ramps. 

TABLE 7 

Ranges for Minimum Shoulder Width 

Type of Roadway Rural Urban 

US (feet) Metric (meters) US (feet) Metric (meters) 

Freeway 4–12 1.2–3.6 4–12 1.2–3.6 

Ramps (1-lane) 1–10 0.3–3.0 1–10 0.3–3.0 

Arterial 2–8 0.6–2.4 2–8 0.6–2.4 

Collector 2–8 0.6–2.4 2–8 0.6–2.4 

Local 2–8 0.6–2.4 — — 

Source: A Policy on Geometric Design of Highways and Streets, AASHTO 

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Mitigation Strategies For Design Exceptions 

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Clarification: Usable and Paved Shoulders 

Design values in the adopted criteria refer to both usable and paved shoulders. A usable 

shoulder width is the actual width available for the driver to make an emergency or parking 

stop. This is measured from the edge of traveled way to the point of intersection of the 

shoulder slope and mild slope (for example, 1:4 or flatter) or to beginning of rounding to 

slopes steeper than 1:4. 

Usable shoulders do not have to be paved. The adopted criteria note that rural arterial 

shoulders should be paved. FHWA policy does not require a design exception for shoulder 

type, but rather for the usable shoulder width dimension only. 

Clarification: Minimum Shoulder Widths for Interstate Highways 

One clarification for shoulder width design exceptions relates to the requirements for 

Interstates with six or more lanes. The adopted criteria for Interstates specify that the paved 

width of the right shoulder shall not be less than 10 feet (3.0 meters). Where truck traffic 

exceeds 250 DDHV (the design hourly volume for one direction), a paved shoulder width of 

12 feet (3.6 meters) should be considered. On a four-lane section, the paved width of the left 

shoulder shall be at least 4 feet (1.2 meters). On sections with six or more lanes, a 10-foot 

(3.0-meter) paved width for the left shoulder should be provided. Where truck traffic 

exceeds 250 DDHV, a paved width of 12 feet (3.6 meters) should be considered. 

Regardless of the differences in language used in the adopted criteria (“shall,” “should be 

considered,” etc.) all of the shoulder widths described above have become standards for the 

Interstate System by virtue of their adoption by FHWA, and they are the minimum values 

for each condition described. Therefore, a project designed for the Interstate System that 

does not provide the applicable shoulder widths would require a formal design exception. 

In addition, the incorporation of high-occupancy vehicle (HOV) lanes is now common 

practice on many urban freeways. Lower-cost design solutions have in many cases resulted 

in the conversion of an existing full-width (12-foot) shoulder to a designated HOV lane. 

Where conversion of a shoulder to HOV use is being considered and replacement or 

construction of a new shoulder is not proposed, a design exception is required (potentially 

for both shoulder width and lateral offset to obstruction). 

Substantive Safety 

Figure 11 illustrates how variations in shoulder width can affect safety on rural two-lane 

highways. Note that the substantive safety effects of incremental shoulder widths are less on 

multilane arterials and on lower-speed urban arterials. 

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Chapter 3—The 13 Controlling Criteria 

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FIGURE 11 

Accident Modification Factors for Shoulder Width on Rural Two-Lane Highways. 

(Source: Prediction of the Expected Safety Performance of Rural Two-Lane Highways, FHWA) 

Traffic Operations 

Shoulder width has a measurable effect on traffic operations and highway capacity, 

particularly for high-speed roadways. The interaction of shoulder width with other 

geometric elements, primarily lane width, also affects operations. 

When determining highway capacity, adjustments are made to reflect the effect of shoulder 

width on free-flow speeds. Table 5 summarizes these effects for rural two-lane highways 

and Table 8 summarizes effects for freeways. 

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Mitigation Strategies For Design Exceptions 

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TABLE 8 

Operational Effects of Freeway Shoulder Widths 

Reduction in Free-Flow Speed (mi/h) 

Right-Shoulder Lanes in One Direction 

Lateral Clearance 

(ft) 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 

Reduction in Free-Flow Speed (km/h) 

Right-Shoulder Lanes in One Direction 

Lateral Clearance 

(m) 2 3 4 ≥5 

≥1.8 0.0 0.0 0.0 0.0 

1.5 1.0 0.7 0.3 0.2 

1.2 1.9 1.3 0.7 0.4 

0.9 2.9 1.9 1.0 0.6 

0.6 3.9 2.6 1.3 0.8 

0.3 4.8 3.2 1.6 1.1 

0.0 5.8 3.9 1.9 1.3 

Source: Highway Capacity Manual 

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Chapter 3—The 13 Controlling Criteria 

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Summary 

Table 9 summarizes the potential adverse impacts to safety and operations of a design 

exception for shoulder width. 

TABLE 9 

Shoulder Width: Potential Adverse Impacts to Safety and Operations

Safety & Operational Issues Freeway Expressway Rural 

Two-Lane 

Urban 

Arterial 

Run-off-road crashes X X X 

Cross-median crashes X X 

Cross-centerline crashes X 

Pavement edge dropoffs X X X 

Rear-end crashes if operations deteriorate 

(abrupt speed reduction) X X X 

Lane blockage from incidents X X X 

Reduced free-flow speeds X X X 

Shying away from the edge of the roadway X X X 

Inadequate space for enforcement activities and 

emergency response 

X X X 

Inadequate space for emergency pullover X X X 

Inadequate space to avoid crashes or objects on 

the travel lanes X X X 

Lack of storage space for disabled vehicles X X X 

Bicyclists forced onto the travel lanes. X X X 

Inadequate space for maintenance activities X X X 

Assumed 

cross section 

with curb and 

gutter (no 

shoulders) 

Freeway: high-speed, multi-lane divided highway with interchange access only (rural or urban). 

Expressway: high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban). 

Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, or local). 

Urban Arterial: urban arterials with speeds 45 mi/h (70 km/h) or less. 

Shoulder Width Resources 

• A Policy on Design Standards Interstate System, AASHTO, 2005. 

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

• A Guide for Achieving Flexibility in Highway Design, AASHTO, 2004. 

• A Guide for Addressing Head-On Collisions, NCHRP Report 500, Volume 4, Transportation 

Research Board, 2003. 

• A Guide for Addressing Run-Off-Road Collisions, NCHRP Report 500, Volume 6, 

Transportation Research Board, 2003. 

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Mitigation Strategies For Design Exceptions 

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• Roadside Design Guide, AASHTO, 2002. 

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

2001.

• Highway Capacity Manual, Transportation Research Board, 2000. 

• Guide for the Development of Bicycle Facilities, AASHTO, 1999. 

• Highway Safety Design and Operations Guide, AASHTO, 1997. 

• Use of Shoulders and Narrow Lanes to Increase Freeway Capacity, NCHRP Report 369, 

Transportation Research Board, 1995. 

• Roadway Widths for Low-Traffic Volume Roads, NCHRP Report 362, Transportation 

Research Board, 1994. 

• FHWA Roadside Hardware Web site

http://safety.fhwa.dot.gov/roadway_dept/road_hardware/index.htm

Bridge Width 

Bridge width is the total width of all lanes and shoulders on the bridge, measured between 

the points on the bridge rail, curb, or other vertical elements that project the farthest onto 

the roadway (Figure 12). A bridge width that meets adopted criteria maintains the 

minimum acceptable lane and shoulder width for the particular design condition as defined 

by area, functional class, design speed, and traffic volume. A design exception is required 

when a bridge is proposed to be constructed with narrower lanes, shoulders, or both. 

Potential problems associated with narrow bridges are twofold. Relatively short bridges 

represent a discontinuity that may affect driver behavior. The narrowed cross section can 

make some drivers uncomfortable and cause them to dramatically reduce speed, increasing 

the risk of rear-end crashes and degrading operations on high-speed, high-volume facilities. 

The bridge rail may be close enough to the travel lanes to cause drivers to shy towards the 

centerline or into adjacent lanes (Figure 13). The bridge infrastructure itself is closer to the 

edge of pavement and thus represents a roadside hazard. Even when properly designed and 

delineated, there is an increased risk of a roadside collision with a bridge end closer to the 

edge of traveled way. 

A second set of concerns is evident for longer bridges (say, greater than 500 feet in length). 

The safety and operational concerns at narrow bridges are similar to those on roads with 

narrow shoulders. There may be inadequate space for storage of disabled vehicles, 

enforcement activities, emergency response, and maintenance work. The lack of shoulder 

width on the bridge may make it impossible to avoid a crash or object on the roadway 

ahead. In addition, options are limited for non-motorized users such as bicyclists, forcing 

them onto the traveled lanes or close to the bridge rail. 

Narrow bridges on horizontal curves can have limited horizontal stopping sight distance 

past the bridge rail (Figure 10). Operations can be degraded, particularly on long bridges on 

high-speed roadways, because of speed reductions as drivers enter the narrowed cross 

section as well as a decrease in driver comfort on the bridge. 

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Chapter 3—The 13 Controlling Criteria 

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FIGURE 12 

Bridge width. 

FIGURE 13 

Vehicle shying 

towards the 

centerline on a 

narrow bridge. 

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Mitigation Strategies For Design Exceptions 

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Summary 

Table 10 summarizes the potential adverse impacts to safety and operations of a design 

exception for bridge width. 

TABLE 10 

Bridge Width: Potential Adverse Impacts to Safety and Operations

Safety & Operational Issues Freeway Expressway Rural 

Two-Lane 

Urban 

Arterial 

Collision with bridge rail or approach guardrail X X X X 

Rear-end crashes (abrupt speed reduction) X X X 

Cross-centerline crashes X X 

Degraded operations because of abrupt speed 

reduction as drivers approach bridge X X X 

Reduced free-flow speeds X X X X 

Inadequate space for enforcement activities and 

emergency response (long bridges) X X X X 

Lane blockage from incidents (long bridges) X X X X 

Shying away from the bridge rail X X X X 

Inadequate space for bicyclists X X X X 

Inadequate space for emergency pullover (long 

bridges) X X X X 

Inadequate space to avoid crashes or objects on 

the travel lanes X X X X 

Lack of storage space for disabled vehicles (long 

bridges) X X X X 

Freeway: high-speed, multi-lane divided highway with interchange access only (rural or urban). 

Expressway: high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban). 

Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, or local). 

Urban Arterial: urban arterials with speeds 45 mi/h (70 km/h) or less. 

Substantive Safety 

In evaluating the potential substantive safety of narrow bridges, the designer should 

consider the two types of conditions described above. For short bridges, the safety risk can 

be modeled by use of the Roadside Safety Analysis Program (see the AASHTO Roadside 

Design Guide). Based on traffic volumes and the widths in question, a designer can estimate 

the relative increased risk of the bridge end closer to the traveled way. 

For longer bridges, the designer can reference information in the shoulder width section, 

such as Figure 11, to gain an understanding of the incremental increase in safety risk with a 

narrower dimension for the combination of lane and shoulder width. 

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Chapter 3—The 13 Controlling Criteria 

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Bridge Width Resources 

• A Policy on Design Standards Interstate System, AASHTO, 2005. 

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

• A Guide for Addressing Head-On Collisions, NCHRP Report 500, Volume 4, Transportation 

Research Board, 2003. 

• Roadside Design Guide, AASHTO, 2002. 

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

2001.

• Highway Safety Design and Operations Guide, AASHTO, 1997. 

• FHWA Roadside Hardware Web site 

http://safety.fhwa.dot.gov/roadway_dept/road_hardware/index.htm

Horizontal Alignment

In terms of the 13 controlling criteria, the term horizontal alignment refers only to the 

horizontal curvature of the roadway (Figure 14). The adopted design criteria specify a 

minimum radius for the selected design speed, which is calculated from the maximum rate 

of superelevation (set by policy from a range of options) and the side friction factor 

(established by policy through research). Superelevation is considered a separate criterion 

and is discussed below. Horizontal alignment influences another primary controlling 

criterion, stopping sight distance. 

Curve design policy published by AASHTO is based on a series of assumptions of driver 

behavior and operations. Drivers are assumed to track the curve in a passenger car at design 

speed. The combination of superelevation, side friction, and radius are established to 

provide for an acceptable level of comfort for the majority of drivers. The design model 

applies to the full range of highway types and conditions. 

The radii of curves are one variable that affects the risk of lane-departure crashes on highspeed roadways. Other contributing factors may include the amount of superelevation, the 

surface friction of the pavement, and the horizontal and vertical alignments preceding the 

curve. Inadequate superelevation or pavement friction can contribute to vehicles skidding 

as they maneuver through a curve. The alignment preceding a curve influences approach 

speeds. The expected crash frequency increases as the speed differential from the approach 

tangent to the curve increases. This may occur if the curve is preceded by a long segment of 

tangent roadway (versus a continuously curvilinear alignment that encourages lower 

speeds), if the approach is on a significant downgrade, or if the curve is not visible to the 

driver on the approach. 

At ramps and loops, a lack of deceleration length can contribute to drivers running off the 

first curve after exiting a freeway. 

Horizontal curves can present special safety problems for trucks and other large vehicles. 

Because of their higher center of mass, large vehicles are more susceptible to overturning at 

curves. Research confirms that such overturning can occur at speeds only slightly greater 

than the design speed of the curve. As discussed in the lane width section, off-tracking of 

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Mitigation Strategies For Design Exceptions 

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large vehicles onto the adjacent lane or shoulder at horizontal curves can affect the safety of 

drivers and bicyclists and degrade operations. 

The risk of lane-departure crashes at curves is significantly influenced by speed, which is 

why curves in reduced-speed urban environments generally present fewer safety and 

operational concerns for the horizontal alignment criterion. 

FIGURE 14 

Horizontal alignment. 

Traffic Operations 

Curves influence speed behavior. Curvilinear roads will have lower speeds, which can 

negatively affect highway capacity. However, for some highway types and contexts, lower 

speeds can be beneficial—for example, reduced-speed urban environments where lower 

speeds increase safety for pedestrians. On rural two-lane highways, curves will limit 

available passing zones and thereby influence capacity. 

A curve that is nominally unsafe (has a radius less than the minimum for the selected design 

speed) may or may not present an unusual operational or safety risk. Such risk depends on 

the site conditions. One approach to characterizing this risk for two-lane rural highways is 

through use of the Design Consistency Module of FHWA’s IHSDM (see Chapter 1). The 

design consistency module predicts the 85th percentile speed along an alignment as a 

function of grade, horizontal alignment, roadway width, and direction of travel. 

Designers can estimate speeds produced on the approach to a sharp curve to determine the 

extent of concern over its use or acceptability. A designer can estimate both the 85th

percentile speed through the curve, as well as the change in speeds produced by the 

alignment of both approaches. Marginal speed reductions and/or differences between 

operating and design speed (say, less than 10 mi/hr) may be considered acceptable. 

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Chapter 3—The 13 Controlling Criteria 

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Substantive Safety 

The substantive safety performance of a roadway is influenced by the presence and design 

characteristics of horizontal curvature, including both the length of curve and radius. Other 

factors contributing to substantive safety of curves include the cross section and the 

character of the roadside through the curve. The following AMF can be used to predict how 

variations in horizontal alignment will affect the expected safety performance of rural twolane highways: 

AMF = (1.55Lc + 80.2/R – 0.012S)

 1.55Lc

Where, 

Lc = length of horizontal curve (mi) 

R = radius of curvature (ft) 

S = 1 if spiral transition curve is present 

 = 0 if spiral transition curve is not present 

The difference in substantive safety between two designs can be estimated by comparing the 

result of exercising this function for the two cases and comparing the results. Note that at a 

given location the curve’s central angle will be fixed, and hence a milder curve than the 

alternative will be longer. Note that the effect on total safety risk will vary with traffic 

volume as well. Designers may accept a design exception for curvature on a roadway with a 

design volume of 750 vehicles per day (vpd), but reach a different conclusion for a road with 

a design volume of 8,000 vpd. 

Summary 

Table 11 summarizes the potential adverse impacts to safety and operations of a design 

exception for horizontal alignment. 

TABLE 11 

Horizontal Alignment: Potential Adverse Impacts to Safety and Operations

Safety & Operational Issues Freeway Expressway Rural 

Two-Lane 

Urban 

Arterial 

Run-off-road crashes X X X 

Cross-median crashes X X 

Cross-centerline crashes X X 

Large vehicle rollover crashes X X X 

Large vehicles off-tracking into 

adjacent lane or shoulder X X X X 

Skidding X X X X 

Rear-end crashes if operations 

deteriorate (abrupt speed reduction) X X X 

Reduced free-flow speeds X X X X 

Freeway: high-speed, multi-lane divided highway with interchange access only (rural or urban). 

Expressway: high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban). 

Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, or local). 

Urban Arterial: urban arterials with speeds 45 mi/h (70 km/h) or less. 

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Mitigation Strategies For Design Exceptions 

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Horizontal Alignment Resources 

• Low-Cost Treatments for Horizontal Curve Safety, FHWA, 2006. 

• Communicating Changes in Horizontal Alignment, NCHRP Report 559, Transportation 

Research Board, 2006. 

• A Policy on Design Standards Interstate System, AASHTO, 2005. 

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

• A Guide for Reducing Collisions on Horizontal Curves, NCHRP Report 500, Volume 7, 

Transportation Research Board, 2004. 

• A Guide for Reducing Collisions Involving Heavy Trucks, NCHRP Report 500, Volume 13, 

Transportation Research Board, 2004. 

• Review of Truck Characteristics as Factors in Roadway Design, NCHRP Report 505, 

Transportation Research Board, 2003. 

• A Guide for Addressing Head-On Collisions, NCHRP Report 500, Volume 4, Transportation 

Research Board, 2003. 

• A Guide for Addressing Run-Off-Road Collisions, NCHRP Report 500, Volume 6, 

Transportation Research Board, 2003. 

• Roadside Design Guide, AASHTO, 2002. 

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

2001.

• FHWA Roadside Hardware Web site 

http://safety.fhwa.dot.gov/roadway_dept/road_hardware/index.htm

Superelevation 

Superelevation is the rotation of the pavement on the approach to and through a horizontal 

curve. Superelevation is intended to assist the driver by counteracting the lateral 

acceleration produced by tracking the curve. Superelevation is expressed as a decimal, 

representing the ratio of the pavement slope to width, ranging from 0 to 0.12 foot/feet. The 

adopted criteria allow for the use of maximum superelevation rates from 0.04 to 0.12. 

Maximum superelevation rates for design are established by policy by each State. 

Selection of a maximum superelevation rate is based on several variables, such as climate, 

terrain, highway location (urban vs. rural), and frequency of very slow-moving vehicles. 

For example, northern States that experience ice and snow conditions may establish lower 

maximums for superelevation than States that do not experience these conditions. Use of 

lower maximum superelevation rates by policy is intended to address the perceived 

problem created by vehicles sliding transversely when traveling at very low speeds when 

weather conditions are poor. 

The adopted criteria provide complete tables expressing the appropriate superelevation rate 

consistent with the established policy for all curves and all design speeds. 

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Chapter 3—The 13 Controlling Criteria 

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FIGURE 15 

Superelevation 

Clarifications 

A formal design exception is required if the State’s superelevation policy cannot be met in 

design of any curve on the NHS. Thus, if a State’s maximum policy is set at 0.06 and a 

design is proposed that would use a superelevation rate greater than 0.06 (but within overall 

AASHTO guidance) this is considered an exception. A design exception is also required if a 

superelevation rate is proposed that is different from the published rate per the State’s 

policy for that curve, regardless of whether the curve is a controlling one (minimum radius 

for a design speed) or not. 

Note that no design exception is required for superelevation transition lengths. Also, some 

States employ spiral curves for high speed and sharper curves to help develop 

superelevation. For States that use spiral transitions, the inability or decision to not use a 

spiral does not require a design exception. 

Safety and Operational Considerations 

The safety and operational concerns related to inadequate superelevation are similar to 

those discussed in the horizontal alignment section. Inadequate superelevation can cause 

vehicles to skid as they travel through a curve, potentially resulting in a run-off-road crash. 

Trucks and other large vehicles with high centers of mass are more likely to roll over at 

curves with inadequate superelevation. 

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Mitigation Strategies For Design Exceptions 

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Substantive Safety 

Table 12 reports how variations in superelevation affect safety on rural two-lane highways. 

A superelevation deficiency is one in which there is insufficient superelevation compared to 

that specified by the appropriate design policy and values. 

TABLE 12 

Accident Modification Factors for Superelevation on Rural Two-Lane Highways 

Superelevation Deficiency Accident Modification Factor 

0.02 1.06 

0.03 1.09 

0.04 1.12 

0.05 1.15 

Source: Prediction of the Expected Safety Performance of Rural Two-Lane Highways, FHWA

Summary 

Table 13 summarizes the potential adverse impacts to safety and operations of a design 

exception for superelevation. 

TABLE 13 

Superelevation: Potential Adverse Impacts to Safety and Operations

Safety & Operational Issues Freeway Expressway Rural 

Two-Lane 

Urban 

Arterial 

Run-off-road crashes X X X 

Cross-median crashes X X 

Cross-centerline crashes X 

Skidding X X X X 

Large vehicle rollover crashes X X X 

Freeway: high-speed, multi-lane divided highway with interchange access only (rural or urban). 

Expressway: high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban). 

Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, or local). 

Urban Arterial: urban arterials with speeds 45 mi/h (70 km/h) or less. 

Superelevation Resources 

• Low-Cost Treatments for Horizontal Curve Safety, FHWA, 2006. 

• A Policy on Design Standards Interstate System, AASHTO, 2005. 

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

• A Guide for Reducing Collisions Involving Heavy Trucks, NCHRP Report 500, Volume 13, 

Transportation Research Board, 2004. 

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Chapter 3—The 13 Controlling Criteria 

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• Review of Truck Characteristics as Factors in Roadway Design, NCHRP Report 505, 

Transportation Research Board, 2003. 

• A Guide for Addressing Head-On Collisions, NCHRP Report 500, Volume 4, Transportation 

Research Board, 2003. 

• A Guide for Addressing Run-Off-Road Collisions, NCHRP Report 500, Volume 6, 

Transportation Research Board, 2003. 

• Roadside Design Guide, AASHTO, 2002. 

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

2001.

• FHWA Roadside Hardware Web site 

http://safety.fhwa.dot.gov/roadway_dept/road_hardware/index.htm

Vertical Alignment 

In terms of the 13 controlling criteria, vertical alignment includes grade as well as vertical 

curvature (both crest and sag); grade is considered separately and discussed below. Vertical 

curvature influences another primary controlling criterion, stopping sight distance. The 

geometric design basis for minimum length of crest vertical curvature is to provide the 

minimum stopping sight distance for the combination of grades and design speed. Sag 

vertical curves are normally designed so the curve does not restrict the distance of roadway 

illuminated by vehicle headlights, which would reduce stopping sight distance at night. The 

influence of and design considerations regarding design exceptions for vertical curvature 

are discussed below in the section on stopping sight distance. 

Refer to the sections on grade and stopping sight distance for more information on vertical 

alignment. 

Vertical Alignment Resources 

• A Policy on Design Standards Interstate System, AASHTO, 2005. 

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

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

2001.

Grade 

Grade is the rate of change of the vertical alignment. Grade affects vehicle speed and vehicle 

control, particularly for large trucks. The adopted criteria express values for both maximum 

and minimum grade. The inability to meet either a maximum or minimum value may 

produce operational or safety problems. 

A primary safety concern is the potential for drivers of heavy trucks to lose control as they 

descend steep grades. A design exception is required if the maximum grade is exceeded. 

Minimum grades to achieve proper drainage have also been established, and a design 

exception is required for highway segments that are flatter than the minimum grade. 

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Speed differential on highways with steep grades can contribute to safety and operational 

problems. Trucks and other heavy vehicles lose speed on steep, ascending grades and may 

be unable to reach full highway speed until they have passed the crest of the steep grade. 

Vehicles behind them are slowed, degrading operations at the least, and contributing to 

rear-end conflicts and in some cases risky passing maneuvers at the worst. Truck drivers 

may also choose to descend grades at slower speeds to maintain better control of their 

vehicles. Operations may be degraded for faster-moving vehicles from behind, creating an 

increased risk of rear-end crashes and risky passing maneuvers. 

Another potential safety concern is present when a horizontal curve lies at the bottom of a 

steep grade (Figure 16). This combination of alignments increases the risk of severe run-offroad crashes. 

FIGURE 16 

Horizontal curve at 

the base of a steep 

grade. 

Clarification 

The adopted criteria also include achieving a minimum grade. Grades of at least 0.30 

percent are considered necessary to achieve appropriate drainage of the pavement. Where 

very mild grades are used for significant lengths of highway, care should be taken to assure 

the combination of cross slope (see discussion below) and grade are sufficient for good 

drainage. A design exception is required when either the maximum grade for a design 

condition is exceeded, or when the minimum grade cannot be achieved. 

Traffic Operations 

The combination of grades, including length of grade, and horizontal curvature can have a 

demonstrable influence on vehicle speeds. One tool for assessing this operational condition 

is the Design Consistency Module of FHWA’s IHSDM (see Chapter 1). This module 

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Chapter 3—The 13 Controlling Criteria 

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produces a speed profile for continuous alignment by direction of travel. It can be used to 

test alignment variations, and provide a direct operational measure of a design exception for 

maximum grade. 

Substantive Safety 

Table 14 illustrates how variations in grade may affect safety on rural two-lane highways. 

TABLE 14 

Accident Modification Factors for Grade on Rural Two-Lane Highways 

Grade (%) Accident Modification Factor 

0 1.00 

2 1.03 

4 1.07 

6 1.10 

8 1.14 

Source: Prediction of the Expected Safety Performance of Rural Two-Lane Highways, FHWA

Summary 

Table 15 summarizes the potential adverse impacts to safety and operations of a design 

exception for grade. 

TABLE 15 

Grade: Potential Adverse Impacts to Safety and Operations

Safety and Operational Issues Freeway Expressway Rural 

Two-Lane 

Urban 

Arterial 

Trucks losing control descending grade X X X 

Risky passing maneuvers X X 

Reduced speeds ascending grade X X X X 

Reduced speeds descending grade X X X X 

Run-off-road crashes, particularly where steep 

grades are combined with horizontal curves X X X 

Rear-end crashes descending grade X X X 

Slick pavement (flat grades) X X X X 

Water ponding on the pavement surface (flat 

grades) X X X X 

Water spreading onto the traveled lanes (flat 

grades) X 

Freeway: high-speed, multi-lane divided highway with interchange access only (rural or urban). 

Expressway: high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban). 

Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, or local). 

Urban Arterial: urban arterials with speeds 45 mi/h (70 km/h) or less. 

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Grade Resources 

• A Policy on Design Standards Interstate System, AASHTO, 2005. 

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

• A Guide for Reducing Collisions on Horizontal Curves, NCHRP Report 500, Volume 7, 

Transportation Research Board, 2004. 

• A Guide for Reducing Collisions Involving Heavy Trucks, NCHRP Report 500, Volume 13, 

Transportation Research Board, 2004. 

• A Guide for Addressing Head-On Collisions, NCHRP Report 500, Volume 4, Transportation 

Research Board, 2003. 

• A Guide for Addressing Run-Off-Road Collisions, NCHRP Report 500, Volume 6, 

Transportation Research Board, 2003. 

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

2001.

• Highway Drainage Guidelines, AASHTO, 2000.

Stopping Sight Distance 

Stopping sight distance is defined as the distance needed for drivers to see an object on the 

roadway ahead and bring their vehicles to safe stop before colliding with the object. The 

distances are derived for various design speeds based on assumptions for driver reaction 

time, the braking ability of most vehicles under wet pavement conditions, and the friction 

provided by most pavement surfaces, assuming good tires. A roadway designed to criteria 

employs a horizontal and vertical alignment and a cross section that provides at least the 

minimum stopping sight distance through the entire facility. 

Stopping sight distance is influenced by both vertical and horizontal alignment. For vertical 

stopping sight distance, this includes sight distance at crest vertical curves (Figure 17), 

headlight sight distance at sag vertical curves (Figure 18), and sight distance at 

undercrossings (Figure 19). 

For crest vertical curves, the alignment of the roadway limits stopping sight distance 

(Figure 17). Sag vertical curves provide greater stopping sight distance during daylight 

conditions, but very short sag vertical curves will limit the effective distance of the vehicle’s 

headlights at night. If lighting is provided at sag vertical curves, a design to the driver 

comfort criteria may be adequate. The length of sag vertical curves to satisfy the comfort 

criteria over the typical design speed range results in minimum curve lengths of about half 

those based on headlight criteria. 

For horizontal curves, physical obstructions can limit stopping sight distance (Figure 20). 

Examples include bridge piers, barrier, walls, backslopes, and vegetation. 

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FIGURE 17 

Vertical stopping sight distance at a crest 

vertical curve. 

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FIGURE 18 

Headlight sight 

distance at a sag 

vertical curve. 

FIGURE 19 

Sight distance at an 

undercrossing. 

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Chapter 3—The 13 Controlling Criteria 

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FIGURE 20 

Horizontal stopping 

sight distance.

Clarifications 

In addition to stopping sight distance, the Green Book provides design criteria for decision 

sight distance, passing sight distance (applies to two-lane roads only) and intersection sight 

distance. FHWA requires a formal design exception wherever stopping sight distance 

cannot be provided. Because stopping sight distance is influenced by both vertical and 

horizontal alignment, a design exception may be required, based on a range of geometric or 

roadside conditions limiting sight lines in three dimensions. 

For sag vertical curves, formal design exceptions are required for curves that meet the 

comfort criteria but not the headlight criteria, unless lighting is provided. 

Safety Effects 

The adopted criteria for stopping sight distance apply to the entire length of a highway. 

Clearly though, the relative risk of limited sight distance can vary significantly, based on the 

circumstances. A simple ‘model’ for evaluating locations with limited sight distance 

involves the following questions: 

• What roadway or other conditions or features are within the segment with limited sight 

distance? 

• How significant is the deficiency in sight distance (as measured by length of highway as 

well as amount of deficiency relative to that required per adopted criteria)? 

• What is the traffic volume through the location with limited sight distance? 

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Mitigation Strategies For Design Exceptions 

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For example, the risk associated with a crest vertical curve with non-standard sight distance 

is greater at a location with intersections or driveways or other roadway features (Figure 21) 

within the area of the sight restriction compared with a similar location with no such 

features. Table 16 summarizes the relative safety risk of combining various geometric 

elements and other roadway features with non-standard stopping sight distance. 

A stopping sight distance profile (see Figure 22) can be a useful tool for understanding 

location-based risk of limited stopping sight distance. The profile shows the amount of 

stopping sight distance at each location along the roadway, thereby illustrating the 

magnitude of sight distance restrictions and where they occur. This information can help 

designers understand the severity of a sight distance restriction, how the restriction may 

interact with other roadway conditions or features, and how/where to implement 

mitigation strategies. The IHSDM (see Chapter 1) creates stopping sight distance profiles 

for rural two-lane highways. 

TABLE 16 

Relative Safety Risk of Various Conditions in Combination with Non-Standard Stopping Sight Distance 

Geometric Condition Relative Safety Risk 

Tangent horizontal alignment 

Mild curvature 

>2000 ft (600m) radius 

Mild downgrade (<3%) 

Minor 

Low-volume intersection 

Intermediate curvature 

1000 ft (300 m) to 2000 ft (600 m) radius 

Moderate downgrade (3–5%) 

Structure 

Significant 

High volume intersection 

Y-diverge on road 

Sharp curvature 

<1000 ft (300 m) radius 

Steep downgrade (>5%) 

Narrow bridge 

Narrow pavement 

Freeway lane drop 

Exit or entrance downstream along freeway 

Major 

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Chapter 3—The 13 Controlling Criteria 

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FIGURE 21 

Not all locations with 

limited stopping sight 

distance are the same in 

terms of safety risk. In this 

example, the intersecting 

roadway in the 

background creates the 

illusion of a straight 

alignment and may 

increase the risk of run-offroad crashes.

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FIGURE 22 

Stopping sight 

distance profile 

(Source: A Guide for 

Achieving Flexibility 

in Highway Design, 

AASHTO). 

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Chapter 3—The 13 Controlling Criteria 

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Summary 

Table 17 summarizes the potential adverse impacts to safety and operations of a design 

exception for stopping sight distance. 

TABLE 17 

Stopping Sight Distance: Potential Adverse Impacts to Safety and Operations

Safety & Operational Issues Freeway Expressway 

Rural 

TwoLane 

Urban 

Arterial 

Collisions with vehicles stopped or slowed on the 

roadway X X X X 

Collisions with objects on the roadway X X X X 

Collisions with vehicles entering from intersecting 

roadways X X X 

Freeway: high-speed, multi-lane divided highway with interchange access only (rural or urban). 

Expressway: high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban). 

Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, or local). 

Urban Arterial: urban arterials with speeds 45 mi/h (70 km/h) or less. 

Stopping Sight Distance Resources 

• A Policy on Design Standards Interstate System, AASHTO, 2005. 

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

• A Guide for Reducing Collisions on Horizontal Curves, NCHRP Report 500, Volume 7, 

Transportation Research Board, 2004. 

• A Guide for Addressing Run-Off-Road Collisions, NCHRP Report 500, Volume 6, 

Transportation Research Board, 2003. 

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

2001.

• Determination of Stopping Sight Distances, NCHRP Report 400, Transportation Research 

Board, 1997. 

Cross Slope 

Pavement cross slope is an important cross-sectional design element. The cross slope drains 

water from the roadway laterally and helps minimize ponding of water on the pavement. 

This prevents maintenance problems and also minimizes icing from occurring on poorly 

drained pavement. On roadways with curbed cross sections, the cross slope moves water 

to a narrower channel adjacent to the curb, away from the travel lanes, where it can be 

removed. Cross slopes that are too steep can cause vehicles to drift, skid laterally when 

braking, and become unstable when crossing over the crown to change lanes. These 

conditions are exacerbated by icy, snowy, or windy conditions. Both maximum and 

minimum criteria exist for cross slope. A formal design exception is required wherever 

either cannot be met. 

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Clarifications 

Cross slope criteria apply to typical tangent alignments. On high-speed roadways, normal 

cross slope is 1.5–2.0 percent, with the cross-slope break (the algebraic difference in slopes 

between the lanes) at the centerline not exceeding 4 percent. In areas of intense rainfall and 

where there are three or more lanes in each direction, additional cross slope may be 

necessary for adequate drainage. Accomplishing other design features (superelevation 

transitions, pavement warping at intersections, etc.) will inevitably require removal of cross 

slope in spot locations. These cases are routine and necessary in design and a design 

exception is not required. 

In addition to the cross slope of the lanes, the cross-slope break on the high side of 

superelevated curves should not exceed 8 percent (Figure 23). A formal design exception is 

required when this condition is not met. 

FIGURE 23 

Cross-slope 

break on the 

high side of 

superelevated 

curve. 

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Chapter 3—The 13 Controlling Criteria 

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Summary 

Table 18 summarizes the potential adverse impacts to safety and operations of a design 

exception for cross slope. 

TABLE 18 

Cross Slope: Potential Adverse Impacts to Safety and Operations

Safety & Operational Issues Freeway Expressway Rural 

2-Lane 

Urban 

Arterial 

Run-off-road crashes X X X 

Slick pavement X X X X 

Water ponding on the pavement surface X X X X 

Water spreading onto the traveled lanes X 

Loss of control when crossing over a high crossslope break X X X 

Freeway: high-speed, multi-lane divided highway with interchange access only (rural or urban). 

Expressway: high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban). 

Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, or local). 

Urban Arterial: urban arterials with speeds 45 mi/h (70 km/h) or less. 

Cross Slope Resources 

• A Policy on Design Standards Interstate System, AASHTO, 2005. 

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

• A Guide for Reducing Collisions on Horizontal Curves, NCHRP Report 500, Volume 7, 

Transportation Research Board, 2004. 

• A Guide for Reducing Collisions Involving Heavy Trucks, NCHRP Report 500, Volume 13, 

Transportation Research Board, 2004. 

• A Guide for Addressing Run-Off-Road Collisions, NCHRP Report 500, Volume 6, 

Transportation Research Board, 2003. 

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

2001.

• Highway Drainage Guidelines, AASHTO, 2000.

Vertical Clearance

The adopted criteria provide vertical clearance values for the various highway functional 

classifications (Table 19). These criteria are set to provide at least a 1-foot differential 

between the maximum legal vehicle height and the roadway, with additional allowances for 

future resurfacing. These clearances apply to the entire roadway width (traveled way and 

shoulders). A formal design exception is required whenever these criteria are not met for the 

applicable functional classification. 

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Clarifications 

The specific standards for vertical clearance adopted for the Interstate System maintain its 

integrity for national defense purposes. On Interstates, the clear height of structures shall 

not be less than 16 feet (4.9 meters) over the entire roadway width, including the useable 

width of shoulder. In urban areas, the 16-foot (4.9-meter) clearance shall apply to at least a 

single routing. On other urban Interstate routes, the clear height shall not be less than 14 feet 

(4.3 meters). A design exception is required if this standard is not met. Exceptions on the 

Interstate must also be coordinated with the Military Surface Deployment and Distribution 

Command Transportation Engineering Agency of the Department of Defense. 

TABLE 19 

Ranges for Minimum Vertical Clearance

Type of Roadway Rural Urban 

US (feet) Metric (meters) US (feet) Metric (meters) 

Freeway 14–16* 4.3–4.9* 14–16* 4.3–4.9* 

Arterial 14–16 4.3–4.9 14–16 4.3–4.9 

Collector 14 4.3 14 4.3 

Local 14 4.3 14 4.3 

*17 feet (5.1 meters) for sign trusses and pedestrian overpasses. 

Source: A Policy on Geometric Design of Highways and Streets, AASHTO 

Substantive Safety 

The adverse effects of structures with insufficient vertical clearance are obvious (see Figure 

24). Impacts to low bridges create risk for the driver of the vehicle, others on both 

roadways, and in extreme situations can result in closure of the bridge for lengthy periods 

and necessitating costly repairs. 

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Chapter 3—The 13 Controlling Criteria 

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FIGURE 24 

Interstate closure 

after an impact with a 

bridge. 

Summary 

Table 20 summarizes the potential adverse impacts to safety and operations of a design 

exception for vertical clearance. 

TABLE 20 

Vertical Clearance: Potential Adverse Impacts to Safety and Operations

Safety & Operational Issues Freeway Expressway Rural 

Two-Lane 

Urban 

Arterial 

Collision with overhead structure X X X X 

Rear-end crashes (vehicles following the vehicle 

that collided with the structure) X X X X 

Debris on the roadway X X X X 

Long delays as a result of a closed roadway or 

lanes X X X X 

Freeway: high-speed, multi-lane divided highway with interchange access only (rural or urban). 

Expressway: high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban). 

Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, or local). 

Urban Arterial: urban arterials with speeds 45 mi/h (70 km/h) or less. 

Vertical Clearance Resources 

• A Policy on Design Standards Interstate System, AASHTO, 2005. 

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Mitigation Strategies For Design Exceptions 

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• Federal Aid Policy Guide, FHWA, 2005. 

http://www.fhwa.dot.gov/legsregs/directives/fapg/0625sup.htm

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

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

2001. 

Lateral Offset to Obstruction 

The lateral offset to obstruction is defined as the distance from the edge of traveled way, 

shoulder, or other designated point to a vertical roadside element. Examples of these 

elements are curbs, walls, barriers, bridge piers, sign and signal supports, trees, and utility 

poles (Figure 25). 

Lateral offset can be thought of as an operational offset—vertical roadside elements offset to 

the extent that they do not affect a driver’s speed or lane position. Adequate clearance from 

these elements should be provided for mirrors on trucks and buses and for opening 

curbside doors where on-street parking is provided. 

The adopted criteria specify a minimum operational offset for all roadway conditions and 

classifications of 1.5 feet. 

Clarification 

Lateral offset should not be confused with the clear zone—a clear recovery area, free of rigid 

obstacles and steep slopes, which allows vehicles that have run off the road to safely recover 

or come to a stop. While lateral offset can be thought of as an operational offset, the clear 

zone serves primarily a substantive safety function. 

FIGURE 25 

Lateral offset to 

obstruction is an 

operational offset and 

is not the same as 

clear zone. 

Lateral offset to obstructions is one of the 13 controlling criteria that require a formal design 

exception per FHWA Policy. Clear zone is not. 

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Chapter 3—The 13 Controlling Criteria 

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Although clear zone is not one of the controlling criteria that require a formal design 

exception, its importance should still be recognized. The AASHTO Roadside Design Guide

provides ranges for clear zone based on speed, traffic, and roadside slopes. The Guide states 

that “the values suggest only the approximate center of a range to be considered and not a 

precise distance to be held as absolute.” Designers are expected to exercise judgment in 

selecting an appropriate clear zone, taking into account the variables listed above as well as 

the location (urban vs. rural), the type of construction (new construction/reconstruction/ 

3R), and the context. Chapter 10 of the Guide provides guidance on roadside safety in urban 

and restricted environments and emphasizes the need to look at each location and its 

particular site characteristics individually. 

According to FHWA, a clear zone should be established for projects or project segments 

based on a thorough review of site conditions, constraints, and safety considerations. Once 

a clear zone has been established, decisions to deviate from it for particular roadside 

obstacles should be identified, justified, and documented. 

Summary 

Table 21 summarizes the potential adverse impacts to safety and operations of a design 

exception for lateral offset. 

TABLE 21 

Lateral Offset to Obstruction: Potential Adverse Impacts to Safety and Operations

Safety and Operational Issues Freeway Expressway Rural 

2-Lane 

Urban 

Arterial 

Shying away from obstructions X X X X 

Reduced free-flow speeds X X X X 

Difficulty for parked vehicles X 

Freeway: high-speed, multi-lane divided highway with interchange access only (rural or urban). 

Expressway: high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban). 

Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, or local). 

Urban Arterial: urban arterials with speeds 45 mi/h (70 km/h) or less. 

Lateral Offset to Obstruction Resources 

• Clear Zone and Horizontal Clearance, Frequently Asked Questions, FHWA, 2005, 

http://www.fhwa.dot.gov/programadmin/clearzone.htm

• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 

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

2001.

• Highway Capacity Manual, Transportation Research Board, 2000. 

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Mitigation Strategies For Design Exceptions 

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Structural Capacity 

The 13th controlling criterion is structural capacity. This refers only to the load-carrying 

capacity of the bridge. Because it is not strictly an element of geometric design, structural 

capacity will not be covered in detail in this guide. Designers should be aware, however, 

that the inability to design for the designated structural capacity requires a design 

exception. There is also information in the Green Book on conditions under which existing 

bridges may remain in place. 

Clarification 

Bridge rail that is structurally sound and meets current crash test standards is an important 

safety consideration, and updating substandard barrier is an important safety improvement 

on 3R and other projects. However, the type or condition of bridge rail is not considered to 

be one of the 13 controlling criteria that require a formal design exception. 

Summary 

Each of the 13 controlling design criteria is established to reflect a desired operational 

and/or safety benefit. Designer understanding of the nature of the benefits and the design 

sensitivities will lead to good decisions regarding design exceptions. 

Based on the topics discussed in this chapter, designers should appreciate that the inability 

to meet a minimum threshold criterion should not be made lightly, and that the expected 

performance for a lesser design may be based on many conditions. Designers should expect 

that to some extent adverse operational and/or safety effects may occur with a design 

exception. The next chapters of this Guide discuss how designers can mitigate potential 

adverse effects and deliver a design with acceptable performance. 

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67

CHAPTER 4 

Mitigation Strategies 

Table 22 lists potential mitigation strategies for FHWA’s 13 controlling criteria. Additional 

information is provided on the following pages. The list is not meant to include every 

possible mitigation strategy for each criterion. Rather, it is intended to initiate a thought 

process by presenting some common as well as innovative mitigation strategies to consider. 

Every design exception location is unique, so the photos and examples presented in this 

chapter and the case studies that follow are not meant to imply a best solution for any 

particular location. The recommended approach is to consider the mitigation strategies 

presented in this chapter as well as other ideas and new approaches. If available, consult 

current research to gain additional information. Then customize one or more strategies to 

address the unique concerns and site conditions at the design exception location. 

The known effectiveness of the mitigation strategies varies. Some, such as shoulder rumble 

strips, have been used for many years and are well proven. Others are new ideas that have 

been tried, but their effectiveness is still being studied. The body of knowledge on these 

strategies will continue to grow, so designers should consult the most recent research 

available to assess the effectiveness of particular strategies.

TABLE 22 

Potential Mitigation Strategies

Design Element Objective Potential Mitigation Strategies 

1. Design Speed Reduce operating speeds to the 

design speed. 

Cross-sectional elements to manage speed. 

Optimize safety and operations by 

distributing available crosssectional width. 

Select optimal combination of lane and shoulder 

width based on site characteristics. 

Provide advance warning of lane 

width reduction. 

Signing. 

Wide pavement markings. 

Recessed pavement markings. 

Raised pavement markings. 

Delineators. 

Lighting. 

Centerline rumble strips. 

Shoulder rumble strips. 

Improve ability to stay within the 

lane. 

Painted edgeline rumble strips. 

Paved or partially-paved shoulders. 

2. Lane Width & 

3. Shoulder Width 

Improve ability to recover if driver 

leaves the lane. 

Safety edge. 

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Mitigation Strategies for Design Exceptions 

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TABLE 22 (CONTINUED) 

Potential Mitigation Strategies

Design Element Objective Potential Mitigation Strategies 

Remove or relocate fixed objects. 

Traversable slopes. 

Breakaway safety hardware. 

Reduce crash severity if driver 

leaves the roadway. 

Shield fixed objects and steep slopes. 

Provide space for enforcement 

and disabled vehicles. 

Pull-off areas. 

Signing. 

Reflectors on approach guardrail and bridge rail. 

Post-mounted delineators. 

Object markers. 

High-visibility bridge rail. 

Bridge lighting. 

Provide advance warning and 

delineation of narrow bridge. 

Improve visibility of narrow bridge, 

bridge rail, and lane lines. 

Enhanced pavement markings. 

Maintain pavement on bridge that Skid-resistant pavement. 

will provide safe driving 

conditions. Anti-icing systems. 

Reduce crash severity if driver 

leaves the roadway. 

Crashworthy bridge rail and approach guardrail. 

Provide space for disabled 

vehicles or emergencies on long 

bridges. 

Pull-off areas. 

4. Bridge Width 

Provide quick response to 

disabled vehicles or emergencies 

on long bridges. 

Surveillance. 

Signing. 

Pavement marking messages. 

Provide advance warning. 

Dynamic curve warning systems. 

Chevrons. 

Post-mounted delineators. 

Provide delineation. 

Reflectors on barrier. 

Widen the roadway. 

Skid-resistant pavement. 

Enhanced pavement markings. 

Lighting. 

Centerline rumble strips. 

5. Horizontal 

Alignment & 

6. Superelevation Improve ability to stay within the 

lane. 

Shoulder rumble strips. 

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TABLE 22 (CONTINUED) 

Potential Mitigation Strategies

Design Element Objective Potential Mitigation Strategies 

Painted edgeline rumble strips. 

Improve ability to recover if driver Paved or partially paved shoulders. 

leaves the lane. 

Safety edge. 

Remove or relocate fixed objects. 

Traversable slopes. 

Breakaway safety hardware. 

Potential Mitigation 

Strategies Reduce crash severity if driver 

leaves the roadway. 

Shield fixed objects and steep slopes. 

7. Vertical 

Alignment 

See (8) Grade and (9) Stopping Sight Distance. 

Provide advance warning. Signing. 

Improve safety and operations for Climbing lanes. 

vehicles ascending or descending 

steep grades. Downgrade lanes. 

Capture out-of-control vehicles 

descending steep grades. 

Escape ramps. 

Enhanced pavement markings. 

Delineators. 

Centerline rumble strips. 

Shoulder rumble strips. 

Improve ability to stay within the 

lane. 

Painted edgeline rumble strips. 

Improve ability to recover if driver Paved or partially-paved shoulders. 

leaves the lane. 

Safety edge. 

Remove or relocate fixed objects. 

Traversable slopes. 

Breakaway safety hardware. 

Reduce crash severity if driver 

leaves the roadway. 

Shield fixed objects and steep slopes. 

Adjusting gutter profile on curbed cross sections. 

8. Grade 

Address drainage on flat grades. 

Continuous drains. 

Signing and speed advisory plaques (crest vertical 

curves). 

Lighting (sag vertical curves). 9. Stopping Sight 

Distance 

Mitigate sight distance restrictions. 

Adjust placement of lane within the roadway cross 

section (horizontal). 

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TABLE 22 (CONTINUED) 

Potential Mitigation Strategies

Design Element Objective Potential Mitigation Strategies 

Cross-sectional elements to manage speed. 

Improve ability to avoid crashes. Wide shoulders. 

Wider clear recovery area. 

Advanced warning signs. 

Dynamic warning signs. 

Larger or additional STOP/YIELD signs. 

Improve driver awareness on 

approach to intersections. 

Intersection lighting. 

Provide warning of slick 

pavement. 

Signing. 

Improve surface friction. Pavement grooving (PCC pavement). 

Open-graded friction courses (HMA pavement). 

Transverse pavement grooving (PCC pavement). 

Open-graded friction courses (HMA pavement). 

Improve drainage. 

Pavement edge drains. 

10. Cross Slope 

Mitigate cross-slope break on the 

high side of superelevated curves. 

Modified shoulder cross slope. 

Advance warning. Signing. 

Alternate routes. 11. Vertical 

Clearance Preventing impacts with low 

structures. 

Large vehicle restrictions. 

Improve visibility of objects near Delineate objects. 

the roadway. 

Lighting. 

Optimize operations by 

distributing available crosssectional width. 

Provide full outside lane width and/or additional 

offset. 

12. Lateral Offset to 

Obstruction 

Improve visibility of the lane lines. Enhanced pavement markings. 

13. Structural 

Capacity Not addressed in this Guide. 

1. Design Speed 

As discussed in Chapter 3, design speed is a design control, and the chosen design speed 

affects many of the geometric elements of a highway. Design exceptions for design speed 

are also rare, for two reasons: 1) the adopted criteria encompass a range of design speeds, 

which provides a great deal of design flexibility; and 2) design exceptions, when needed, are 

normally prepared for the specific design elements and not the design control. 

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Target areas: Highways with limited 

cross-sectional width. 

Strategy: Optimized lane and shoulder 

widths. 

Target areas: High-speed roadways 

with narrow lanes. 

Strategy: Advance signing of narrow 

lanes. 

Target areas: Any highway where a 

design exception is used for design 

speed. 

Strategy: Cross-sectional elements to 

manage speed. 

In the rare cases when a design exception is used for design speed, one mitigation measure 

to consider is choosing cross-sectional elements and dimensions that serve to manage 

operating speeds so they are at or below the design speed. For example, on a transitional 

roadway between a rural and urban environment, a more-enclosed urban cross-section with 

curb and gutter gives drivers a visual cue that they are entering a reduced-speed 

environment. It may also feel less comfortable for a driver 

to maintain high speeds on such a cross section compared 

to a more-open, rural cross section with full-width lanes 

and wide shoulders. Just as design speed is selected by the 

designer, cross-sectional elements can be chosen that help 

manage operating speeds. 

2. Lane Width and 3. Shoulder Width 

Lane and shoulder width strategies have been combined in this discussion because normally 

they are evaluated in combination when there is limited cross-sectional width. The two 

criteria are also interrelated in terms of their effects on safety and operations. 

Distribute Cross-Sectional Width 

In locations where cross-sectional width is 

constrained, evaluating how that width can be 

distributed most effectively between the lane and 

shoulder should be evaluated. This strategy is 

basically an exercise in trade-offs—taking some of the lane width to use for additional 

shoulder width or vice versa, depending on the location and the objectives. 

The optimal distribution will depend on site-specific characteristics. For example, on a rural 

two-lane roadway with no shoulders and a history of run-off-road crashes, an effective 

strategy may be to distribute some of the available width to accommodate a narrow paved 

shoulder and rumble strips, at the expense of narrower lanes. The objective would be to 

reduce the probability of run-off-road crashes. For another highway, with heavy truck 

volumes and a curvilinear alignment, maintaining full 12-foot lanes at the expense of some 

of the shoulder width may be a more-optimal design. The objective would be minimizing 

truck off-tracking into adjacent lanes or the shoulder. The key is to look at the site-specific 

characteristics such as highway type, traffic and truck volumes, geometry, crash history, 

and crash type. With this information, various combinations of lane and shoulder widths 

can be evaluated with the goal of optimizing safety and traffic operations at the design 

exception location. 

Case Study 1 (presented in Chapter 5) illustrates how one State evaluated multiple 

combinations of lane and shoulder width on a segment of urban freeway where the cross 

section was constrained. 

Provide Advance Warning of Lane Width 

Reduction 

Signs can be used to warn drivers in advance of a 

change in lane width. Messages such as a ROAD 

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Mitigation Strategies for Design Exceptions 

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Target areas: High-speed roadways 

with narrow lanes or shoulders. 

Strategy: Enhanced pavement 

markings. 

Target areas: High-speed roadways with 

narrow lanes or shoulders. 

Strategy: Delineators. 

Target areas: Urban freeways and other 

high-speed urban roadways, or segments 

of high-speed rural roadways with a high 

crash history or a higher probability of 

run-off-road crashes. 

Strategy: Lighting. 

NARROWS sign (Figure 26) may be used alone or in combination with an advisory speed 

plaque. The Manual of Uniform Traffic Control Devices (MUTCD) provides guidance on the 

size of warning signs for various highway types but notes that larger signs may be used 

when appropriate. Larger warning signs should be considered for design exception 

locations. 

Use of advance warning signs as a stand-alone measure is unlikely to sufficiently mitigate a 

design exception for lane width, but at some locations it can be an effective component of a 

more comprehensive approach. 

Improve Ability to Stay Within the Travel 

Lane 

Another category of mitigation strategies for 

both lane and shoulder widths is aimed at 

enhancing a driver’s ability to stay within the 

lane. One method is to provide clear delineation 

and better visibility of the lanes. Wide pavement 

markings (Figure 27), recessed pavement 

markings with high retroreflectivity (Figure 28), 

and raised pavement markings (Figure 29) can 

help drivers 

stay within their 

lane—

particularly at 

night, when the 

pavement is wet 

or when visibility is poor. Both raised and 

recessed pavement markings will have higher 

costs than standard painting. Recessed 

pavement markings may provide extra 

advantages in areas of the country where snow 

and ice removal can cause additional wear on 

 painted or raised markings. 

Roadside delineators (Figure 30) can help drivers 

see changes in roadway geometry. Lighting 

(Figure 31) will have higher up-front costs and 

ongoing utility costs, but is another strategy that 

can enhance a driver’s ability to see and stay 

within the travel lane. Depending on the type of 

highway, traffic volumes, crash history, and other 

site-specific characteristics, lighting may be appropriate for the entire length of the design 

exception location, or it may be appropriate only for selected segments. For example, for a 

high-speed rural roadway with narrow lane or shoulder widths, lighting could be installed 

along horizontal curves or along segments with a history of lane-departure crashes. 

FIGURE 26 

Signs can be used to warn drivers in advance of a 

change in lane width. 

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FIGURE 27 

Wide pavement 

markings. 

FIGURE 28 

Recessed pavement 

markings. 

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Mitigation Strategies for Design Exceptions 

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FIGURE 29 

Raised pavement 

markings. 

FIGURE 30 

Post-mounted 

delineators. 

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Target areas: High-speed rural 

highways. 

Strategy: Shoulder rumble strips. 

Target areas: Two-lane, undivided, rural 

highways. 

Strategy: Centerline rumble strips. 

FIGURE 31 

Lighting. 

In addition to visible delineation, shoulder and centerline rumble strips improve a driver’s 

ability to stay within the lane by providing both an audible warning and a slight vibration 

within the vehicle that a driver can feel. On rural two-lane roadways with narrow lane 

widths, drivers may have a tendency to shy to the outside when meeting other vehicles. 

Shoulder rumble strips (Figure 32) warn drivers that they are outside the lane. Another 

concern on two-lane undivided roadways are cross-centerline head-on or sideswipe crashes. 

Similar to shoulder rumble strips, centerline rumble 

strips (Figure 33) can be used to warn drivers that 

they are driving near the centerline and are close to 

encroaching on the opposing lane. Centerline 

rumble strips are normally used on high-speed 

rural two-lane highways. Shoulder rumble strips are an effective strategy on any highspeed rural highway. Agencies are encouraged to work in cooperation with local and state 

bicycle groups on shoulder rumble strip issues. By involving bicyclists early in the process, 

designs can be developed that achieve the safety benefits of rumble strips while at the same 

time accommodating the needs of bicyclists. The gap pattern illustrated in Figure 32 is one 

method that can be used to better accommodate bicyclists. 

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Target areas: High-speed rural highways 

and areas where snow removal operations 

are causing deterioration of pavement 

markings. 

Strategy: Painted edgeline rumble strips. 

FIGURE 32 

Shoulder rumble 

strips. 

FIGURE 33 

Centerline rumble strips. 

An emerging strategy that has been tried in several States is combining edgeline pavement 

markings with shoulder rumble strips (Figure 34). The rumble strips are placed at the edge 

of the travel lane. This allows rumble strips to be 

placed on roadways with very limited cross-sectional 

width and narrow paved shoulders. The edgeline 

marking is then painted directly over the rumble 

strips. Several advantages of this strategy have been 

observed. 

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Target areas: All high-speed highways. 

Strategy: Paved or partially paved 

shoulders. 

Target areas: High-speed highways, 

especially those with no paved shoulder 

or narrow paved shoulders. 

Strategy: Safety edge. 

First, the pavement marking on the near-vertical face of the rumble strip reflects more light 

back towards the driver at night, creating a more-visible edgeline. Second, in northern 

states, the paint and beads that are in the depressed portion of the rumble strip are less 

prone to wear from snow plowing. This can extend the life and performance of the painted 

edgeline. 

FIGURE 34 

Painted edgeline 

rumble strips. 

Improve Ability to Recover if Driver Leaves the Lane 

When a driver leaves the lane or the paved portion of the roadway at high speeds, there is a 

significant safety risk. As discussed in Chapter 3, pavement edge dropoffs can increase this 

risk. 

Paved or partially paved shoulders (Figure 35) move 

the pavement edge and potential dropoffs farther 

from the travel lane. Another strategy is to construct 

the pavement edge to allow safer recovery for 

drivers who leave the paved section of the roadway. 

The safety edge (Figure 36) accomplishes this by 

providing a beveled edge of pavement instead of a near-vertical edge. This strategy can be 

used with both hot mix asphalt (HMA) and portland cement concrete (PCC) pavements. 

Working with contractors is recommended because some modifications to paving 

equipment will be necessary. The safety edge is 

particularly worth considering for areas with very 

limited cross-sectional width, where there is not 

enough width for paved or partially paved 

shoulders. Many roadways on the local system fit 

this description. 

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Mitigation Strategies for Design Exceptions 

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Target areas: Any high-speed or rural 

highway. 

Strategy: Clear recovery area, traversable 

slopes, breakaway safety hardware, and 

barriers where appropriate. 

FIGURE 35 

Partially paved 

shoulders. 

Reduce the Crash Severity if the Driver Leaves the Roadway 

Because the probability of run-off-road crashes is higher at locations with design exceptions 

for lane or shoulder width, special attention should be paid to providing clear recovery 

areas and implementing measures to reduce the severity of these crashes. 

Fixed objects should be removed (Figure 37) or 

relocated to a place where they are less likely to be 

hit—at or beyond the clear zone, if possible. Signs, 

light poles, and other necessary roadside 

hardware should be installed with crashworthy 

breakaway supports (Figure 38). Foreslopes, 

transverse slopes, and drainage structures should be made traversable. In some cases, fixed 

objects or steep slopes should be shielded with barriers (Figure 39). Although the use of 

barriers may increase crash frequency, crash severity is expected to decrease. 

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FIGURE 36 

Safety edge (top) 

and after the 

shoulder has been 

graded over the 

edge (bottom). 

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Mitigation Strategies for Design Exceptions 

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FIGURE 37 

Fixed object removal. Separate box culverts 

were extended, connected, and covered at 

this interchange. 

FIGURE 38 

Breakaway 

light poles. 

FIGURE 39 

Shielding fixed objects with barrier. 

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Target areas: High-speed roadways 

with narrow shoulders. 

Strategy: Pull-off areas. 

Provide Pull-Off Areas where Shoulder Width is Limited 

Where shoulder width is limited, another mitigation strategy is to provide regularly spaced 

pull-off areas (Figure 40). Pull-off areas provide several advantages. First, they provide 

room to store disabled vehicles, which is particularly important for maintaining operations 

on high-volume highways. A disabled vehicle can be 

parked or quickly removed from a travel lane to a 

pull-off area, allowing traffic to flow in all available 

traffic lanes as quickly as possible. Second, pull-off 

areas provide an area for law enforcement to detain 

vehicles in areas with narrow shoulders. This 

increases safety for law enforcement personnel, the stopped driver, and passing drivers. 

Operations are likely to be improved as well because drivers are more likely to maintain 

normal speeds and stay within their lane if law enforcement activities are being conducted a 

sufficient distance from the travel lanes in a pull-off area. 

If possible, pull-off areas should be located where lane departure crashes are less likely, 

such as tangent sections or on the inside of horizontal curves. 

Case Study 4 (presented in Chapter 8) illustrates how one State is using pull-off areas on a 

historic urban freeway with extremely narrow shoulders. 

FIGURE 40 

Pull-off area on 

the inside of a 

horizontal curve. 

4. Bridge Width 

The strategies for mitigating narrow bridges are aimed primarily at improving a driver’s 

ability to see the narrowed cross section on the bridge, the bridge rail, and the lane lines. 

Safety benefits are a reduced probability of sideswipe or head-on crashes with other vehicles 

on the bridge, as well as fewer impacts with the bridge rail and approach guardrail. 

Operational benefits may result from an increase in driver comfort. A driver who can 

clearly see these cross-sectional elements is more likely to maintain normal operating speeds 

or at least not dramatically reduce speeds at the bridge. This is particularly important for 

maintaining efficient traffic flow on urban freeways and can also reduce the probability of 

rear-end crashes on high-speed, high-volume highways. 

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Target areas: Any narrow bridge 

location. 

Strategy: Advance signing of narrow 

bridge. 

Target areas: Any narrow bridge 

location. 

Strategy: Delineation. 

Signing 

Signs can be used to warn drivers in advance of a narrow bridge (Figure 41). In some 

situations, flashers installed in conjunction with the 

sign may further increase driver awareness. The 

MUTCD provides guidance on the size of warning 

signs for various highway types but notes that larger 

signs may be used when appropriate. Larger 

warning signs should be considered for design 

exception locations. 

Use of advance warning signs as a stand-alone measure is unlikely to sufficiently mitigate a 

design exception for bridge width, but at some locations it can be an effective component of 

a more comprehensive approach. 

Delineation 

Delineation of 

the narrowed 

cross section at 

the bridge is another strategy for providing advance 

warning. One method that provides very good 

delineation at night is reflectors or reflector tabs that 

are placed on the approach guardrail and along the 

bridge rail (Figure 42). Post-mounted delineators 

approaching the bridge are another option. Instead of 

providing just a single point of delineation, such as an 

object marker, reflectors and delineators allow the 

driver to better see the cross section narrowing as well 

as the most narrow segment of the cross section—the 

bridge. 

FIGURE 42 

Reflector tabs on guardrail.

FIGURE 41 

Signs can be used to warn drivers in advance 

of a narrow bridge. 

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Target areas: Any narrow bridge 

location. 

Strategy: High-visibility bridge rail. 

Object markers placed at the ends of the bridge rail is a common treatment (Figure 43). In 

areas where agricultural equipment or other wide vehicles are using the bridge, one issue to 

consider when using object markers or other post-mounted signs at the ends of the bridges 

is that they may prevent this type of equipment from being able to cross the bridge. In these 

cases, using reflectors on the approach guardrail and the bridge rail or other methods to 

achieve delineation of the narrow bridge should be considered instead of post-mounted 

delineation. 

FIGURE 43 

Object markers and post-mounted 

delineators at a narrow bridge. 

Installing high-visibility bridge rails are another method for delineating narrow bridges. 

White concrete has been used by some agencies to enhance the visibility of bridge rail at 

night or when visibility is poor (Figure 44). There are also 

proprietary products on the market with features that make 

bridge rails more visible. 

FIGURE 44 

White concrete 

bridge rail. 

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Mitigation Strategies for Design Exceptions 

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Target areas: Narrow bridges in urban 

areas; bridges in areas with a high 

number of pedestrians and other nonmotorized users; bridges where traffic 

volumes are high; bridges with a history 

of crashes or operational problems. 

Strategy: Lighting at narrow bridges. 

Target areas: Any narrow bridge. 

Strategy: Skid-resistant pavement. 

Target areas: Bridges on high volume, 

high-speed highways or bridges with a 

history of safety problems. 

Strategy: Anti-icing systems. 

Bridge Lighting 

Lighting is another way to make narrow bridges more 

visible to drivers. Although most often used in urban 

areas, lighting may be appropriate on some rural bridges, 

particularly if there is a history of safety problems. 

Skid-Resistant Pavement and Anti-Icing Systems 

Particularly in northern regions of the country where 

icing on bridges is a common problem, measures to 

maintain skid-resistant pavement should be considered to help drivers maintain control on 

slick pavement. Pavement grooving and other 

textures (Figures 62 and 63) can be placed at the time 

the bridge deck and bridge approach is constructed. 

Textures can also be milled into existing pavement. 

Although relatively expensive to deploy, automated 

anti-icing systems (Figure 45) may be appropriate, at 

especially problematic locations. 

FIGURE 45 

Anti-icing system on 

a bridge. 

Crashworthy Bridge Rail and Approach Guardrail 

Because of the higher probability of impacts with the bridge rail and approach guardrail at 

narrow bridge locations, crashworthy barrier that meets or exceeds NCHRP Report 350 

crash test criteria should be used (Figure 46). This includes the bridge rail, the guardrail, the 

stiffened guardrail transition that connects to the bridge rail, and the guardrail terminal. 

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Target areas: Any narrow bridge 

location. 

Strategy: Enhanced pavement markings 

and lane delineation. 

Target areas: Any narrow bridge 

location. 

Strategy: Crashworthy bridge rail and 

approach guardrail. 

Target areas: Long bridges. 

Strategy: Emergency pull-off areas. 

Safety hardware that complies with Report 350 criteria is 

required on new installations on the NHS. Upgrading 

older systems, regardless of highway system, is 

encouraged—particularly at design exception locations. 

In areas with high volumes of large vehicles, barrier 

that has passed test-level 4 or 5 criteria should be considered. Test-levels 4 and 5 include 

crash tests with single-unit trucks and tractor-semi-trailers, respectively. 

The AASHTO Roadside Design Guide provides guidance on barrier flare rates. Flared 

approach guardrail, particularly when combined with reflectors, can visually transition a 

driver into the narrowed cross section of the bridge (Figure 42). 

FIGURE 46 

Bridge rail and 

guardrail transition in 

compliance with 

NCHRP Report 350.

Pavement Markings and Lane Delineation 

Other mitigation strategies for narrow bridge width 

that are discussed in other sections of this chapter 

include enhanced pavement markings; see the Lane 

Width and Shoulder Width section. 

In addition to the safety benefits of helping drivers see and stay within the lane, improved 

lane delineation is expected to increase driver comfort at narrow bridges and improve 

operations. 

Emergency Pull-off Areas 

If a design exception for bridge width cannot be 

avoided for long bridges, emergency pull-off areas 

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Mitigation Strategies for Design Exceptions 

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Target areas: Any highway, particularly 

high-speed highways, at the approach to 

sharp or unexpected horizontal curves. 

Strategy: Advance warning with signing 

and pavement markings. 

Target areas: Long bridges. 

Strategy: Surveillance. 

should be considered. Pull-off areas on bridges should be safely terminated, either by 

flaring the bridge rail at an appropriate rate or through the use of an impact attenuator on 

any blunt end facing traffic. 

Surveillance 

Another strategy for long bridges is to use intelligent 

transportation systems (ITS) such as cameras to monitor 

long bridges for crashes, disabled vehicles, or other 

problems. This will allow law enforcement and other 

emergency responders to get to the scene as quickly as possible, which may prevent a crash. 

It also allows a disabled vehicle to be removed from the narrow bridge as quickly as 

possible, which will improve safety as well as minimize the amount of time a lane is 

blocked. 

5. Horizontal Alignment and 6. Superelevation 

Horizontal alignment and superelevation strategies have been combined in this discussion 

because they are normally evaluated in combination. The two criteria are also interrelated 

in terms of their effects on safety and operations. 

Signing and Pavement Marking Messages 

Signs can be used to warn drivers in advance of sharp horizontal curves and where there is 

non-standard superelevation (Figures 47 and 48). The most commonly used are the curve 

warning sign (for advisory speeds of 30 mi/h or greater) and the turn warning sign (for 

advisory speeds less than 30 mi/hr). Advisory speed plaques mounted below the warning 

sign are often used. In some situations, flashers installed in conjunction with the sign may 

further increase driver awareness. The MUTCD provides guidance on the size of warning 

signs for various highway types but notes that larger 

signs may be used when appropriate. Larger 

warning signs should be considered for design 

exception locations. 

Another consideration, besides the radius of the 

curve and the rate of superelevation, is the roadway 

alignment leading up to the curve. For example, a curve on a highway with a 

predominantly curvilinear alignment is more expected by the driver. Conversely, a sharp 

curve along a highway with a predominantly straight alignment or at the end of a long 

tangent is more likely to surprise a driver. Advance warning is especially important in 

these situations. 

Curve warning messages painted on the pavement are another method for providing 

advance warning of horizontal curves. One example is the painted message SLOW, along 

with a painted turn arrow. 

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Target areas: Curves with a history of 

safety problems. A common application 

is to mitigate truck rollover crashes on 

sharp curves at interchange ramps and 

loops. 

Strategy: Dynamic message signs. 

Dynamic Message Signs 

At some curves, signs that provide dynamic messages 

to drivers may be an effective countermeasure (Figure 

49). Changeable, real-time information can be 

communicated to the driver, such as the current 

recommended speed and the driver’s current 

operating speed. 

FIGURE 47 

Turn warning sign 

with flashing beacon. 

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FIGURE 48 

Curve warning sign. 

Note how vertical 

alignment can affect 

visibility of the curve. 

FIGURE 49 

Dynamic curve 

warning system. 

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Target areas: Any sharp or unexpected 

horizontal curve. 

Strategy: Delineation. 

Delineation 

In addition to advance warning, delineation is a common mitigation strategy for horizontal 

curves. There are several ways to effectively delineate horizontal curves: 

• Chevrons (Figure 50). The MUTCD provides 

guidance on chevron size for various highway 

types but notes that larger signs may be used 

when appropriate. Larger chevrons should be 

considered for design exception locations. 

• Post-mounted delineators (Figure 51). 

• Reflectors on barrier. If barrier is used along the horizontal curve, low-cost delineation 

can be provided with reflectors installed along the barrier (Figure 52). 

FIGURE 50 

Delineation with large 

chevrons. 

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Target areas: Curves on highways with 

large truck volumes, cross-centerline 

crashes, or run-off-road crashes. 

Strategy: Widen the roadway. 

FIGURE 51 

Delineation with postmounted delineators. 

FIGURE 52 

Delineation with 

reflectors on barrier. 

Widen the Roadway 

Widening the travel lanes at horizontal curves can 

mitigate off-tracking of trucks and other large 

vehicles into adjacent lanes. Additional lane width 

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Target areas: Any horizontal curve. 

Strategy: Preventing or reducing the 

severity of lane departure crashes. 

Target areas: Any horizontal curve. 

Strategy: Grooved, textured, or opengraded pavements to improve surface 

friction and skid resistance. 

Target areas: Any highway with steep 

grades. 

Strategy: Signing. 

will make it easier for all drivers to maneuver through the curve without leaving the travel 

lane. If cross-centerline crashes are a problem at a curve, a narrow median, preferably with 

centerline rumble strips, can provide some separation between the directions of traffic. If 

run-off-road crashes are more prevalent, widening the shoulder will help a driver that has 

left the travel lanes safely recover. Lane widening can also be beneficial on ramps and 

loops, particularly where there is a history of run-off-road crashes. The AASHTO Policy on 

Geometric Design of Highways and Streets provides design guidance on lane widening through 

curves. 

Skid-Resistant Pavement 

Another strategy aimed at keeping drivers on the roadway is to provide pavement 

treatments to improve surface friction and skid resistance such as grooving of PCC 

pavement and open-graded friction courses for HMA 

pavement. Pavement grooving and other textures 

(Figures 62 and 63) can be placed at the time 

pavement is constructed or they can be milled into 

existing pavement. See the Cross Slope section for 

more information. 

Other Horizontal Curve Strategies 

Because horizontal curves are a contributing factor to lane departure crashes, many of the 

strategies for preventing or reducing the severity of these crashes are applicable. See the 

Lane and Shoulder Width discussion earlier in this chapter for additional information on the 

following strategies: 

• Enhanced pavement markings 

• Lighting 

• Shoulder, centerline, and painted edgeline rumble 

strips 

• Paved or partially paved shoulders 

• Safety edge 

• Clear recovery area, traversable slopes, breakaway safety hardware, and barrier where 

appropriate 

7. Vertical Alignment 

Most design exceptions for vertical alignment are related to grade and stopping sight 

distance. The following two sections discuss these elements. 

8. Grade 

The strategies for mitigating grade are aimed at 

providing drivers with advance warning as they 

approach a steep grade, improving the ability of 

traffic to safely ascend and descend steep grades, 

and improving drainage in locations with flat 

grades. 

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Target areas: High-speed highways 

with steep grades (most common on rural 

highways). 

Strategy: Climbing lanes and 

downgrade lanes. 

Steep Grades 

Signs can be used to warn drivers in advance of steep grades (Figure 53). The MUTCD 

provides guidance on the size of warning signs for various highway types but notes that 

larger signs may be used when appropriate. Larger warning signs should be considered for 

design exception locations. Use of advance warning signs as a stand-alone measure is 

unlikely to sufficiently mitigate a design exception for grade, but it can be an effective 

component of a more comprehensive approach. 

Climbing lanes are a common strategy for improving safety and operations on uphill grades 

(Figure 54). From an operations standpoint, traffic can continue at free-flow speeds by 

passing trucks and other slow-moving vehicles. From a safety perspective, providing 

passing opportunities with a climbing lane reduces 

the probability of risky passing maneuvers. Similarly, 

adding a lane on the downgrade side of the facility 

may also be beneficial in some situations, where large 

trucks or other slow-moving vehicles create additional 

risk for faster-moving vehicles approaching from 

behind. 

FIGURE 53 

Advance warning of a 

steep grade. 

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Target areas: High-speed highways 

with steep grades and high truck volumes 

(most common in regions with 

mountainous terrain). 

Strategy: Escape ramps. 

Target areas: Any highway with steep 

grades. 

Strategy: Preventing or reducing the 

severity of lane departure crashes. 

FIGURE 54 

Climbing lane. 

For steep downhill grades with large truck volumes, 

escape ramps can be an effective strategy for 

capturing heavy vehicles that have lost control 

(Figure 55). Case Study 2 (presented in Chapter 6) 

illustrates an innovative truck escape ramp 

constructed in a mountainous region with very 

severe grades. 

Strategies should be considered for improving drivers’ ability to stay within the lane or their 

ability to recover if they leave the lane, and reducing crash severity if the vehicle leaves the 

roadway. The Lane and Shoulder Width discussion earlier in this chapter has additional 

information on the following strategies: 

• Enhanced pavement markings 

• Delineation 

• Shoulder, centerline, and painted edgeline 

rumble strips 

• Paved or partially-paved shoulders 

• Safety edge 

• Clear recovery area, traversable slopes, breakaway safety hardware, and barrier where 

appropriate 

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Target areas: Urban arterials, normally 

with speeds of 45 mi/h or less. 

Strategy: Adjusting the gutter profile. 

Target areas: High-speed roadways 

with flat grades; areas where fast 

removal of surface water and minimizing 

spread onto the roadway is especially 

important. 

Strategy: Special drainage systems. 

FIGURE 55 

Truck escape ramp. 

Flat Grades 

For proper drainage of the pavement surface, there 

needs to be adequate slope in the transverse 

direction (cross slope) and in the longitudinal 

direction (grade). To mitigate grades that are too 

flat, measures should be considered that will 

improve drainage on the highway. 

In areas with curbed cross sections, the profile of the 

gutter can be adjusted by slightly varying the cross 

slope of the lanes. This creates a “rolling” gutter 

profile that increases the grade along the curb 

between inlets, thereby creating more efficient flow 

and removal of water in the gutter. 

In some areas, more expensive drainage systems 

may be appropriate. Continuous drainage systems can be installed in areas with flat grades 

(Figure 56). These drains capture the water along the length of the highway segment with 

flat grades, and the pipe or channel underlying the drain can be sloped to move water 

efficiently through the system. 

FIGURE 56 

Continuous drainage system. 

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Target areas: Crest vertical curves. 

Strategy: Signing. 

Target areas: Sag vertical curves. 

Strategy: Lighting. 

Target areas: Horizontal curves. 

Strategy: Lower-height barrier. 

Target areas: Horizontal curves. 

Strategy: Adjusting placement of lane 

within the roadway cross section. 

FIGURE 57 

Sign for crest vertical curve with inadequate 

stopping sight distance.

9. Stopping Sight Distance 

The strategies for mitigating sight distance problems are aimed at mitigating sight distance 

restrictions, improving drivers’ ability to avoid crashes, and improving driver awareness on 

the approach to intersections. 

Stopping Sight Distance on Vertical Curves 

Advance signing (Figure 57) should be considered 

in areas with design exceptions for stopping sight 

distance at crest vertical curves. The MUTCD 

recommends this sign be supplemented with a 

speed advisory plaque. 

The MUTCD provides guidance on the size of 

warning signs for various highway types but notes 

that larger signs may be used when appropriate. 

Larger warning signs should be considered for 

design exception locations. 

Use of advance warning signs as a stand-alone 

measure may not be sufficient to mitigate a design 

exception for stopping sight distance, but at some 

locations it can be an effective component of a 

more comprehensive approach. 

Because headlight sight distance is the control at 

sag vertical curves, the most common mitigation measure at these locations is to install 

lighting. 

Horizontal Stopping Sight Distance 

One common horizontal sight obstruction is concrete barrier. Lower-height barrier should 

be considered in these situations. There are vertical-shaped concrete barriers in the height 

range of 29 to 32 inches that are compliant with 

NCHRP Report 350 criteria at test-level 4 (crash 

testing with a single-unit truck at 60 mi/h). Case 

Study 4 (presented in Chapter 8) illustrates how one 

State is using a lower-height median barrier to 

maximize horizontal sight distance. 

In some cases, slight adjustments to lane width or 

the placement of the lane within the roadway cross 

section can increase horizontal stopping sight 

distance. This strategy must be evaluated carefully to ensure that it does not create other 

safety or operational problems, particularly if the lanes are narrowed. 

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Target areas: Any location with limited 

stopping sight distance. 

Strategy: Select cross-sectional 

elements to manage speed. 

Target areas: Any location with limited 

sight distance to an intersection. 

Strategy: Static or dynamic warning of 

intersection or entering traffic. 

Select Cross-Sectional Elements to Manage 

Speed 

In some locations, mitigation measures to consider 

for either vertical or horizontal sight distance design 

exception locations are cross-sectional elements and dimensions that manage operating 

speeds so they are at or below the speeds corresponding to the available sight distance. For 

example, an urban cross section with curb and gutter gives the driver a visual cue that they 

are in a reduced-speed environment. A more-closed cross section may also affect driver 

comfort and cause drivers to slow down. This strategy should not create additional design 

exceptions. 

Improve Ability to Avoid Crashes 

Where there is insufficient sight distance to vehicles or other objects on the roadway ahead, 

a fundamental strategy is to design shoulders and a roadside that will improve a driver’s 

ability to avoid a crash. Wider shoulders will give 

drivers a better chance to safely avoid a crash and 

remain on the roadway. Providing additional clear 

recovery area on the roadside will reduce the 

probability of a severe run-off-the-road crash if the 

driver leaves the roadway. 

Improve Driver Awareness on Approach to Intersections 

At some locations, the visibility of approaching intersections and associated traffic control 

devices may be restricted because of inadequate horizontal or vertical sight distances. 

Mitigation measures can be implemented to make the driver more aware of the intersection. 

Advance signing can be installed to warn drivers of the intersection before it is clearly 

visible. In some situations, flashers installed in conjunction with the sign may further 

increase driver awareness. At intersections with a high crash history, high traffic volumes, 

severe sight restrictions, or other concerns, ITS applications may be appropriate strategies. 

For example, detectors can be placed in the pavement 

on a minor road approach to a major highway. A 

flasher on the major highway can be installed to warn 

drivers that vehicles are at the minor road approach, 

entering the intersection (Figure 58). 

FIGURE 58 

Intersection warning sign with flashers 

activated by vehicles entering from the side 

road. 

Target areas: Any location with limited 

stopping sight distance. 

Strategy: Provide wider shoulders and 

wider clear zones. 

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Target areas: Any location with limited 

sight distance to an intersection, 

particularly intersections with a history of 

night crashes. 

Strategy: Intersection lighting. 

Target areas: Any location with limited 

sight distance to intersection signs. 

Strategy: Repositioning, adding, or 

enhancing intersection signs. 

Measures can also be taken if the sight distance to 

traffic control devices at an intersection is limited. 

Examples include: installing larger STOP or YIELD 

signs, installing STOP signs on both sides of the 

roadway, adding a STOP sign on the left side near the 

centerline within an island, and installing a flasher on 

top of the STOP sign to improve visibility because of 

limited vertical sight distance (Figure 59). 

FIGURE 59 

A STOP sign with a flashing beacon 

improves visibility of the sign at this 

intersection with limited vertical sight 

distance. 

Another strategy for improving intersection 

recognition, particularly where there is a history of 

night crashes, is intersection lighting (Figure 60). 

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Target areas: High-speed roadways with 

insufficient cross slope. 

Strategy: SLIPPERY WHEN WET signing. 

FIGURE 60 

Intersection lighting. 

10. Cross Slope 

The primary concern for locations with 

insufficient cross slope is inadequate drainage 

and ponding of water on the travel lanes. 

SLIPPERY WHEN WET signs may be used to 

warn drivers of pavements with insufficient 

cross slope that may become more slick than 

sections with normal cross slope (Figure 61). 

The MUTCD provides guidance on the size of 

warning signs for various highway types but 

notes that larger signs may be used when appropriate. 

Larger warning signs should be considered for design exception locations. 

FIGURE 61 

SLIPPERY WHEN WET sign. 

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Target areas: Any highway with cross 

slopes that are either too flat or too steep. 

Strategy: Grooved, textured, or opengraded pavements to improve surface 

friction. 

Target areas: Any highway with 

insufficient cross slope. 

Strategy: Improve drainage through 

transverse grooving on PC pavement 

and open-graded surface courses on 

HMA pavement. 

Another strategy aimed at helping drivers maintain control on slick pavements is pavement 

grooving and other textures that improve surface friction (Figures 62 and 63). This strategy 

is appropriate for pavements with cross slopes that are either too flat or too steep. For PCC 

pavement, textures can be placed at the time 

of construction or milled into existing 

pavement. Longitudinal grooving will 

minimize noise—both externally and for 

drivers. For HMA pavement, open-graded 

surface courses can be used to improve 

surface friction. 

FIGURE 62 

Longitudinal texture 

applied to fresh 

pavement to improve 

surface friction. 

Improving drainage should be considered for roadways 

with insufficient cross slope. Transverse grooving on 

PCC pavement can improve surface drainage 

(Figure 63). On HMA pavement, open-graded friction 

courses with a higher percentage of voids allows water 

to drain more quickly through the surface course to an 

impervious intermediate course, and out into an edge 

drain or the ditch. This strategy should be considered on 

resurfacing projects in situations where the cross slope cannot be increased to the acceptable 

range. In some locations, more expensive continuous drainage systems may be appropriate 

(Figure 56). 

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Mitigation Strategies for Design Exceptions 

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Target areas: Highly superelevated 

highways where the cross-slope break 

exceeds 8 percent. 

Strategy: Adjustment of the high-side 

shoulder cross slope. 

FIGURE 63 

Transverse grooving to improve surface 

drainage and friction. 

On the high side of superelevated curves, the cross-slope break should not exceed 8 percent. 

One mitigation strategy to consider is to move the breakpoint outward in the transverse 

direction (Figure 64), reducing the probability of a driver crossing over the breakpoint. 

Another strategy is to slope the shoulder in the same direction as the traveled lanes through 

the area with high superelevation. In northern regions, however, a downside to this 

strategy is that any ice or snow on the shoulder will drain onto the roadway as it melts 

during the day, creating the potential for ice to form 

on the traveled lanes as temperatures fall. Figure 64 

illustrates how the cross slope of the shoulder can be 

transitioned to mitigate a steep cross-slope break. In 

this example, a portion of the shoulder is paved flat 

(no cross slope), adjacent to the steep cross slope of 

the travel lanes. The remainder of the shoulder is 

sloped in the opposite direction. This is an effective method for non-paved shoulders to 

prevent gravel or soil from washing onto the travel lanes and for controlling drainage across 

the travel lanes. There are additional ways to modify the cross-slope break, including 

rounding over the breakpoint on HMA pavements. 

FIGURE 64 

An example of transitioning the cross slope of the shoulder to mitigate a cross-slope break greater than 8%. Rounding at 

the breakpoint is an option with HMA pavement. 

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Target areas: Any highway with a 

structure that has low vertical clearance. 

Strategy: Signing. 

Target areas: Highways with a nearby 

detour route that is designed to carry 

heavy vehicles. 

Strategy: Detours. 

Target areas: Highways where an 

alternate route for large vehicles exists— 

Non-interstate highways. 

Strategy: Prohibiting large vehicles. 

11. Vertical Clearance 

Signing is the most common mitigation strategy for vertical clearance (Figures 65 and 66). 

Whenever vertical clearance criteria are not met, advance warning should be placed at the 

nearest intersecting road or wide point in the road at which a vehicle can detour or turn 

around. The MUTCD provides guidance on the size of warning signs for various highway 

types but notes that larger signs may be used 

when appropriate. Larger warning signs should 

be considered for design exception locations. In 

some locations, electronic message signs have 

been used to provide enhanced warning. 

An innovative strategy for providing additional warning is to combine the sign with chimes 

that are hung from a sign truss at the same height as the vertical clearance of the structure 

(Figure 67). If a truck hits the chimes, the driver is alerted that the truck will not clear the 

structure. 

FIGURE 65 

Vertical clearance 

signing. 

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FIGURE 66 

Vertical clearance 

signing. 

FIGURE 67 

Warning sign with 

hanging chimes 

installed at the same 

height as the vertical 

clearance of the 

structure. 

In some locations, it may be appropriate to provide marked detours for trucks and other 

large vehicles that allow them to bypass the low structure. Similarly, it may be appropriate 

to prohibit large vehicles on certain routes to prevent impacts with low structures. 

12. Lateral Offset to Obstruction 

As discussed in Chapter 3, a lateral offset to obstruction is not the same as the clear zone. A 

lateral offset, by definition, deals with objects so close to the roadway that there may be 

adverse impacts to the operation of the highway. Some examples of these objects include 

walls, barriers, bridge piers, sign and signal supports, trees, and utility poles. The clear 

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Target areas: Any highway with 

roadside obstacles near the traveled 

lanes—most commonly, urban arterials. 

Strategy: Delineate roadside obstacles. 

zone is a clear recovery area, free of rigid obstacles and steep slopes, which serves a safety 

function. 

Assuming an object cannot be removed or 

relocated, the primary mitigation strategy is to 

make the objects highly visible to drivers. 

Delineation with reflectors or reflective sheeting 

(Figures 68 and 69) is one method to make the 

objects more visible, particularly at night. Another strategy to consider is lighting. In 

addition to making roadside objects more visible, lighting has many other benefits in urban 

areas where design exceptions for lateral offset are most common—from public safety 

benefits to improved pedestrian safety. 

FIGURE 68 

Reflective sheeting 

on utility poles. 

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Target areas: Any highway with roadside 

obstacles near the traveled lanes—most 

commonly, urban arterials. 

Strategy: Narrow selected cross-sectional 

elements to provide additional offset to the 

obstruction. 

Target areas: Any highway with 

roadside obstacles near the traveled 

lanes—most commonly, urban arterials. 

Strategy: Enhanced pavement 

markings. 

FIGURE 69 

Reflective sheeting 

on utility poles.

On urban arterials with more than two lanes, 

another strategy to consider is distributing the 

available cross-sectional width to provide additional 

offset to the obstruction. For example, through 

lanes, turn lanes, or medians could be narrowed 

slightly in order to provide additional offset or 

additional space for on-street parking. With this 

strategy, care must be taken to ensure that any operational benefits gained in the outside 

lanes are not lost to poorer performance on the inside lanes. Each site will have unique 

characteristics that need to be evaluated before determining an optimal distribution of the 

cross section—traffic volumes, traffic composition, the available cross-sectional width, speed 

studies, and offset distance to the obstruction. 

Another mitigation strategy for lateral offset is clear 

delineation of the lane lines. See the Lane and 

Shoulder Width section for information on 

enhanced pavement markings. 

13. Structural Capacity 

Mitigation strategies for structural capacity are not addressed in this Guide. 

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105 

CHAPTER 5 

Case Study 1 - Interstate 235 

Reconstruction 

Project Location 

Des Moines, Iowa 

FIGURE 70 

The project runs through

Des Moines, Iowa. 

FIGURE 71 

Approximate project 

limits of design 

exception.

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Project Description and Context 

Interstate 235 is an urban freeway approximately 14 miles (22 km) long that runs through 

the heart of Iowa’s capital city. The freeway serves downtown Des Moines, the state capital 

complex, Drake University, and other local destinations. 

The original freeway, constructed in the 1960s, was reconstructed in the mid 2000s. The 

design speed selected for the reconstruction project was 60 mi/hr (100 km/h), with a posted 

speed limit of 55 mi/hr. The design year (2025) traffic volume was 151,000 vehicles per day. 

Because the project involved full reconstruction, design criteria from the current AASHTO 

guidance (A Policy on Design Standards, Interstate System) were used. 

The reconstruction project focused on improving safety and operations through the 

corridor, as well as replacing aging infrastructure. The project elements included: 

• New bridges with full vertical clearances, improved pedestrian and bicycle 

accommodations, and aesthetic enhancements. 

• Capacity improvements, with the final cross section consisting of six to eight basic lanes, 

as well as auxiliary lanes between some of the interchanges. 

• Complete interchange reconstruction, with new or upgraded configurations to improve 

capacity, ramp spacing, lane balance, and lane continuity and to remove left-hand 

entrances and exits. 

• New barriers—through the median, at side obstacles, and on bridges. 

• Pavement replacement or strengthening. 

• Landscaping improvements. 

FIGURE 72 

Interstate 235 before 

reconstruction.

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Chapter 5 — Case Study 1 – Interstate 235 Reconstruction 

107 

FIGURE 73 

Interstate 235 after 

reconstruction. 

Site Constraints 

The design cross section that was used through much of the freeway corridor is illustrated 

in Figures 74 and 75. Twelve-foot (3.6-m) lanes, 12-foot (3.6-m) outside shoulders, and 

12-foot (3.6-m) inside shoulders were provided, consistent with FHWA’s adopted criteria 

from AASHTO’s A Policy on Design Standards, Interstate System. 

FIGURE 74 

Cross section within the unconstrained locations. Note the 12-foot (3.6-m) lanes, the 12-foot (3.6-m) outside shoulders, and 

12-foot (3.6-m) inside shoulders.

Through most of the corridor, the existing median width was 50 feet (15.2 m), which meant 

that the added lanes, full shoulders, ramp connections, and median barrier could be 

accommodated within the median. However, over a length of about 4.6 miles (7.4 km), the 

existing median was about 10 feet (3.0 m) narrower than the rest of the corridor, which was 

not enough space to meet full design criteria for all of the cross-sectional elements. 

Within this constrained area, providing a cross section that fully met criteria would have 

significantly increased the costs and impacts of the project on adjacent land uses. In addition 

to the right-of-way impacts, nearly $28 million would have been incurred for construction 

costs to widen the freeway to the outside, including significant utility relocation. 

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Mitigation Strategies for Design Exceptions 

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FIGURE 75 

The unconstrained cross 

section.

The Design Exception 

Because of the limited cross-sectional width and the unacceptable impacts of widening to 

the outside, the Iowa DOT decided to pursue a design exception for either shoulder width 

alone or both lane and shoulder widths. Working cooperatively with the FHWA Division 

Office, the designers investigated various combinations of lane and shoulder widths for the 

available width that would optimize safety and operations (see the following section on 

Mitigation Measures). 

The cross section that was eventually selected narrowed the two inside traveled lanes on 

both sides by 6 inches (Figure 76). This provided enough space to provide 8-foot inside 

shoulders. The resulting design represented a compromise in both the lane width and 

shoulder width values. The consensus was that although this design did not meet the 

adopted design criteria, it would function well operationally and would most effectively use 

the available cross-sectional width to optimize safety. A design exception was prepared for 

both the lane and shoulder widths. 

FIGURE 76 

Cross section within the area of restricted width. Note the 11.5-foot (3.5-m) inside lanes and 8-foot (2.4-m) inside shoulders.

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Chapter 5 — Case Study 1 – Interstate 235 Reconstruction 

109 

Mitigation Measures 

Mitigation Measure 1: Lane Width Reduction to Provide Wider Inside Shoulder 

After concluding that providing an inside shoulder that met full criteria was not feasible, the 

designers initially considered full 12-foot (3.6-m) lanes with 7-foot (2.1-m) inside shoulders. 

The first mitigation measure investigated was to slightly reduce lane width to provide a 

wider inside shoulder. The safety goal was to provide enough inside shoulder width to 

store disabled vehicles and allow drivers to maneuver onto the shoulder to avoid a crash or 

an object in front of them on the traveled lanes. An 8-foot (2.4-m) shoulder was determined 

to be the minimum width for safe storage of disabled vehicles next to the concrete median 

barrier. The wider shoulder also slightly increased horizontal stopping sight distance 

around the 44-inch median barrier. 

As listed in Table 23, a number of combinations were considered, and there was much 

discussion about the tradeoffs associated with each of the alternatives. As part of the 

analysis, the Iowa DOT consulted a study by the Texas Transportation Institute related to 

lane and shoulder widths on urban freeways. 

TABLE 23 

Combinations of Lane and Shoulder Widths Considered 

Lane Width Inside Shoulder Width Comments 

12 feet (3.6 m)—all thru lanes 12 feet (3.6 m) Meet all design criteria. Widen to 

the outside. 

12 feet (3.6 m)—all thru lanes 7 feet (2.1 m) 

Use available width to 

accommodate full lane widths. 

Design exception for shoulders 

only. 

11.5 feet (3.5 m)—all thru lanes 9 feet (2.7 m) 

11.0 feet (3.4 m)—all thru lanes 10 feet (3.0 m) 

11.5 feet (3.5 m)—inner 2 lanes in 

each direction 8 feet (2.4 m) Selected design. 

11.0 feet (3.4 m)—inner two lanes in 

each direction 9 feet (2.7 m) 

Alternatives were also discussed that used some of the outside shoulder width. 

Parts of the freeway project did not include full pavement replacement, including the design 

exception area. A thick HMA overlay was placed over the existing portland cement concrete 

lanes. With the possibility of reflective cracking, one issue that needed to be considered was 

how such cracks would line up with the proposed lane lines. Because narrowing the lanes 

would place the new lane lines at a different transverse position than the longitudinal joints 

of the underlying PC pavement, there was some concern that if cracking were to occur, 

drivers would position their vehicles relative to the crack instead of the pavement markings. 

This could be even more prevalent when sun glare or inclement weather obscures the 

pavement markings. 

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Mitigation Strategies for Design Exceptions 

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This issue influenced which lanes were narrowed and by how much, as the designers 

strived to line up the proposed lane lines with the underlying joints as closely as possible 

(Figure 77), while at the same time achieving a wider and more useable inside shoulder 

(Figure 78). 

FIGURE 77 

Narrower lane widths would place the lane lines in a different transverse position than the underlying longitudinal joints.

In addition to the reflective cracking issue, there were several other reasons that 11.5-foot 

lanes on the two inside lanes were ultimately chosen. First, the designers concluded that 

more than a 6-inch deviation from the adopted criteria for lane width was not appropriate 

for a high-volume urban interstate or for the specific characteristics of this location. Second, 

6-inch lane width reductions, on a total of four lanes, provided enough space for inside 

shoulders wide enough to accomplish the safety objectives. Third, the right-side lanes 

would experience more maneuvering and lane changing as vehicles entered and exited the 

freeway than would the inside lanes. Therefore, full lane widths were maintained on the 

two outside lanes in each direction. 

Other elements that were considered in the analysis included the following: 

• Horizontal alignment. A relatively straight horizontal alignment, with no non-standard 

horizontal curvature through the design exception area, supported the concept of 

narrowed lanes. The potential for large vehicles to off-track along curves and encroach 

into adjacent lanes is an issue that should be considered when lane widths are being 

studied. 

• Volume of trucks and other large vehicles. Wider vehicles such as trucks, buses, and 

recreational vehicles need adequate lane width for driver comfort. Operationally, lane 

widths that are too narrow may cause drivers of large vehicles to drive slower than the 

prevailing operating speeds. This issue is also interrelated with horizontal alignment, 

because larger vehicles will off-track to a greater degree around curves. Although there 

is some large vehicle traffic on Interstate 235, much of it bypasses the city on Interstates 

80 and 35 along the north and west sides of the metro area (Figure 71). This fact also 

supported the concept of slight lane width reductions. Large vehicles in the innermost 

lane will also be adjacent to the 8-foot inside shoulder, which increases the comfort level 

for drivers in that lane. 

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Chapter 5 — Case Study 1 – Interstate 235 Reconstruction 

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• Speeds. The posted speed limit for Interstate 235 is 55 mi/hr, with operating speeds 

slightly higher during non-peak hours. The lower speeds of an urban freeway, as 

compared to most rural freeways and expressways, also supported the proposed design. 

• Substantive safety. Crash data from the 1990s were evaluated and showed that shoulder 

width was not a contributing factor to the crashes within the design exception area. 

FIGURE 78 

Inside lane and 

shoulder widths within 

the constrained areas 

were narrowed. 

Mitigation Measure 2: Enhanced Pavement Markings 

To deal with the concern of the adjusted lane lines not matching up with potential reflective 

cracks, high-reflective pavement markings were used to better delineate edges of the lanes. 

Because these pavement markings are recessed, they also require less frequent reapplication compared to flush, painted markings that are more prone to wear from snow 

and ice removal. 

Mitigation Measure 3: Lighting Placement 

The 8-foot inside shoulders provide space for emergencies and incidents. It is insufficient, 

however, for most maintenance activities requiring vehicles in the median. A final 

mitigation measure involved the elimination of one important maintenance operation in the 

median. Overhead lighting was shifted to the outside of the freeway through the design 

exception area, instead of on top of the median barrier (Figure 79). This allows lighting 

maintenance to take place on the outside, where there is more space, instead of within the 

8-foot inside shoulder, which would require closure of the inside lane. Another reason for 

doing this was to prevent vehicle intrusion with light poles when the barrier is struck by 

large vehicles. In the area with the greatest cross-sectional width and a split median barrier, 

lighting was placed down the median (Figure 80). 

8-foot 

shoulder 

11.5-foot lanes 

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Mitigation Strategies for Design Exceptions 

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FIGURE 79 

Lighting was placed 

on the outside of the 

freeway through the 

design exception 

area. 

FIGURE 80 

Lighting was placed 

down the center of 

the median where 

more cross-sectional 

width was available. 

Discussion 

The Iowa design exception illustrates the use of tradeoffs as a mitigation measure. Instead of 

simply using full lane widths and writing a design exception for shoulder width, the Iowa 

DOT went through a thoughtful design process that led to mitigating the narrow shoulders 

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Chapter 5 — Case Study 1 – Interstate 235 Reconstruction 

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by using a small amount of width from several lanes on the freeway to provide a wider and 

more useable shoulder. 

Even though the slightly narrower lanes were considered appropriate for this location, this 

treatment may not be appropriate for every location with limited cross-sectional width. It is 

important to analyze each location and its particular conditions individually. 

Issues such as highway type, truck volumes, horizontal alignment, terrain, speed, and crosssectional width available would all be variables that influence lane width. In this case study, 

even the method of pavement rehabilitation was a site-specific issue that influenced the lane 

and shoulder widths. 

Good sources of information related to lane and shoulder widths on urban freeways include 

HSIS Summary Report, Safety Effects of Using Narrow Lanes and Shoulder-Use Lanes to Increase 

the Capacity of Urban Freeways (http://www.tfhrc.gov/safety/pubs/05001/05001.pdf) and 

NCHRP Report 369, Use of Shoulders and Narrow Lanes to Increase Freeway Capacity. 

The process that was followed for the Iowa design exception also makes this a particularly 

good model: 

1. The Iowa DOT evaluated concepts that would meet design criteria. The designers 

identified the impacts and costs of meeting criteria before developing design 

exception alternatives. This included a benefit/cost analysis of widening the freeway 

to the outside. A clear understanding of both the social and economic costs was 

developed. 

2. The Iowa DOT worked cooperatively with the FHWA Division Office. A number of 

ideas and alternatives were shared and discussed between the two agencies to 

determine the optimal lane and shoulder widths for this particular location. 

3. Research was consulted to better understand the potential impacts of narrowing the 

traveled lanes. 

4. The crash history of the location was evaluated, with particular emphasis on crashes 

that would be sensitive to shoulder width. 

5. Several mitigation measures were evaluated and implemented. 

6. The design exception was clearly documented. Initial review was inherent in the 

decisionmaking process that took place between staff working on the issue from 

both the DOT and the FHWA. Final review and approval was by the Director of the 

DOT’s Office of Design and the FHWA Division Director. 

Acknowledgements 

Mike Kennerly, Director, Office of Design, Iowa DOT 

Bill Lusher, I-235 Project Manager, Iowa DOT 

Andy Wilson, Transportation Engineer, FHWA Iowa Division 

Jerry Roche, Safety Engineer, FHWA Iowa Division 

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115 

CHAPTER 6 

Case Study 2 - Tensleep-Buffalo 

Highway (U.S. 16) 

Project Location 

Buffalo, Wyoming 

FIGURE 81 

The project is located 

 in north-central Wyoming. 

FIGURE 82 

Approximate 

project limits. 

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Mitigation Strategies For Design Exceptions 

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Project Description and Context 

The Tensleep-Buffalo Highway (U.S. 16) is a rural two-lane highway that leads into the Big 

Horn National Forest in north-central Wyoming. The roadway is classified as a rural 

arterial, with a posted speed limit of 55 mi/hr and reduced speed limits in some locations. A 

wide variety of motorists use the highway, including logging operators and drivers of other 

heavy trucks, U.S. Forest Service personnel, school bus drivers, tourists, outdoor recreation 

users, and bicyclists. The design year (2019) traffic volume was 2,080 vehicles per day, with 

23 percent truck traffic. 

The Tensleep-Buffalo Highway is situated within the Rocky Mountains in extremely 

challenging topography. Much of the highway lies between a steep cut on the north side 

and a deep canyon on the south (Figure 83). The area has immense natural resources and 

spectacular scenery and views. The highway is designated as the Cloud Peak Skyway Scenic 

Byway through the southern part of the Big Horn National Forest (Figure 84). 

A 9-mile (14.4-km) segment of the highway was reconstructed in 2004. Reasons for the 

project included pavement replacement and safety improvements, because the highway had 

a higher–than-state-average crash rate. 

FIGURE 83 

The steep cut on the 

north side of the 

highway and deep 

canyon on the south 

illustrate the difficult 

terrain faced by 

designers. 

Site Constraints 

The site constraints of the Tensleep-Buffalo Highway project area were the steep, 

mountainous terrain and sensitive environmental areas (Figure 85). Challenging soil and 

geologic conditions, including slide areas, were also present. 

One of the major project stakeholders was the U.S. Forest Service, which is the steward of 

the Big Horn National Forest. The Forest Service requested that the project cause minimal 

disturbance or impact to vegetation, wildlife, aquatic life, waterways, and natural terrain 

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Chapter 6—Case Study 2 - Tensleep-Buffalo Highway (U.S. 16) 

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formations within the forest. The surrounding area is an elk winter refuge and game 

migration route. Minimal disruption to recreational users of the forest was also requested. 

FIGURE 84 

The Tensleep-Buffalo 

Highway leads into the 

Big Horn National 

Forest in the Rocky 

Mountains. 

Alternative alignments were considered but were determined to be infeasible. Flattening the 

grades and horizontal curves to meet design criteria would have involved massive cuts into 

the mountains. The environmental impacts would have been severe and the construction 

costs exorbitant. For these reasons, the highway was essentially reconstructed along existing 

alignment. 

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Mitigation Strategies For Design Exceptions 

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FIGURE 85 

A massive retaining 

wall illustrates the 

difficult site constraints 

that were 

encountered. 

The Design Exceptions 

The design speed selected for the highway was 65 mi/hr (100 km/h). 

Grade 

For the selected design speed and mountainous terrain, the design criteria specified 5 

percent maximum grades. Grades exceeded 5 percent for approximately 4.5 miles (7.3 km), 

or about 52 percent of the project length, with significant lengths exceeding 7 percent. The 

steepest grade reached 7.9 percent. 

Horizontal Alignment 

For the selected design speed, the minimum radius was 1,434 feet (437 m) with maximum 

superelevation of 6 percent. There were 11 horizontal curves that could not be flattened to 

this extent. The sharpest of these had a radius of 590 feet (180 m) and a corresponding 

design speed of 40 mi/hr (60 km/h). Five of the curves had radii with corresponding design 

speeds of 45 mi/hr (70 km/h). 

Shoulder Width 

Design criteria specified shoulder widths of 8 feet (2.4 m) for rural arterials with traffic 

volumes of more than 2000 vehicles per day. Six-foot (1.8-m) shoulders were used. 

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Chapter 6—Case Study 2 - Tensleep-Buffalo Highway (U.S. 16) 

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Mitigation Measures 

Because of the extremely difficult environmental constraints, Wyoming DOT was faced with 

design exceptions of large magnitude—both in terms of the length of highway that was 

affected and the degree of deviation from design criteria. Because meeting all design criteria 

was neither feasible nor appropriate at this location, the challenge was to make the highway 

as substantively safe as possible. 

Mitigation Measure 1: Advance Signing 

A major mitigation measure implemented by the Wyoming DOT was advance signing. 

Signing with clear, simple messages was provided throughout the project to give drivers 

adequate warning of the steep grades (Figure 86) and sharp horizontal curves (Figure 87), as 

well as upcoming safety features such as brake-test areas and runaway truck ramps 

(Figure 88), discussed later in this section. Both conventional and electronic signing was 

used throughout the project. The electronic sign shown in Figure 87 is equipped with a 

radar speed detection device. The message on the black panel below the sign displays 

SLOW DOWN, when the measured speed of vehicles is too high. 

FIGURE 86 

Signing for the steep 

grades is provided 

throughout the 

project. 

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Mitigation Strategies For Design Exceptions 

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FIGURE 87 

Electronic signing provides 

enhanced warning for the nonstandard horizontal curves. 

FIGURE 88 

Advance signing for 

the truck escape 

ramp. 

Mitigation Measure 2: Truck Brake-Check Area and Other Pullout Areas 

Another mitigation measure taken for the steep grades was a designated area where 

truckers can pull off the highway and check their brakes (Figure 89). In addition to the 

brake-check area, there are several other pullout areas: 

• An interpretive site for tourist information and views (Figure 90). 

• Three small pullout areas where drivers can pull off the roadway completely, if 

necessary. 

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Chapter 6—Case Study 2 - Tensleep-Buffalo Highway (U.S. 16) 

121 

• A small pullout area for viewing geologic formations. 

• A game-check station. 

• A road closure turnaround (used to close the road during winter storms and redirect 

drivers back to Buffalo). 

The mitigative safety effect of these pullout areas is that they give drivers a place to 

completely pull off the roadway if they have car trouble or other difficulties while they are 

traveling a highway with steep, mountainous terrain. 

FIGURE 89 

Advance signing for 

the brake-check 

turnout. 

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Mitigation Strategies For Design Exceptions 

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FIGURE 90 

A pull-off area provides tourist 

information and spectacular 

views. Several other small 

turnout areas provide safe 

places for drivers to pull off the 

highway. 

Mitigation Measure 3: Truck Arrestor System 

An innovative mitigation measure that was implemented by the Wyoming DOT was an 

escape ramp that captures out-of-control trucks with a proprietary arrestor system 

(Figures 91 and 92). This measure was chosen through the value engineering process. 

Thirteen methods were evaluated, with five evaluated in depth. The system that was 

eventually constructed was selected based on its overall improvement to safety, preferable 

location, fewer environmental impacts, constructability, lower construction costs, and ease 

of maintenance and repair. 

FIGURE 91 

Runaway truck ramp 

with arrestor system. 

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Chapter 6—Case Study 2 - Tensleep-Buffalo Highway (U.S. 16) 

123 

Before the reconstruction project, a truck escape ramp had been located on the other side of 

the roadway, near the same location as the new ramp. Trucks that lost control had to cross 

the opposing lane of traffic to use the ramp. Finding a suitable location on the right-hand 

side was challenging and with the canyon on this side of the highway, an innovative 

method was needed for capturing trucks. The end result was a safer ramp that no longer 

required crossing the opposing lane of traffic. 

FIGURE 92 

Runaway truck ramp. 

A nearby ramp on the 

other side of the 

roadway was 

replaced because it 

required crossing the 

opposing lane of 

traffic. 

Electronic signing is used to alert drivers when the escape ramp is inoperable. When a truck 

is captured by the arrestor system, a signal notifies local law enforcement personnel, who 

then activate the flashing beacons (Figure 93). Wyoming DOT maintenance personnel stop 

truck traffic while the system is being repaired or when snow is being removed. 

FIGURE 93 

Electronic signing provides advance 

warning when the truck ramp is 

closed. 

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Mitigation Strategies For Design Exceptions 

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Mitigation Measure 4: Climbing Lanes 

With the very steep grades, high truck volumes, and limited passing opportunities, climbing 

lanes (Figure 94) were added throughout the project to improve operations and prevent 

dangerous passing maneuvers. 

FIGURE 94 

Climbing lane. 

Mitigation Measure 5: Guardrail 

The steep topography also made it difficult to provide an adequate clear zone to allow 

drivers who have run off the road to safely recover. Steep grades and horizontal curves are 

factors that contribute to run-off-the-road crashes, so the Wyoming DOT placed guardrail 

strategically throughout the project to prevent these crashes. 

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Chapter 6—Case Study 2 - Tensleep-Buffalo Highway (U.S. 16) 

125 

FIGURE 95 

Guardrail on the 

outside of a horizontal 

curve. 

Discussion 

Mitigation of non-standard design elements and incorporating safety improvements was an 

integral part of the design of the Tensleep-Buffalo Highway. The Wyoming Case Study 

illustrates the value of implementing multiple mitigation strategies in a location with 

extremely difficult environmental constraints. The following mitigation measures were 

implemented: 

• Advance signing, both conventional and dynamic, was provided for the steep grades 

and sharp horizontal curves. 

• A brake-test pullout area was provided for trucks. Other pullout areas along the route 

allow drivers to pull completely off the roadway if necessary. 

• An innovative truck escape ramp was provided at a critical location. 

• Climbing lanes were provided to allow safe passing on the steep grades. 

• Guardrail was constructed to prevent run-off-the-road crashes. 

The Wyoming Case Study is also a good model of context-sensitive design. In a 

mountainous area with immense natural resources and natural beauty, the Wyoming DOT 

reconstructed the highway in a way that blends in with the surroundings and follows the 

natural topography. Meeting all design criteria would have been neither feasible nor 

appropriate in such a setting. By making safety improvements and implementing mitigation 

measures for the design exceptions, the project is expected to have a positive effect on the 

substantive safety of the Tensleep-Buffalo Highway. 

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Mitigation Strategies For Design Exceptions 

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Acknowledgements 

Paul Bercich, State Highway Development Engineer, Wyoming DOT 

Sue Palmer, Project Designer, Wyoming DOT 

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127 

CHAPTER 7 

Case Study 3 - State Route 99 

Reconstruction

Project Location 

Several Communities—Seattle, Washington Area 

FIGURE 96 

State Route 99 is an 

urban arterial in 

Seattle, Washington. 

FIGURE 97 

State Route 99 

parallels Interstate 5 

through the Seattle 

metro area. 

Segments of the 

highway have been 

reconstructed through 

the cities of 

Shoreline, SeaTac, 

Des Moines, Kent, 

and Federal Way. 

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Mitigation Strategies For Design Exceptions 

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Project Description and Context 

State Route (SR) 99 is a major urban arterial in Seattle and surrounding communities. One 

of its functions is to provide regional mobility. It is an alternative, high-capacity route to 

Interstate 5 and serves major regional destinations, including the Seattle-Tacoma 

International Airport. Traffic volumes are high along the entire corridor. In SeaTac, for 

example, average daily traffic is 38,000 vpd. 

For the cities and towns in the Seattle area through which SR 99 passes, the highway serves 

additional functions. It is considered a “Main Street” and gateway by the communities that 

provides access to local businesses, residential areas, and bus transit along the highway, and 

creates an impression for drivers entering the communities. 

The existing highway consisted primarily of a five-lane cross section with two-way left-turn 

lanes (TWLTLs) (Figure 98). In many areas, strip-commercial development was the 

predominant land use adjacent to the highway. Sidewalks and other pedestrian 

accommodations were limited, and a lack of access control along the highway contributed to 

safety and operational problems. Posted speeds in the areas of reconstruction were 40 or 45 

mi/hr, with slightly higher operating speeds in some areas. 

Improving safety for both pedestrians and motorized users was a major impetus for the 

reconstruction projects along SR 99. When compared with highways of similar functional 

classification, SR 99 had some of the highest crash rates in the State. Crash severities were 

also high. 

Beginning in the 1990s, several cities along SR 99 developed comprehensive plans for the 

corridor that proposed reconstructing the highway and redeveloping adjacent land. The 

goal of these plans was to develop a corridor that would enhance the area economically, 

improve safety, and create a more pleasant and attractive “Main Street” through their 

communities. The fundamental vision for SR 99 was to transform the wide, asphalt cross 

section with uncontrolled access and limited pedestrian facilities (Figure 99) into a tree-lined 

boulevard that would provide a safe, welcoming, and attractive environment for both 

pedestrians and drivers (Figure 100). 

At the same time, there was a consensus that the highway’s important function as a major 

arterial for providing regional mobility had to be maintained. With the highway’s multiple 

functions, a cooperative relationship between the communities and Washington DOT was 

critical for resolving complex trade-offs and finding the proper balance between regional 

mobility and local goals. 

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Chapter 7—Case Study 3 - State Route 99 Reconstruction 

129 

BEFORE 

AFTER 

FIGURE 98 

SR 99 before and 

after reconstruction 

in Des Moines. The 

five-lane cross 

section with a twoway left-turn lane 

was common 

throughout the 

corridor. 

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Mitigation Strategies For Design Exceptions 

130 

FIGURE 99 

SR 99 before 

reconstruction in 

SeaTac (top) and 

Shoreline (bottom). 

Note the wide paved 

shoulders, lack of 

access control, and 

no sidewalks. One of 

the primary goals of 

the communities 

along the highway 

was to create a more 

attractive, pedestrianfriendly local 

environment.

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131 

Improvements to SR 99 

With the goals of increasing safety, improving pedestrian accommodation, and creating a 

more attractive corridor, the cities’ redevelopment plans included several specific proposed 

improvements to SR 99: 

• Replacing the center TWLTLs with a raised median to accommodate tree plantings and 

other landscaping, provide a refuge for pedestrians, and improve access control 

(Figure 100). Left turns would be restricted to major intersections, and some U-turn 

areas would be provided at selected locations (Figure 101). 

• Providing additional improvements for pedestrians, such as pedestrian lighting, 

improved crossing points, and improved transit stop areas (Figures 102 and 103). 

• Adding HOV or bus transit lanes on each side, as well as tree plantings, sidewalks, and 

lighting (Figures 102 and 103), through use of the wide paved shoulders as well as some 

additional right-of-way acquisition. 

• Consolidating and defining driveways and other access points (Figure 103). 

• Placing utilities underground (Figure 103). 

• Aesthetic enhancements (Figure 100). 

FIGURE 100 

Drawing of proposed 

improvements to SR 

99 in Shoreline. 

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FIGURE 101 

Left-turn lane and Uturn areas after 

reconstruction in 

Federal Way. A 

much greater level of 

access control was 

achieved. 

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Chapter 7—Case Study 3 - State Route 99 Reconstruction 

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FIGURE 102 

New transit stop in 

Des Moines. 

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Chapter 7—Case Study 3 - State Route 99 Reconstruction 

134 

FIGURE 103 BEFORE 

SR 99 before-andafter reconstruction in 

Des Moines. 

Conditions for 

pedestrians along the 

corridor were greatly 

improved. 

AFTER 

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Chapter 7—Case Study 3 - State Route 99 Reconstruction 

135 

The Design Variance 

One area of concern for the Washington DOT with the cities’ redevelopment plans was the 

inclusion of trees near the roadway, both within the median (Figure 104) and along the 

outside, particularly for a high-volume highway such as SR 99 with speeds in the range of 

40 to 50 mi/hr. Plantings this close to the roadway did not meet Washington DOT’s clear 

zone criteria. There were also concerns about the trees obstructing the drivers’ views of 

pedestrians. 

Even though clear zone is not one of FHWA’s 13 controlling criteria requiring a formal 

design exception, the Washington DOT followed a similar approach. The existing crash 

problem along SR 99—with both high crash rates and severities—was a major motivation 

for improvements for both the DOT and the cities, so exceptions to any design criteria that 

could affect safety were carefully evaluated. Working with the cities, the DOT developed 

and implemented several measures to monitor and mitigate the potential adverse safety 

impacts of the proposed designs. 

FIGURE 104 

Median tree plantings 

along SR 99 in 

SeaTac. 

Monitoring and Mitigation 

In-Service Evaluation 

Because the proposed designs did not meet Washington DOT’s clear zone criteria, the DOT 

entered into the “In-Service Evaluation of Landscaped Medians Agreement” with the cities 

along SR 99. A key provision of the agreement stipulated that the cities participate in the 

data collection. The cities agreed to provide the DOT researchers with records of any 

median intrusions, tree strikes (Figure 105), and tree replacements related to tree health 

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Mitigation Strategies For Design Exceptions 

136 

within their project areas. This part of the agreement was critical because many of the trees 

were small in diameter at the time of data collection. City maintenance personnel could 

provide information on impacts with small trees that broke off and where the crash severity 

was low enough that drivers left the scene without a crash report being filed (Figure 106). 

Another key provision was a commitment to implement mitigation measures—up to and 

including tree removal—if warranted by the incoming crash information. The goal would 

be to implement other mitigation strategies first, if possible, before asking the cities to 

remove trees. 

Results of the in-service evaluation will also be used to evaluate the Department’s urban 

design criteria and make modifications, if appropriate. 

The data collected for the in-service evaluation are summarized in Table 24. Crash records 

were collected for 3 years before the reconstruction project and for 3 years after 

construction. Because of the rare and random nature of crashes, short before-and-after 

studies are often ineffective. The longer the collection period, the higher the probability that 

the data are truly measuring the results attributable to the changes in the before–and-after 

conditions and not just the random variation in crashes from year to year. Although an 

even longer collection period would further increase this probability, 3 years was selected as 

a reasonable and practical time frame for this study. 

FIGURE 105 

Impact with tree in 

median (SeaTac). 

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Chapter 7—Case Study 3 - State Route 99 Reconstruction 

137 

FIGURE 106 

Information from City 

maintenance 

personnel provided 

data on tree impacts 

that were not 

reported. Because 

many of the trees 

were still small in 

diameter, this 

information was 

critical to developing 

an understanding of 

the impact on safety 

as the trees mature 

(SeaTac). 

The evaluation is currently ongoing and the analysis consists of two parts. The first is a 

before- and-after comparison—identification of significant changes in safety performance 

before and after construction and comparison to similar facilities statewide. The second 

phase of the study is development of statistical models designed to explain the factors that 

contribute to the frequency or severity of crashes in the area. 

Several designs were used by the various communities, particularly for the median, which 

will provide control sections and an informative comparison. For example, sections of SR 99 

within Kent did not include trees in the median. SeaTac and Shoreline used trees within 

raised medians of varying widths. Des Moines used a low-profile barrier along the sections 

that had trees in the median (see the following section for more information). 

SeaTac was the first city in which the in-service evaluation was conducted. The crash 

records and maintenance reports indicated that a high percentage of tree hits were occurring 

where the median narrowed adjacent to left-turn lanes. As a result, trees were not planted 

in these narrow-median areas near intersections in subsequent phases of the SeaTac 

reconstruction project (Figure 107). 

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Mitigation Strategies For Design Exceptions 

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TABLE 24 

Types of Before-and-After Data Collected 

Roadway Characteristics 

• Number of lanes 

• Lane widths 

• Vertical alignment 

• Horizontal curvature 

• Shoulders 

• Driveway presence 

• Lane use (including TWLTLs) 

• Intersections 

• Median locations 

• Level of access control 

Median and Roadside Features 

• Median widths 

• Left/U-turn lanes 

• Median and outside tree counts and types 

• Sidewalk presence 

Traffic characteristics 

• Average daily traffic 

• Speed limits 

• 85th percentile speeds (when available) 

Crash Experience 

• Crashes (3 years) 

• Median intrusions and tree replacement 

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Chapter 7—Case Study 3 - State Route 99 Reconstruction 

139 

FIGURE 107 

When the in-service 

evaluation showed 

that many tree hits 

were occurring at the 

narrow-median 

locations adjacent to 

turn lanes, trees were 

no longer planted in 

these areas in 

subsequent project 

phases (SeaTac). 

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Mitigation Strategies For Design Exceptions 

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Low-Profile Barrier 

In Des Moines, an innovative low-profile barrier was used in the median to shield drivers 

from the trees as well as light poles and fixed aesthetic features (Figure 108). The barrier 

had successfully passed the test-level 2 (45 mi/hr) crash test criteria of NCHRP Report 350. 

The barrier terminates with a sloped-down end section where the median narrows adjacent 

to left-turn lanes (Figure 109). No trees or other fixed objects were placed in this narrowmedian area near intersections. 

At a height of only 18 inches, the barrier has a minimal visual impact. As a mitigation 

technique, the barrier is expected to reduce crash severities. It may also have an effect on 

pedestrian movements, potentially discouraging crossings at unmarked, mid-block 

locations. Impacts with the barrier, pedestrian crashes, and speeds are among the variables 

being monitored in Des Moines as part of the in-service evaluation. 

FIGURE 108 

A low-profile barrier was used 

along the median of SR 99 in 

Des Moines. 

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Chapter 7—Case Study 3 - State Route 99 Reconstruction 

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FIGURE 109 

Sloped-down end section 

adjacent to turn lane. 

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Mitigation Strategies For Design Exceptions 

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FIGURE 110 

Two low-profile 

concrete barriers 

have passed NCHRP 

Report 350 test-level 

2 (45 mi/hr) crash 

testing. 

Discussion 

The reconstruction of SR 99 within several communities in the Seattle area illustrates the 

importance of monitoring the performance of design exception locations after construction. 

For these projects, the Washington DOT established a formal in-service evaluation 

agreement with the cities involved. Several characteristics of Washington’s in-service 

evaluation make it a good model: 

• The cooperation between the DOT and the cities was critical for a successful data 

collection effort. Relying on crash reports alone would not have been as effective 

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Chapter 7—Case Study 3 - State Route 99 Reconstruction 

143 

because the trees were small during the data collection period and many impacts were 

not reported. By supplementing crash data with the unreported impacts provided by 

city maintenance personnel, the researchers were able to compile a much more complete 

and accurate data set and gain some insights about future safety performance as the 

trees mature. 

• The DOT and the cities jointly committed to implement mitigation measures, including 

tree removal in some areas, if warranted by the incoming crash information. 

• By monitoring performance, some changes were made quickly, before the study was 

completed. For example, when it became evident in SeaTac that many trees were being 

struck at the narrow-median areas adjacent to left-turn lanes, planting in these areas was 

discontinued in subsequent project phases. As discussed in Chapter 2, some judgments 

on expected performance can be made from speed studies and other driver behaviors 

that can be obtained in a much shorter time frame than crash studies. Quicker, proactive 

mitigation efforts are sometimes appropriate. 

• The Washington DOT is using the information learned from the in-service evaluation to 

evaluate its internal urban design criteria and make modifications, if appropriate. The 

knowledge obtained from this evaluation is also expected to assist in decisionmaking 

when similar proposals for exceptions to DOT design criteria are made for future 

projects. 

Acknowledgements 

Dick Albin, Washington DOT 

Anna St. Martin, Washington DOT 

Dave Olson, Washington DOT 

Samih Shilbayeh, Washington DOT 

References 

St. Martin, Anna, John Milton, Mark Hallenbeck, and Jennifer Nee. 2007. Research Report, 

In-Service Evaluation of Major Urban Arterials with Landscaped Medians—Conditions as of 2004.

Washington State Transportation Center, University of Washington. February 2007. 

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145 

CHAPTER 8 

Case Study 4 - State Route 110 

(The Arroyo Seco Parkway) 

Project Location 

Los Angeles, California 

FIGURE 111 

Los Angeles, California. 

FIGURE 112 

State Route 110. 

The Arroyo Seco 

Parkway. 

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Mitigation Strategies For Design Exceptions 

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Project Description and Context 

State Route (SR) 110 in Los Angeles, constructed in the 1930s, was the first modern freeway 

on the West Coast (Figure 113). Also known as the Pasadena Freeway, SR 110 was 

designated a California Historic Parkway in 1993 and renamed the Arroyo Seco Parkway. 

The Parkway has a number of historic structures (Figure 114) and incorporated many 

innovative highway design features for its time. It was designated a National Civil 

Engineering Landmark by the American Society of Civil Engineers in 1999. 

Along some segments of the highway, drivers can experience scenic views of parks, 

hillsides, and mountains as they travel along the freeway (Figure 115). In 2002, it was 

designated a National Scenic Byway. In other areas, the highway has a more urban and 

industrial context. A corridor management plan is being developed to preserve the historic 

and cultural features along the highway, improve views, and beautify some of the areas in 

disrepair. 

The Arroyo Seco Parkway is a six-lane freeway (three lanes in each direction) and is a major 

commuter route to downtown Los Angeles. The 2004 Annual Average Daily Traffic 

(AADT) within the project limits was 105,000 vehicles per day, with a peak hour volume of 

8,300 vehicles. The corridor has extremely constrained cross-sectional width. The through 

travel lanes are 11 feet (3.35 meters) wide, and there are essentially no shoulders (both 

inside and outside) along much of the corridor (Figure 116). The horizontal alignment is 

also extremely curvilinear. Much of the Parkway has a posted speed limit of 55 mi/hr, with 

even higher average operating speeds. Most curves along the corridor do not meet 

curvature and horizontal stopping sight distance criteria for these speeds and therefore are 

signed with lower advisory speeds (Figures 115 and 116). 

The California Department of Transportation (Caltrans) conducted a 3-year crash analysis 

for the corridor. The data indicated a crash rate about twice the average rate for similar 

highway types. There were 1,217 total crashes over this time period. Of these, 324 crashes 

involved the median barrier, resulting in 111 injuries and 1 fatality. The analysis also 

showed concentrations of crashes at entrance and exit ramps and concluded that a primary 

causal factor is the limited acceleration and deceleration lengths. 

Several projects are being developed to improve the safety of the Parkway. One of the 

projects involves replacement of the w-beam median guardrail with concrete barrier. 

Another project is for geometric improvements at an interchange. There is also a 

beautification project under development that includes contextually appropriate barriers 

with fencing on top for access control and other improvements to enhance the appearance 

and maintainability of the Parkway.

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Chapter 8—Case Study 4-State Route 110 (The Arroyo Seco Parkway) 

147 

FIGURE 113 

SR 110 shortly after 

construction in 1940. 

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Mitigation Strategies For Design Exceptions 

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FIGURE 114 

Historic structures 

along the Arroyo 

Seco Parkway.

 

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Chapter 8—Case Study 4-State Route 110 (The Arroyo Seco Parkway) 

 149 

FIGURE 115 

The San Gabriel 

Mountains on the 

horizon illustrate the 

Parkway’s scenic 

context.

FIGURE 116

The Arroyo Seco 

Parkway has a 

narrow cross section, 

a curvilinear 

alignment, and nonstandard interchange 

geometry.

Site Constraints 

The historic structures and historic context of the highway are a major constraint in terms of 

meeting current design criteria. Caltrans concluded that the impacts to these historic 

elements would not be acceptable. In addition, a channelized river, the Arroyo Seco, 

parallels SR 110 on the east, constraining the available cross-sectional width on that side 

(Figure 117). Development adjacent to the highway and the interchanges further constrains 

major geometric improvements in some locations. 

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Mitigation Strategies For Design Exceptions 

150 

FIGURE 117

The Arroyo Seco 

Channel runs 

adjacent to the 

Parkway, 

constraining its 

width on one side. 

The Design Exceptions 

Of the 13 controlling criteria, elements that do not meet current criteria on the Arroyo Seco 

Parkway include lane width, shoulder width (both inside and outside), horizontal 

alignment, lateral offset to obstruction, and stopping sight distance. 

Mitigation Measures 

Mitigation Measure 1: Evaluation of Barrier Height 

One of the mitigation measures cited in the documentation for the median barrier project 

was to evaluate the use of a shorter concrete median barrier to maximize horizontal 

stopping sight distance. Caltrans’ standard median barrier for urban freeways is a singleslope concrete barrier (Figure 118). Barrier as low as 32 inches has been used in other 

locations and the optimal barrier height is being evaluated for this project. 

Several site-specific characteristics are being taken into account. First, large trucks are not 

permitted on the Arroyo Seco Parkway. Taller barriers typically provide greater 

performance for containing and redirecting large trucks, but with the truck restriction, this 

barrier function is not a factor. An additional advantage of taller barriers is that they can 

shield headlight glare, which can be especially beneficial on a roadway like the Parkway 

with a curvilinear alignment. A consideration for a location with a historic context is a 

barrier shape or type that is conducive to aesthetic treatments. Constructability is another 

factor because some barrier shapes are more efficient to construct through the use of slipforming equipment. 

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Chapter 8—Case Study 4-State Route 110 (The Arroyo Seco Parkway) 

 151 

A shorter median barrier will increase horizontal stopping sight distance. However, a 

careful weighing of the tradeoffs involved with barrier height will be conducted before a 

final selection is made. 

FHWA’s Roadside Hardware Web site provides information and dimensions on barrier that 

meets NCHRP Report 350 crash test criteria. The shortest vertical-shaped concrete barrier 

that meets test-level 3 or 4 criteria is 29 inches. See the following Web site for more 

information: http://safety.fhwa.dot.gov/roadway_dept/road_hardware/index.htm. 

FIGURE 118 

Single-slope concrete 

median barrier at a 

horizontal curve. 

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Mitigation Strategies For Design Exceptions 

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Mitigation Measure 2: Pull-off Areas 

An existing mitigation measure along the Arroyo Seco Parkway for shoulder width is pulloff areas that are provided periodically along the outside (Figure 119). The pull-off areas 

provide a space for disabled vehicles to pull off the highway, which improves safety and 

can prevent blocking of through travel lanes. Call boxes with telephones are provided at 

some of the pull-off areas. 

FIGURE 119

Pull-off areas are 

provided periodically 

along the outside 

lanes. 

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Chapter 8—Case Study 4-State Route 110 (The Arroyo Seco Parkway) 

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Mitigation Measure 3: Delineation 

To provide better delineation of the narrow lane and shoulder widths and the horizontal 

curves, Caltrans has provided enhanced delineation. This includes raised pavement 

markers, pavement markings with high retroreflectivity, and reflectors along the future 

concrete median barrier (Figure 120). 

FIGURE 120 

Enhanced 

delineation 

with raised 

pavement 

markers and 

pavement 

markings with 

high 

retroreflectivity. 

Reflectors will 

also be placed 

along the new 

concrete 

median barrier. 

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Mitigation Measure 4: Signing 

There are a number of advance warning signs along the corridor, both on the mainline and 

on the ramps (Figure 121). Many of the curve warning signs are combined with advisory 

speed plaques. 

FIGURE 121

Warning signs for 

curvature, slowing 

traffic, and vertical 

clearance. 

Mitigation Measure 5: Auxiliary Lane at Exit Ramp 

Another project along the Parkway involves improvements to the exit ramp at State Street 

near the corridor’s northern limits within the city of Pasadena. The ramp has non-standard 

deceleration length (Figure 122), and reconstruction to full criteria is not possible due to the 

site constraints. In order to improve safety for exiting drivers and drivers behind them, an 

auxiliary lane is being added upstream of the ramp, parallel to the through travel lanes. 

This will allow exiting vehicles to decelerate on the auxiliary lane instead of on the outside 

travel lane. 

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FIGURE 122 

Geometry at the 

State Street Exit 

Ramp.

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Discussion 

The Arroyo Seco Parkway illustrates the challenge presented by older highways that were 

constructed at a time when less was known about design criteria and its relationship to 

highway safety and operations. In this case, the highway also has a historic context as well 

as a river and development near the highway right-of-way. The impacts associated with 

reconstructing this type of highway to meet current criteria are often unacceptable. 

The choice of median barrier illustrates the tradeoffs associated with mitigation measures 

themselves. The height and type of barrier affects horizontal stopping sight distance, the 

ability to contain and redirect large vehicles, headlight glare, aesthetic considerations, and 

constructability. Looking at the characteristics of each specific site is important for the 

careful weighing of these tradeoffs. For the Arroyo Seco Parkway, its crash history, traffic 

volume and composition (large trucks are currently prohibited), horizontal alignment, 

future maintenance requirements, and context are all important variables. 

This case study also illustrates that mitigation measures can be implemented on projects 

with a smaller scope—3R projects or safety-improvement projects. Lower-cost, lowerimpact measures such as improved delineation, pull-off areas, or the addition of an 

auxiliary lane for deceleration can have a significant safety impact at some locations. 

Acknowledgements 

Jim Deluca, Project Development Coordinator, Caltrans 

Greg Damico, Senior Transportation Engineer, Caltrans 

Neil Liang, Transportation Engineer, Caltrans 

Melissa De La Pena, Project Engineer, CH2M HILL 

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Bibliography 

American Association of State Highway Transportation Officials. 2005. A Policy on 

Design Standards, Interstate System. Washington, DC. 

American Association of State Highway Transportation Officials. 2004a. A Policy on 

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American Association of State Highway Transportation Officials. 2004b. A Guide for 

Achieving Flexibility in Highway Design. Washington, DC. 

American Association of State Highway Transportation Officials. 2004c. Guide for the 

Planning, Design, and Operation of Pedestrian Facilities. Washington, DC. 

American Association of State Highway Transportation Officials. 2002. Roadside Design 

Guide. Washington, DC. 

American Association of State Highway Transportation Officials. 2001. Guidelines for 

Geometric Design of Very Low-Volume Local Roads (ADT≤400). Washington, DC. 

American Association of State Highway Transportation Officials. 1999. Guide for the 

Development of Bicycle Facilities, Washington, DC. 

American Association of State Highway Transportation Officials. 1997. Highway Safety 

Design and Operations Guide. Washington, DC. 

Federal Highway Administration. Federal-Aid Policy Guide. 

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Federal Highway Administration. The National Highway System. 

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Federal Highway Administration. The Interactive Highway Safety Design Model. 

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Federal Highway Administration. Roadside Hardware.

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U.S. Department of Transportation

Federal Highway Administration, Office of Safety

1200 New Jersey Avenue, SE, HSSD

Washington, D.C. 20590 Publication Number: FHWA-SA-07-011

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