Speed

50 inferred

45 Desi

40 Operating 

35 Limit

Publication No. FHWA-SA-10-001

September 2009

Speed Concepts:

Informational Guide

FOREWORD

The speed at which drivers operate their vehicles directly affects two performance measures 

of the highway system—mobility and safety. Higher speeds provide for lower travel times, a 

measure of good mobility. However, the relationship of speed to safety is not as clear cut. It is 

difficult to separate speed from other characteristics including the type of highway facility. Still, 

it is generally agreed that the risk of injuries and fatalities increases with speed. Designers of 

highways use a designated design speed to establish design features; operators set speed limits 

deemed safe for the particular type of road; but drivers select their speed based on their individual perception of safety. Quite frequently, these speed measures are not compatible and their 

values relative to each other can vary. This guide discusses the various speed concepts to include 

designated design speed, operating speed, speed limit, and a new concept of inferred design 

speed. It explains how they are determined and how they relate to each other.

The purpose of this publication is to help engineers, planners, and elected officials to better 

understand design speed and its implications in achieving desired operating speeds and setting 

rational speed limits. 

Joseph S. Toole

 Associate Administrator,

Office of Safety,

Federal Highway Administration

Notice

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

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document. 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 the 

document.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and 

policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement.

Technical Report Documentation Page

1. Report No.

FHWA-SA-10-001

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

4. Title and Subtitle

Speed Concepts: Informational Guide

5. Report Date

December 2009

6. Performing Organization 

Code

7. Author(s)

Eric T. Donnell, Ph.D., P.E; Scott C. Hines, Kevin M. Mahoney, D. Eng., P.E., Richard J. Porter, 

Ph.D., Hugh McGee, Ph.D., P.E.

8. Performing Organization 

Report No.

9. Performing Organization Name and Address

Thomas D. Larson Pennsylvania Transportation Institute

The Pennsylvania State University 

201 Transportation Research Building

University Park, PA 16802

Vanasse Hangen Brustlin, Inc.

8300 Boone Boulevard

Suite 700

Vienna, Virginia 22182

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

DTFH61-03-D-00105

12. Sponsoring Agency Name and Address

Office of Safety

Federal Highway Administration

U.S. Department of Transportation

1200 New Jersey Ave., S.E.

Washington, DC 20590

13. Type of Report and Period 

Covered

Report

14. Sponsoring Agency Code

15. Supplementary Notes: 

FHWA COTR: Edward Sheldahl, Office of Safety.

Document was prepared by the Pennsylvania State University under subcontract to Vanasse Hangen Brustlin. Inc.

16. Abstract

Traffic speed is an important yet complex topic in the transportation engineering community. Furthermore, speed is of considerable 

interest to enforcement agencies, safety advocates, property owners, users of the transportation system, and the public at-large because of its perceived effect on crash risk. Each of these stakeholders perceives speed measures differently; therefore, many issues 

related to speed are either misunderstood or remain unanswered. 

The objectives of this guide is to:

•	 Define common speed-related terminology so that the guide’s contents can be clearly conveyed.

•	 Explain the differences between designated design speed, inferred design speed, operating speed, and posted speed limits.

•	 Illustrate perceptions and research conclusions related to the effects of speed.

•	 Document speed-based technical processes.

•	 Summarize State and local government agency roles and actions related to traffic speed.

•	 Highlight speed management and mitigation measures.

The informational guide uses a combination of speed profile plots, tabular summaries, illustrative examples, and narrative discussion to describe each of the objectives noted above. 

17. Key Words

Traffic speed, design speed, 

inferred design speed, operating 

speed 

18. Distribution Statement

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

59

22. Price

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

i
ii
TABLE OF CONTENTS

CHAPTER 1. INTRODUCTION AND PURPOSE.................................................................... 1

CHAPTER 2. TERMINOLOGY AND NOTATION................................................................... 3

CHAPTER 3. CONSEQUENCES OF SPEED ........................................................................... 7

SPEED MOBILITY RELATIONSHIP.................................................................................. 7

SPEED SAFETY RELATIONSHIP .................................................................................... 7

CHAPTER 4. ENGINEERING AND TECHNICAL CONCEPTS............................................ 9

DESIGN SPEED.................................................................................................................... 9

SIGHT DISTANCE ............................................................................................................. 10

VERTICAL CURVATURE.................................................................................................. 12

HORIZONTAL CURVATURE AND SUPERELEVATION ............................................... 14

CHAPTER 5. SPEED CHARACTERISTICS.......................................................................... 17

SPEED DISTRIBUTIONS.................................................................................................. 17

SPEED PROFILES.............................................................................................................. 18

CHAPTER 6. GOVERNMENT ROLES RELATED TO TRAFFIC SPEEDS......................... 21

PLANNING AND DESIGN OF ROADS AND STREETS................................................ 21

ESTABLISHING SPEED LIMITS AND ADVISORY SPEEDS........................................ 26

ENFORCING SPEED LIMITS........................................................................................... 27

SPEED BASED TECHNICAL PROCESSES..................................................................... 28

CHAPTER 7. MANAGING TRAFFIC SPEED ....................................................................... 33

IMPROVING THE DESIGN PROCESS ............................................................................ 33

SETTING AND ENFORCING RATIONAL SPEED LIMITS ........................................... 34

SPEED LIMIT SOFTWARE ............................................................................................... 35

MITIGATION MEASURES................................................................................................ 35

CHAPTER 8. SUMMARY....................................................................................................... 43

REFERENCES .......................................................................................................................... 45

APPENDIX: CALCULATING INFERRED DESIGN SPEED FROM HORIZONTAL 

AND VERTICAL CURVATURE .............................................................................................. 47

iii
LIST OF FIGURES

Figure 1. Crest and sag vertical curves. ....................................................................................... 12

Figure 2. Effect of crest vertical curves on sight distance. .......................................................... 13

Figure 3. Effect of sag vertical curves on sight distance. ............................................................ 13

Figure 4. Cross section of vehicle on superelevated road............................................................ 15

Figure 5. Maximum side friction factors for various design speed. Source: Ref (1)................. 16

Figure 6. Normal probability distributions. ................................................................................. 17

Figure 7. Examples of two normal probability distributions, operating speeds, same mean, 

different standard deviations. ....................................................................................... 18

Figure 8. Example cumulative distribution function, operating speeds....................................... 19

Figure 9. Example speed profile. ................................................................................................. 19

Figure 10. Sight distances corresponding to different reaction times.......................................... 23

Figure 11. Speed profile with inferred design speed substantially exceeding designated design 

speed. .......................................................................................................................... 24

Figure 12. Speed limit sign .......................................................................................................... 26

Figure 13. Advisory speed signs.................................................................................................. 27

Figure 14. Conceptually ideal speed relationships ...................................................................... 29

Figure 15. Typical speed relationships contemplated by design process..................................... 29

Figure 16. Speed relationships that sometimes develop on low and moderate design speeds. ... 30

Figure 17. Relationships that can result when speed limit is increased on basis of actual 

operation speeds.......................................................................................................... 30

Figure 18. Mean and 85th percentile speeds versus inferred design speed on horizontal curves. 34

Figure 19. Horizontal curve warning sign with speed advisory plaque....................................... 36

Figure 20. Example bituminous surface treatment. ..................................................................... 38

Figure 21. Example dynamic speed display sign......................................................................... 39

Figure 22. Example stopping sight distance profile from the IHSDM. ....................................... 42

LIST OF TABLES

Table 1. Probability of pedestrian death resulting from various vehicle impact speeds................ 8

Table 2. Comparison of stopping sight distance requirements for various conditions. ............... 11

Table 3. Primary factors used to establish posted speeds ............................................................ 26

Table 4. Recommended advisory speeds with corresponding ball bank reading......................... 27

iv
1

CHAPTER 1. INTRODUCTION AND PURPOSE

Traffic speed is a subject of considerable interest and importance to transportation and traffic 

enforcement agencies, safety advocates, motorists, non-motorist street and highway users (e.g., 

pedestrians and bicyclists), residential and commercial property occupants and the public atlarge. Speed is a controversial and complex subject. Even after significant research, many 

questions remain unanswered about speed relationships. The labels “good” and “bad” are 

difficult to apply to a particular speed condition since these terms are subjective and based on 

preferences, which often vary amongst different individuals and stakeholder groups. 

Speed is often regarded as an indicator or measure of two different transportation performance 

characteristics, mobility and safety. Higher speeds generally translate to lower travel times, 

an indication of good mobility. The relationship between speed and safety is complicated and 

unclear. There are very few points of consensus on the effects of speed on crash probability. 

One of the many complicating factors is that high-speed highways (e.g., Interstates) have low 

crash rates. However, since Interstate highways also have distinguishing design features (e.g., 

limited access control and wide clear zones), it is difficult to separate the effects of speed from 

other characteristics. There is general agreement that the risk of injuries and fatalities increases 

with speed. So, even without a complete understanding, we know that conflicts can develop 

between the mobility and safety objectives. 

Individual vehicle speeds are selected by drivers, a very large (over 200 million in the U.S.) 

and diverse population. They interpret and respond to signals, both explicit and implicit, in the 

driving environment. The roadway alignment, cross section, roadside, advisory speeds and speed 

limit are all elements of the driving environment that are thought to influence speed selection. 

Our knowledge is limited on how speed is affected by specific features, individually and in 

combination. However, it is known that different information sources (e.g., speed limit and 

roadway geometry) often send drivers different signals on appropriate speed. Speed management 

principles and techniques can be applied to clarify and unify the information being provided to 

drivers and to balance safety and mobility objectives. 

The purpose of this publication is to help engineers, planners, and elected officials to better 

understand design speed and its implications in achieving desired operating speeds and setting 

rational speed limits. It answers the following questions:

•	 Is the design speed the maximum safe speed?

•	 Can you expect to achieve target operating speeds if you select a design speed above the 

target speed?

•	 Is it safe to set speed limits above the designated design speed?

•	 How can the design speed concept be applied to better achieve desired speeds?
2 

•	 What are some engineering measures that could be used to mitigate potential problems 

associated with inconsistencies in design, operating and posted speeds?

This document is written such that it can be understood by a broad audience of interested readers. 

Extensive background knowledge is not needed. As a result, some subjects are not covered with 

the depth or precision customary in specialized research publications on speed-related subjects. 
3

CHAPTER 2. TERMINOLOGY AND NOTATION

The following terms and abbreviations are commonly used in speed literature and discussions. 

Other references may use or define these terms somewhat differently than as defined below and 

used in this publication. 

15th percentile speed – the speed at or below which 15 percent of vehicles travel. Also,

see Speed distribution.

85th percentile speed – the speed at or below which 85 percent of vehicles travel. Also,

see Speed distribution.

A – abbreviation for the algebraic difference in vertical alignment grades, in percent. 

AASHTO – abbreviation for the American Association of State Highway and Transportation 

Officials.

Advisory speed – a speed below the speed limit that is recommended for a section of highway. 

The advisory speed is normally determined through an engineering study that considers highway 

design, operating characteristics and conditions. Advisory speeds are displayed on warning signs 

in speed values that are multiples of 5 mph. Advisory speeds cannot be enforced. 

Designated design speed – the speed established as part of the geometric design process for a 

specific segment of roadway. 

e – abbreviation for superelevation rate.

f – abbreviation for side friction factor.

FHWA - abbreviation for the Federal Highway Administration, an operating agency of the U. S. 

Department of Transportation.

g –force of gravity

Green Book – A Policy on Geometric Design of Highways and Streets (1) as published by 

AASHTO.

IHSDM – abbreviation for the Interactive Highway Safety Design Model, a collection of 

software tools that can be used to evaluate the safety and operational effects of geometric designs 

for two-lane rural highways.

Inferred design speed – the maximum speed for which all critical design-speed-related criteria 

are met at a particular location. 
4 

K – abbreviation for the rate of vertical curvature.

L – abbreviation for the length of vertical curve.

Mean speed - the summation of the instantaneous or spot-measured speeds at a specific location 

of vehicles divided by the number of vehicles observed. It is a common measure of central 

tendency.

MUTCD - abbreviation for the Manual on Uniform Traffic Control Devices.

NHTSA - abbreviation for the National Highway Traffic Safety Administration, an operating 

agency of the U. S. Department of Transportation.

Operating speed - the speeds at which vehicles are observed operating during free flow 

conditions. Free flow speeds are those observed from vehicles whose operations are unimpeded 

by traffic control devices (e.g., traffic signals) or by other vehicles in the traffic stream. The 

85th percentile of the distribution of observed speeds is the most frequently used measure of the 

operating speed. 

Posted speed – one of two speed limit types (statutory speed is other type); the maximum lawful 

vehicle speed for a particular location as displayed on a regulatory sign. Posted speeds are 

displayed on regulatory signs in speed values that are multiples of 5 mph.

R – abbreviation for horizontal curve radius.

Side friction – the lateral force developed between the tires and the roadway as a vehicle 

traverses a horizontal curve, expressed as a dimensionless coefficient of vertical force imposed 

by the vehicle’s weight. 

SSD – abbreviation for the stopping sight distance.

Sight distance – the length along a roadway over which a driver has uninterrupted visibility – 

this is known as available sight distance. Different minimum sight distance design criteria exist 

for various operations and maneuvers, including stopping sight distance, passing sight distance 

and intersection sight distance.

Speed deviation – sometimes used to indicate Standard deviation of speed (see definition). Also, 

see Speed distribution.

Speed distribution – an arrangement of speed values showing their observed or theoretical 

frequency of occurrence. 

Speed limit – the maximum lawful vehicle speed for a specific location. There are two types of 

speed limits, posted speed and statutory speed, definitions of each are provided.
5

Speed zone – a speed limit established on the basis of an engineering study for a particular 

section of road, for which the statutory speed limit is not appropriate.

Standard deviation of speed – a statistical measure (standard deviation) of the spread in 

values, applied to speeds. Also, see Speed distribution.

Statutory speed – one of two speed limit types (posted speed is other type). Numerical speed 

limits (e.g., 25 mph, 55 mph), established by state law that apply to various classes or categories 

of roads (e.g. rural expressways, residential streets, primary arterials, etc.) in the absence of 

posted speed limits. 

USLIMITS version 2.0 - a web-based expert speed zoning software advisor. 
6 
7

CHAPTER 3. CONSEQUENCES OF SPEED

Traffic speeds are relevant and of interest to nearly everyone. Our preferences and judgments 

of appropriate speed are strongly influenced by setting and perspective. The speed we prefer as 

a driver or passenger traveling long-distance along a rural Interstate highway is much different 

than what we prefer as a pedestrian crossing an urban street. Context is everything. 

Speeds have fuel consumption, emissions and traffic noise consequences. Although these are 

important matters, most of the public interest and agency actions related to traffic speeds involve 

consideration of mobility and safety.

SPEED MOBILITY RELATIONSHIP

Speed is used directly and indirectly as a measure of mobility. For motorists, shippers and the 

commercial transportation industry, high operating speeds are considered desirable. Several 

other mobility measures (e.g., delay and travel time) are also linked to speed. For example, 

travel time along a specified route can be determined on the basis of distance and average speed. 

For this reason, facilities that emphasize mobility (e.g., Interstate highways and other freeways) 

accommodate high speed.

SPEED SAFETY RELATIONSHIP 

The effects of speed on safety are complex and only partially known. Despite a long and 

sustained research effort, only a limited number of consistent and reliable speed-safety 

relationships have been established, including:

•	 Many factors affect highway safety.

•	 Safety is fundamentally dependent on successful human performance.

•	 Relationships of safety performance to specific driver-related factors (e.g. education, 

impairment, perception-reaction time, visual acuity) have been investigated.

•	 Studies have also investigated the impacts of vehicle characteristics (e.g., size, weight, 

braking), roadway type and configuration, traffic conditions (e.g. vehicle, pedestrian and 

bicycle volumes) on crash probability and severity.

•	 Complicated interactions between human and non-human subsystems have made it 

difficult for researchers to isolate the effect of speed or other selected factor(s) on safety.

A clear understanding of how speed affects safety has also been obscured by the use of different 

speed-related indicators and a wide range of study methods. Different studies have evaluated 

the safety effects of speed limits, operating speeds and design speed, with some studies making 

the unsubstantiated assumption that these characteristics are related to each other. Many of 
8 

the speed related studies are based on data from high-speed highways. Too often, the results 

and conclusions of these studies are inappropriately applied to all roads and streets. Despite 

the complexity and challenge of understanding speed-safety relationships, useful research 

conclusions have been reached. Key research studies and conclusions are summarized below.

Effect of Speed on Crash Severity

There is clear and convincing evidence that crash severity increases with individual vehicle 

speed. This finding is supported by theory and statistical analysis. 

A vehicle’s kinetic energy is proportional to its velocity squared. When a crash occurs, all or part 

of the kinetic energy is dissipated, primarily through friction and mass deformation. As kinetic 

energy increases exponentially with speed, so does the potential for mass deformation, including 

humans that are inside and outside of the vehicle. Analysis of crash statistics (2) have shown that 

the probability of being injured in a crash increases as the change in speed at impact increases, 

particularly when this change in speed occurs over a short time duration.

A direct and positive relationship has been found between speed and the severity of pedestrian 

injury in vehicle-pedestrian crashes. The results of one United Kingdom study (3) are shown in 

the second column of table 1. A different study (4), conducted in the United States, also found a 

positive correlation between speed and severity of pedestrian injuries. The results, stratified by 

pedestrian age group are presented in columns three through five of table 1.

Table 1. Probability of pedestrian death resulting from various vehicle impact speeds.

Vehicle speed 

(mph)

Probability 

of pedestrian 

fatality 

(%) *

Probability of 

pedestrian fatality age ≤ 14 

(%)**

Probability 

of pedestrian fatality 

age 15 to 

59 (%)**

Probability of 

pedestrian fatality age ≥ 60 (%) 

**

20 5 1 1 3

30 45 5 7 62

40 85 16 22 92

*Source: Ref (3); ** Source: Ref (4)
9

CHAPTER 4. ENGINEERING AND TECHNICAL CONCEPTS

A number of technical and engineering techniques are closely associated with traffic speeds. The 

designed driving environment consists primarily of infrastructure and traffic control devices. 

The geometric design process is used to define the location and dimensions of road and street 

infrastructure which consists of the horizontal and vertical alignment, cross section features, 

intersection type and all the associated details. Traffic control devices are used to regulate, warn 

and guide drivers through the use of signs, traffic signals, pavement markings and other devices. 

Geometric design is an engineering discipline that attempts to account for interactions between 

drivers, infrastructure and vehicles. The techniques and criteria reflect both theoretical and 

practical considerations and evolve as new reliable research conclusions are reached. This 

section of the guide introduces and summarizes several technical topics and techniques that are 

closely related to speed.

DESIGN SPEED

The design speed is a selected speed used to determine the various geometric design features 

of the roadway (1). The historical evolution of the design speed definition has been well 

documented in NCHRP Report 504 (5). The design speed concept was first proposed in 1936 as 

“the maximum reasonably uniform speed which would be adopted by the faster driving group of 

vehicle operations, once clear of urban areas (6).” The American Association of State Highway 

Officials (AASHO) accepted a modified definition of design speed in 1938 as “the maximum 

approximately uniform speed which probably will be adopted by the faster group of drivers, but 

not, necessarily, by the small percentage of reckless ones (7).” Beginning with the 1954 version 

of A Policy on Geometric Design of Rural Highways, the definition of the design speed was “the 

maximum safe speed that can be maintained over a specified section of highway when conditions 

are so favorable that the design features of the highway govern. (8).” Inclusion of the “maximum 

safe speed” was included in the design speed definition until release of the 2001 version of the 

AASHTO Green Book. Removal of “the maximum safe speed” from the design speed definition 

was proposed in 1997 as a result of NCHRP Report 400 (9). It was recognized that operating 

speeds can be greater than the design speed, and the term safe was removed to avoid the 

perception that speeds greater than the design speed were “unsafe” (5). The revised definition as 

a result of NCHRP Report 400 is the definition that is presented at the beginning of this section, 

and is the definition used today. For the purposes of this informational guide, the design speed 

concept described in this section is referred to as the “designated design speed.” The designated 

design speed is used explicitly for determining minimum values for highway design such as 

horizontal curve radius and sight distance. The designated design speed is typically included 

within design documentation and is often shown on the cover sheet of design plans.

A second design speed term is also used in this informational guide. It is referred to as the 

“inferred design speed.” Inferred design speed is applicable only to features and elements that 

have a criterion based on (designated) design speed (e.g., vertical curvature, sight distance, 

superelevation). The inferred design speed of a feature will be different than the designated 

design speed when the actual value is different than the criterion-limiting (minimum or 
10 

maximum) value. The inferred design speed for a radius-superelevation combination is the 

maximum speed for which the limiting speed-based side friction value is not exceeded for the 

designed rate of superelevation and the inferred design speed (determined through an iterative 

process). The inferred design speed for a horizontal curve may also be limited by horizontal 

offsets to sight obstructions on the inside of a horizontal curve. The inferred design speed for a 

crest vertical curve is the maximum speed for which the available stopping sight distance is not 

exceeded by the required stopping sight distance. The inferred design speed may also be limited 

by a combination of lane width and average daily traffic. The inferred design speed can be 

greater than, equal to, or less than the designated design speed. 

SIGHT DISTANCE

To maintain vehicle paths and avoid conflicts, drivers need visibility of road and traffic 

conditions. Drivers continuously and (after a certain level of driving experience is gained)

subconsciously process visual information through observation, interpretation and responsive 

action. Visibility needs are related to the operating environment and vehicle speeds. These 

are the key factors used in developing geometric criteria for sight distance. The Green Book 

identifies four types of sight distance: decision, intersection, passing (on two-lane roads) 

and stopping and provides guidance on where each type is recommended. The Green Book 

establishes minimum design values for sight distance and above minimum values are encouraged 

where feasible. As a result, the inferred design speed resulting from the highway geometric 

design process (i.e., actual roadway geometry that is constructed) is often higher than the 

designated design speed. 

The methods of determining the criterion for a particular situation are also included in the Green 

Book and are related to speed. A set of assumptions, usually based on research or practical 

experience, are incorporated into the criteria. One of the key assumptions relates to the object 

or condition that should be seen. In the case of passing sight distance, the object is a vehicle 

approaching the passing vehicle in the same lane and opposite direction. The assumption is that 

the approaching vehicle is a passenger car and the object is assumed to be 3.5 feet above the 

pavement elevation. This is one of many assumptions used to formulate the relationship between 

speed and passing sight distance criteria.

Stopping sight distance should be provided along the entire length of every road and street. In 

that sense, it is the most common type of sight distance. The first and second columns in table 

2 indicate the relationship between design speed and the Green Book stopping sight distance 

criteria on relatively level roadways. The criteria assume a driver perception-reaction time of 

2.5 seconds, driver eye height of 3.5 feet and an object height (i.e., tail lights of another vehicle) 

of 2.0 feet. The assumptions for driver eye height and tail light height reflect the 5th percentile 

statistics (9). This means that 95 percent of vehicles have a driver eye height and tail light height 

greater than these values. The assumed perception-reaction time exceeds the 90th percentile 

driver reaction time. However, normal perception-reaction times vary from about 0.75 to 1.5 

seconds (10) depending on the alertness, fatigue level, alcohol consumption, and age of the 

driver.
11

Table 2. Comparison of stopping sight distance requirements for various conditions.

Design speed 

(mph)

AASHTO design 

stopping sight 

distance criteria 

(feet)

Estimated stopping 

sight distance for 

mean driver

(feet)

Estimated 

stopping sight 

distance for 

dry roadway 

conditions 

(feet)

15 80 40 75

20 115 60 110

25 155 80 150

30 200 105 190

35 250 135 235

40 305 165 285

45 360 200 340

50 425 235 400

55 495 275 465

60 570 320 535

65 645 365 605

70 730 415 680

75 820 465 765

80 910 520 850

Another assumption used in developing Green Book stopping sight distance criteria is that 

braking will result in a vehicle deceleration rate of 11.2 feet/second2

 [0.35g]. This deceleration 

rate was developed through research in consideration of developments in vehicle technologies 

and driver performance. It also assumes that the friction provided by the driving surface is 

adequate to support this rate. Some vehicles with good tires on good pavement can brake at 

0.85g or higher with most able to brake in emergency stops at 0.7g or better. Although most 

pavements provide adequate friction for the assumed deceleration rate, inclement weather 

conditions (e.g., snow and ice) may not. On wet pavement, most vehicles can stop at 0.4g or 

higher even with bald tires. The deceleration rate on ice [0.15g] and snow [0.22g] are lower 

(11). It has also been shown that heavy vehicles achieve the same deceleration rate as passenger 

cars on wet pavement when the heavy vehicle is equipped with antilock brakes (12). Since the 

AASHTO design criteria accounts for wet pavement and allows for controlled braking, it has 

been shown that the design stopping sight distance criteria are adequate for heavy vehicles. 

In addition to the Green Book stopping sight distance design criteria shown in table 2, the third 

column is the estimated stopping sight distance based on the mean driver with a reaction time 

of 1.1 seconds and a vehicle deceleration rate of 17.71 ft/sec2

 (9). This is intended to show 

how large the difference in required stopping sight distance is between the average driver and 

the design or 90th percentile driver. The final column in table 2 also shows the stopping sight 
12 

distance required for the 90th percentile driver on dry roadway conditions. The design criteria 

assume that the available friction is that of a wet roadway. The difference between estimated 

wet and dry stopping sight distance is not based upon available friction, but drivers’ tendency 

to decelerate at a higher rate on dry pavement conditions. However, in an emergency stop, the 

difference between wet and dry stopping distance is due to friction.

Sight distance requirements also depend on the vertical alignment (or profile) of the roadway. 

On inclines (upgrades), the effect of gravity is to reduce vehicle speeds. Conversely, a vehicle 

descending a grade is being accelerated by gravity. For steep grades, uphill or down, gravity 

has a real and practical effect on the stopping sight distance. Generally, no adjustment to the 

base condition (level road) criteria is made for small grades (less than three percent), positive or 

negative.

VERTICAL CURVATURE

Vertical curves are used to transition from one grade (i.e., slope in the direction of travel) to another. Figure 1 illustrates two types of vertical curves, crest and sag.

Crest Sag

Figure 1. Crest and sag vertical curves.

The rate of change in grades affects both driver comfort and sight distance. Crest vertical curves 

can limit sight distance by restricting a driver’s line of sight, as shown in figure 2. In figure 2, 

the point of vertical curve (beginning point of curve), point of vertical intersection (intersection 

of initial and final grades), and point of vertical tangent (end point of curve) are shown as PVC, 

PVI, and PVT, respectively. The initial and final grades are G1 and G2, respectively. The unobstructed horizontal distance between the driver eye height (H1) and object height above the 

roadway surface (H2) defines the available stopping sight distance. 

Sag vertical curves affect how headlight beams intersect the roadway and reveal an object as 

shown in figure 3. If a vertical curve is long enough to provide adequate sight distance, it will 

also provide adequate driver comfort. 
13

Figure 2. Effect of crest vertical curves on sight distance.

Figure 3. Effect of sag vertical curves on sight distance.

Minimum rates of vertical curvature (K) are included in the Green Book for different categories 

of sight distance, design speed and vertical curve type (i.e., crest, sag). The minimum length of 

vertical curve can be determined by using these rates and equation 1, shown below.
14 

HORIZONTAL CURVATURE AND SUPERELEVATION

Road and street alignments are comprised of two basic elements, curves and tangents. The most 

common type of horizontal alignment curve, and the only one discussed here, is the simple curve 

which has a single (constant) radius. Tangents are straight sections between curves. 

The forces associated with vehicles and occupants moving along a curved path are more 

complicated than for a straight path. A vehicle and its contents (hereafter simplified to “vehicle”) 

traversing a curved roadway at a constant speed are accelerating toward the center of the curve. 

This is known as centripetal acceleration. The magnitude of the acceleration is a function of the 

vehicle speed and radius of curvature.

A vehicle can only follow a circular roadway alignment if a centripetal force (Fc) is applied. The 

direction of this force is the same as the acceleration, toward the center of the curve. With some 

simplifying assumptions, equation 2 can be derived. The left side of the equation represents two 

external forces and the right side is the resulting acceleration. 

The two left-hand side components represent superelevation and friction. Superelevation is 

another term for “banking” the roadway. As shown in figure 4, superelevation results in part 

of the vehicle’s mass being accelerated laterally, toward the low side of the banked section (the 

vehicle’s weight, W, is shown in Figure 4). Higher rates of superelevation increase the lateral 

acceleration provided by this component. 
15

Figure 4. Cross section of vehicle on superelevated road.

It should be noted that superelevation and friction are “signed.” They have magnitude and 

direction. Friction and superelevation do not necessarily act in the same direction. For purposes 

of this discussion, the positive direction is defined as toward the center of the curve. Negative 

superelevation is actually quite common. Think of a road with a normal crown (i.e., high point 

in center and sloping outward in both directions). If the normal crown is maintained through a 

curve, both sides have superelevation but with opposite signs. For a curve to the left, the right 

lane of a normal crown section has negative superelevation.

Friction usually, but not always, acts toward the center of the curve which has been defined as 

the positive direction. Exceptions occur, such as a vehicle traveling at low speed along a road 

with a high rate of superelevation. Once a road is constructed, the radius and superelevation 

(both direction and magnitude) are known and constant. The required side friction will vary with 

speed. 

Drivers and other vehicle occupants moving on a turning roadway “feel” the side friction 

developed between the tire and pavement. It is often referred to as unbalanced lateral 

acceleration and is a direct indication of driver discomfort. A passenger (or driver) traveling 

along a curve at 70 mph will be less comfortable than one traversing the same curve at 55 

mph. The use of friction factors in geometric design, particularly limiting or maximum values, 

is often misunderstood. Design criteria for horizontal curves (minimum radii, design rate of 

superelevation) are established so that maximum design side friction factors are not exceeded 

at the design speed. Maximum design side friction factors decrease with design speed. Values 

used in the 2004 Green Book are shown in figure 5. The values are established so that the 

friction required to maintain a vehicle’s path along the curved alignment will nearly always 
16 

be achievable. Maximum design side friction factors are not determined solely as a matter of 

physics or engineering mechanics. Turning maneuvers become more demanding for drivers as 

the unbalanced lateral acceleration (side friction) increases. In recognition of this, human factors 

come into play in establishing maximum design side friction factors.

 

The term “comfort” is used to describe the human response to different levels of unbalanced 

lateral acceleration. The maximum design side friction factors were set so that unbalanced 

lateral acceleration levels do not cause discomfort to vehicle occupants. The values currently 

in use are based on research of passenger comfort (blindfolded passengers) conducted in the 

1940s. Subsequent advancements in vehicle technologies call into question the validity of 

the historic research and its continued use for design and analysis purposes. This leads to two 

important conclusions. First, the maximum design side friction factors do not represent the limit 

of available friction based strictly on material properties and mechanics. Second, the research 

on human response to unbalanced lateral acceleration (i.e., demand friction) is dated and may no 

longer apply.

Observed speeds on horizontal curves routinely exceed designated design speeds. The maximum 

design side friction factors in use adequately accommodate the design speed under a wide 

range of conditions (i.e., related to tire, pavement and driver variability), including very poor 

conditions. Therefore, higher curve speeds can be attained under good conditions, and even 

average conditions. This condition is so prevalent that design values are suspected of being 

unrealistic or inappropriate. 

Figure 5. Maximum side friction factors for various design speed. Source: Ref (1)
17

CHAPTER 5. SPEED CHARACTERISTICS

 SPEED DISTRIBUTIONS

A few basic statistical concepts are introduced in this chapter to help with an understanding of 

speed terms and principles.

Speeds are selected by the driver. Different drivers select different speeds, dependent upon many 

variables (vehicle limitations, roadway conditions, driver ability, etc.). No single speed value can 

accurately represent all the speeds at a particular location. A speed distribution (see definition 

in chapter 2) provides that information. Operating speeds have been found to be normally 

distributed (sometimes called a “bell-shaped curve”). This is fortunate since using that premise 

(probability is normally distributed) allows for some straightforward calculations. Even though 

a variable (e.g., speed, height) may be normally distributed, properties of the distribution may 

vary. Two different normal probability density functions are shown in figure 6. The vertical axis 

indicates the probability that the variable will take on the corresponding value on the horizontal 

axis. The speed distribution provides useful information but it is not a convenient way to 

describe speeds. 

Figure 6. Normal probability distributions.

The unique properties of the normal probability distribution allow it to be conveniently described 

with just two characteristics, mean and standard deviation. These two characteristics also have 

practical importance for traffic speeds. Figure 7 includes plots of two different normal speed 

distributions. These distributions have the same mean but different standard deviations. 
18 

Figure 7. Examples of two normal probability distributions, operating speeds, same mean, 

different standard deviations.

For reasons stated elsewhere in this Informational Guide, the 85th percentile speed is a frequently 

used characteristic. The mean speed plus one standard deviation approximates the 85th percentile 

speed for a normally distributed sample of speeds. For an observed sample of speed data, 

determining the 85th percentile value is fairly easy, either manually (for relatively small samples) 

or a spreadsheet. To get a graphical sense of the 85th percentile speed, the cumulative distribution 

function can be plotted. An example is included as figure 8. To find the values associated with 

any Nth percentile (e.g., 25th, 60th or 85th percentile) level, find that percentile level on the vertical 

axis. Then move horizontally until the line representing the cumulative distribution function is 

intersected, project a line from that point down to the intersection with the horizontal axis. The 

value on the horizontal axis is associated with the Nth percentile. For the figure 8 example, the 

85th percentile speed is 59 mph. 

SPEED PROFILES

Speed profiles are a graphical representation of speed features plotted by location. Figure 9 is 

an example. Speed profiles are useful in studies and evaluations. The contents of speed profiles 

vary but features that are commonly included are the designated design speed, operating speed 

and posted speed limit; inferred design speed can be displayed as well. 
19

Figure 8. Example cumulative distribution function, operating speeds.

Figure 9. Example speed profile.
20 
21

CHAPTER 6. GOVERNMENT ROLES RELATED TO TRAFFIC 

SPEEDS

Government agencies, mostly at the state, county and local levels, own and operate 

approximately four million miles of roads and streets in the U.S. Through road ownership 

and construction, statutes, traffic regulation and enforcement, government agencies create the 

physical and legal driving environment. Not all drivers respond in the same way to the same 

driving environment and these differences extend to speed selection. An understanding of speed 

issues requires some basic insights of agency functions and human behavior in a regulated 

environment.

Before a new road or street is built, plans are prepared by engineers. Except for minor streets 

and roads, the design process considers speed by adopting a designated design speed. The road 

is then constructed in accordance with the design plans and put into service for the motoring 

public, which is regulated by a set of laws and regulations, including an established speed limit. 

A more detailed description of these processes and their relationships is provided below.

 PLANNING AND DESIGN OF ROADS AND STREETS

Roadway geometry is one of the informal information sources that drivers interpret when 

selecting a speed. However, roadway geometry can provide deceptive, conflicting and confusing 

speed cues. This is unfortunate but true and because the existing network is so extensive, 

substantial modifications are often impractical. 

The Green Book is the dominant reference publication for geometric design in the U.S. and its 

application involves selecting a “design speed.” The definition of design speed has changed 

over time but the way it is used in the design process has not. The Green Book guidance on 

designating design speed does not address speed limits. The Green Book recommends that 

topography, anticipated operating speed, adjacent land use and functional classification be 

considered and as high a design speed as practical be selected, except for local streets. The 

selected (designated) design speed is used explicitly to determine:

•	 minimum sight distances.

•	 minimum horizontal curve radii.

•	 maximum grade.

•	 minimum rate of vertical curvature.

•	 minimum width of selected roadway cross section features.

•	 superelevation rate.

•	 maximum gradient.

•	 roadside clearance.

•	 freeway nose taper.

•	 acceleration/deceleration lane length.
22 

Chapter 4 provides background for many of these speed-based design criteria. Some of these 

criteria, such as sight distance and vertical curvature, are related. 

The Green Book provides minimum or limiting values for the design criteria. It also 

recommends “above-minimum design values should be used, where practical.” The underlying 

rationale for this guidance is that above-minimum features will safely accommodate a condition 

beyond the assumed parameters. Historically, this was thought to be “conservative” and 

consistent with other engineering disciplines that use factors of safety to intentionally “over 

design” critical components. The direct effect of utilizing above-minimum design elements 

is that it affords drivers greater comfort to travel at higher speeds, and thereby leads to an 

“inferred” design speed greater than the designated design speed that may be inappropriate 

for surrounding conditions. A definition for “inferred design speed” is included earlier in 

this publication and will be referenced in the next paragraph. An example is provided on the 

following page to complement the definition and clarify the term’s meaning.

Frequently, roads and streets designed for a particular speed appear suitable for much higher 

speeds. Drivers read the road, not the design plans. Some roadway segments, such as a long 

straight section, look the same regardless of designated design speed. When these features are 

combined with over-designed speed sensitive features (i.e., from using above-minimum values 

as recommended by highway geometric design policy) the visible cues on appropriate speed may 

be in sharp contrast to the designated design speed. What was contemplated by the designer as 

a factor of safety (with respect to the designated design speed) is often negated by driver speed 

choice. The differing perceptions of speed by designer and driver are especially problematic 

where a single minimum-value feature (e.g., minimum radius curve) is located within a segment 

that generally has an inferred design speed much higher than the designated design speed. This 

condition is contrary to the design objective of “design consistency.” However, the Green Book 

includes no specific design criteria for design consistency and the condition described technically 

complies with the geometric design policy. 

Highway geometry is not the only cue drivers are receiving. Landscape, development, perceived 

conflicts, and intersections also contribute to the driver’s selection of speed. Just as selecting 

a design that is higher than necessary is inappropriate, it is also inappropriate to select a lower 

design speed if other cues will lead drivers to select a speed that is too high for the design 

geometry. We do not want the highway geometry to be a surprise to the driver.

Design policy recognizes that a wide range of performance levels exist amongst the large and 

diverse population of drivers. The need to integrate human performance and engineering design 

gives rise to another speed conundrum. Green Book criteria attempt to safely accommodate 

below-average driver proficiencies. Many drivers routinely drive at speeds faster than the speed 

limit and faster than other drivers. Overconfident drivers may come to believe they possess 

superior abilities and skills that enable them to operate safely at higher speeds. 

Figure 10 illustrates an example relationship between speed, geometric design and differing 

levels of driver performance. Minimum stopping sight distance design criteria are based on 

several assumptions, as were previously discussed in chapter 4, including 2.5 seconds of driver 
23

perception-reaction time. The solid line in figure 10 is a plot of sight distance versus speed using 

a 2.5-second reaction time. The sight distance values coincide with Green Book stopping sight 

distance criteria, which are also listed in table 2. The broken-line plot is based on a reaction time 

of 1.5 seconds. Drivers do not use formulae to select their speeds; consequently some drivers 

drive faster because they feel capable of reacting quickly. However, the probability of dealing 

successfully with unexpected circumstances is reduced with higher speeds.

Figure 10. Sight distances corresponding to different reaction times.

The 85th percentile speed is used extensively in the field of traffic engineering and safety. Since 

the majority of drivers are considered reasonable and should be accommodated, some numerical 

definition for this segment of the driver population is needed. Over time, the 85th percentile 

driver (or speed) has been used to characterize reasonable and prudent behavior.

For a variety a reasons, including those summarized in this chapter, policy-compliant designs 

often appear adequate for speeds far above the designated design speed. The speed profile 

in figure 11 is an example of a case where the 85th percentile speed and posted speed exceed 

the designated design speed. These data were collected along an existing multi-lane highway 

in central Pennsylvania. The geometric features are in keeping with Green Book guidance 

recommending higher-than-minimum values for design features (e.g., vertical curve lengths, 

horizontal curve radii). The inferred design speed is substantially higher than the designated 

design speed. When the facility was first opened to traffic, the posted speed of 40 mph was 

“mismatched” with the geometric features. Based on a review of operating speeds, the speed 

limit was changed to 50 mph. Many researchers have noted that the design speed process has no 

systematic check for consistency between operating and design speeds. (13,14,15) 
24 

Figure 11. Speed profile with inferred design speed substantially exceeding designated 

design speed.

Even though the Green Book is the principal reference for geometric design, for two basic 

reasons there are many street and highway segments that do not conform to its current 

provisions:

•	 Road design methods have changed substantially over time but when design methods 

change existing roads are not necessarily updated,

•	 Some agencies with road and street design responsibility, especially municipal 

governments, may use different methods.
25

Many city streets and some of the first freeways were designed prior to development of the 

design speed concept. Many designs were created without explicit consideration of vehicle 

speeds. AASHO, which was the predecessor to AASHTO, published its first design-related 

policies in the 1940s. The original AASHO policy used a design speed but it did not apply to 

urban streets. Over the years, AASHO, and then AASHTO, periodically updated and expanded 

its geometric design policy. As the policy became more complete (e.g., included procedures for 

urban streets) it reached its current status as the principal reference for designing U.S. roads and 

streets. When geometric design policy is updated, it is often not applied retroactively. In other 

words, facilities that were designed under an earlier policy are not automatically revised to meet 

the most current policy. 

The use of AASHTO policy is only mandatory in the design of roads that are part of the 

National Highway System, which includes all Interstate highways and other selected arterial 

highways. For other roads and streets, state DOTs and local government units (i.e. counties, 

cities, boroughs, towns), have the discretion to develop and apply the geometric design process 

and criteria they consider most appropriate. State DOTs use design processes and criteria that 

are the same or similar to AASHTO’s. Local agency practice varies widely, with larger agencies 

typically adopting policies similar to their state DOTs, and therefore those of AASHTO. Local 

governments with limited roadway responsibilities (i.e., limited mileage and no arterials) may 

use very simple guidelines, such as those incorporated into subdivision regulations. These 

guidelines typically assume low, but unspecified, speeds. Minimum or typical values are 

provided for features such as curve radii, grades and cross section features, without reference to a 

design speed. 

The previous discussion focused on the case where inferred design speeds exceed the designated 

design speed. Cases where the opposite is true can also occur—see Appendix A which provides 

an example of how to calculate the inferred design speed of horizontal and vertical curvature. 

In such cases, design speed-related criteria are less than the minimum value associated with 

a particular designated design speed. For example, a radius-superelevation combination on 

a horizontal curve may be less than the minimum radius for the designated design speed. 

Use of geometric design criteria that do not meet the minimum value based on a designated 

design speed may occur for a variety of reasons. First, the roadway alignment may have been 

constructed using geometric design criteria that have since been revised. As noted earlier, design 

policy is typically not applied retroactively and therefore the below-minimum design feature is 

not re-constructed to comply with the most recent design policy. Traffic control devices (e.g., 

warning signs with speed advisory plaque) are often placed near below-minimum design features 

to alert drivers of the need to reduce their operating speed. On planned or reconstructed roadway 

facilities, particularly those on the National Highway System, use of below-minimum design 

criteria are often constructed only after granting a design exception. For more information about 

design exceptions, the reader is referred to NCHRP Synthesis 316 (25).

To summarize, government agencies exercise control over the planning and detailed design of 

roads and streets, either by performing these functions or approving the work of consultants and 

third parties, such as developers. The current design process includes consideration of speed. 

However, the design speed process and AASHTO guidance often lead to inferred design speeds 
26 

far in excess of the designated design speed. Further, many roads and streets were not designed 

for any particular anticipated speed, either because they predate the design speed method or the 

jurisdictional agency uses a different approach. The net result is that the geometric features of 

roads and streets have not been designed to encourage a predetermined operating speed. In fact, 

many aspects of the geometric design process are suspected of encouraging operating speeds 

higher than the design speed and higher than desirable.

 ESTABLISHING SPEED LIMITS AND ADVISORY SPEEDS

Speed limits are set in one of two ways: (1) determined specifically for a 

particular road or segment on the basis of an engineering study and displayed 

as a posted speed on a regulatory sign, or (2) a statutory speed limit that 

applies in the absence of a posted speed. Statutory speed limits are set forth 

in state laws. Often, more than one statutory speed limit applies within a 

state. For example, one speed limit may apply everyplace except within 

urban areas for which a different and lower limit applies.

The authority to establish speed limits varies by jurisdiction and can include elected bodies, 

state DOTs and local transportation, public works and police agencies. Posted speeds must be 

reasonable. Speed limits are only meaningful if the majority of motorists comply voluntarily 

and that occurs only if a speed limit is reasonable. An engineering study made in accordance 

with established traffic engineering practices should be conducted as part of setting a posted 

speed (MUTCD 2B.13); this is required in many states. A universal process for conducting 

these studies does not exist. As shown in table 3, different agencies often consider different 

factors. Different studies with similar circumstances may sometimes lead to a different speed 

limit. However, any such difference should be relatively minor since the 85th percentile speed is 

a dominant factor in establishing posted speeds. The (MUTCD) (16) indicates that posted speeds 

“should be within 5 mph of the 85th-percentile speed of free-flowing traffic.”

Table 3. Primary factors used to establish posted speeds

Source: Ref (17)

Figure 12. Speed limit 

sign.

Factor

Percent time used

By state 

agencies

By local 

agencies

85th percentile speed 100 86

Roadside development 85 77

Accident experience 79 81

10 mph pace 67 34

Roadway geometrics 67 57

Average test run speed 52 34

Pedestrian volumes 40 50
27

State or local officials often receive citizen requests for speed limit reductions because of 

perceived excessive speeds. However, research has repeatedly shown that changes in posted 

speeds have little effect on operating speeds. (17).

Advisory speed plaques can bedisplayed on any warning sign to 

inform the road user of the recommended speed for the warning 

condition ahead. The placement of warning signs should be 

based on an engineering study or engineering judgment. Since 

advisory speed plaques often apply to a particular feature, the 

signage may combine information regarding the feature and 

associated advisory speed. An advisory speed plaque is shown 

on the left side of Figure 13. The warning sign on the right side of Figure 13 combines the right 

curve warning symbol with an advisory speed. 

Advisory speeds for horizontal curves are often determined by driving a vehicle equipped 

with a ball-bank indicator through the curve several times at different speeds and applying the 

Green Book guidelines shown in table 4. These guidelines are based on relatively old values 

for driver comfort. It should be noted that the table 4 friction values are less than the maximum 

side friction factors used in design and discussed in chapter 4 (Horizontal Curvature and 

Superelevation).

Table 4. Recommended advisory speeds with corresponding ball bank reading.

Source: Ref (1)

Advisory speeds are displayed on warning signs in speed values that are multiples of 5 mph. For 

further information from the 2009 MUTCD, see Advisory Speed Posting in the next chapter.

ENFORCING SPEED LIMITS

Traffic laws, including speed limits, are enforced by police agencies at the state, county and 

municipal levels. It is extremely rare and generally considered counterproductive to cite drivers 

operating slightly over the speed limit. Since exceeding the speed limit is so common, it is not 

practical to issue a ticket to each and every offending driver. Flagrant violators (i.e., drivers 

operating at very high speeds) pose the greatest risk and are generally the focus of enforcement. 

Police exercise discretion in deciding at what speed and circumstances a citation will be 

Figure 13. Advisory speed signs.

Advisory 

speed 

(mph)

Maximum ball 

bank reading 

(degrees)

Equivalent 

friction factor 

(f)

20 14 0.21

25, 30 12 0.18

35 - 50 10 0.15
28 

issued. Police have no specific knowledge of designated and inferred design speeds. Individual 

vehicle speeds are assessed on the basis of the speed limit, prevailing operating speeds, and 

environmental conditions.

Decisions on when and where to enforce speed limits directly affect driver speed selection. 

Visible and active enforcement reduces operating speeds but the effect diminishes as the distance 

and time from the enforcement increases. Although several automated enforcement programs 

have been instituted in the U.S., speed enforcement is still accomplished primarily by sworn 

police officers. The cost and availability of assigning police officers to this function limits the 

frequency, coverage, and effectiveness of speed enforcement.

SPEED BASED TECHNICAL PROCESSES

As discussed in previous sections of this chapter, there are a variety of decisions related to 

traffic speed. These decisions are made by different groups and people, within different levels 

of government and at different times. In this section, speed-related developments and decisions 

are reviewed over the life cycle of a road or street. The patterns described are common but not 

accurate for each and every facility. 

The conceptual relationship shown in figure 14 between designated design speed, operating 

speed, and speed limit can be thought of as ideal. The concept depicted is not found in any 

authoritative publication or manual on traffic speed, but conforms to the general engineering 

approach. Under this concept, public transportation agencies establish a speed limit less than 

the designated design speed. Drivers, with rare exceptions, heed the limits. In the life cycle 

of transportation facilities, design comes first. Through the design process, concepts are 

converted to operational guidance and then to detailed decisions for implementation. Figure 15 

illustrates an interpretation of how these idealized relationships are pursued through operational 

design guidance. A design speed equal to or higher than the speed limit is usually (but not 

always) designated when the speed limit is known during the design process. Design speeds are 

sometimes designated by determining the speed limit and adding a specified value, such as 5 or 

10 mph, although the Green Book (1) guidance on design speed does not address speed limits. 

The relationship between designated design speed and speed limit varies. Speed limits are not 

necessarily known during the design process. In addition, established speed limits are subject to 

review and revision.
29

Figure 14. Conceptually ideal speed relationships.

Figure 15. Typical speed relationships contemplated by design process.

Designated design speed is explicitly determined during the design process. Inferred design 

speed is determined implicitly (but typically not calculated) as a result of geometric design 

decisions. As noted previously, the designated and inferred design speeds often differ because 

designers are encouraged to exceed minimum values for geometric design features that are 

determined based on the design speed. The result is that many design features meet criteria for 

design speeds far greater than the designated design speed. 

After a road is open to traffic, actual operating speeds may be higher than anticipated as shown 

in figure 16. This often happens when the designated design speed is less than the desired speed, 

a common occurrence for streets and roads with low and moderate designated design speeds. 

The desired speed is defined as the speed, under free-flow conditions, that drivers choose to 

travel when not constrained by roadway design features (18). The MUTCD (16) recommends 

that posted speeds be within 5 mph of the 85th percentile operating (free-flowing) speed. If the 

speed limit is changed to reflect actual operating speeds, the relationship between design speed 

and posted speed will be altered. Figure 17 illustrates a resulting scenario of a speed limit being 

higher than the designated design speed. 
30 

Figure 16. Speed relationships that sometimes develop on low and moderate design 

speeds.

Figure 17. Relationships that can result when speed limit is increased on basis of actual 

operation speeds.

The undesirable conditions shown in figures 16 and 17 occur more frequently on certain roadway 

types. There is a concern that raising the speed limit, as shown in figure 17, will lead to even 

higher operating speeds and thus contribute to a cycle of speed escalation and reduced levels of 

safety. 

Some people have difficulty understanding how it can be acceptable for a speed limit to exceed 

the designated design speed. First, geometric design criteria above minimum values for a 

designated design speed are often used. Secondly, geometric design criteria have been developed 

using a variety of premises. For example, horizontal curve criteria are based on “comfort” levels, 

the sensory response of vehicle occupants to unbalanced lateral acceleration. The values were 

developed in the 1940s. Lastly, the criteria are based on assumed conditions that are improbably 

poor (e.g., actual conditions will likely be better). Consequently, driving at the design speed 

under favorable environmental conditions provides a margin of safety that can compensate for 

unexpected developments, below-average driver performance and less favorable road conditions.
31

Over the life cycle of a road, different groups and individuals make speed-related decisions. 

These groups have different roles and often use different information in their decision processes. 

A set of speed-related references are listed below. 

Each of these references provides information, direction or guidance on a particular facet of a 

complex subject. None provides an overall or unifying approach to managing speed.
32 
33

CHAPTER 7. MANAGING TRAFFIC SPEED

Traffic speeds involve a complex set of interactions between engineering, legal and driver 

performance factors. Currently, knowledge of speed behavior is limited. Although the research 

literature contains a variety of speed prediction models for rural two-lane highways and lowspeed urban streets, accurate speed prediction models for other road and street types is limited. 

As such, the ability to accurately predict speeds on all road and street types does not exist. 

Similarly, there is no reliable guidance on how to attain specific operating speed characteristics 

(e.g., mean, 85th percentile, speed deviation) and speed relationships (e.g., between 85th

percentile and design speeds) during the geometric design process. Until this type of information 

is implemented, safety can be improved through strategies that result in better geometric 

designs and infrastructure conditions, more credible and effective speed control and targeted 

enforcement. Speed management is a strategy for controlling speed through a comprehensive, 

interdisciplinary and coordinated approach that encompasses behavioral, enforcement and 

engineering elements.

IMPROVING THE DESIGN PROCESS

Once constructed, transportation infrastructure is enduring. Roads and streets are public 

investments that establish spatial arrangements for community development and economic 

activity. Alterations may be costly and disruptive. Since the consequences of geometric 

design are significant and long-lasting, decisions should be deliberate. Geometric design is 

one potential influence on traffic speeds. Comprehensive and reliable speed prediction models 

do not exist for all road and street types. However, some information has been developed on 

geometry-speed relationships. The relationships shown in figure 18 were developed from data 

for horizontal curves on rural highways. Studies of similar scopes have reached similar findings. 

A reasonable conclusion from these studies is that drivers will operate as close to their preferred 

speed as possible, which may be below or above the inferred design speed. It is also reasonable 

to deduce that restrictive geometric features, especially tight horizontal curves, will tend to 

restrain operating speeds albeit not to the level of the inferred design speed. 

The Green Book recommends that anticipated operating speed be considered in designating the 

design speed. The strong influence of driver desire and expectations on operating speed should 

be recognized in that determination. Expectations are formed, in part, on the function of the 

facility within the network.

Improving design consistency is another area of speed-related improvement. Desirably, specific 

features and locations along a travel route should not require unexpected speed reductions. 

Isolated speed-restrictive features are likely to violate a driver’s expectation that has developed 

from conditions encountered previously. Developing designs that accommodate a fairly uniform 

speed over an extended distance are preferable to those that will result in substantial speed 

fluctuations. Relative to driver expectancy, geometric features should transition from a higher 

to lower design speed gradually. As an example, long tangents (with an infinite inferred design 

speed) between tight curves should be avoided. 
34 

The Interactive Highway Safety Design Model (IHSDM) is a suite of software packages being 

developed by the FHWA. The IHSDM is planned as an integrated system of modules for 

evaluating the operational and safety effects of highway geometric design alternatives. The 

IHSDM Design Consistency Module evaluates the consistency of a design relative to various 

speed measures.

Figure 18. Mean and 85th percentile speeds versus inferred design speed on horizontal 

curves. 

SETTING AND ENFORCING RATIONAL SPEED LIMITS

The management of speed through appropriate speed limits is an essential element of 

highway safety. Appropriate speed limits are a prerequisite for effective and sustainable speed 

management. Speed limits should reflect the maximum reasonable speed for normal conditions. 

Speed limits should be accepted as reasonable by most drivers. Not all drivers will conform to 

reasonable speed limits. In essence, speed limits separate high-risk and reasonable behavior. If 

lower speed limits are desired, then engineering and other measures should be implemented that 

reduce speeds to a level that would support a lower limit.

NHTSA and FHWA jointly support efforts to demonstrate and evaluate an integrated “three 

E’s” (engineering, enforcement, education) approach to the management of speed and crash 

risk. Rational speed limits are established on the basis of an engineering study of prevailing 

speed and other factors such as pedestrian activity and crash history. The 85th percentile speed 

is typically used as a starting point for setting a rational limit but it may be set as low as the 
35

average speed based on other factors, such as those listed previously in table 3. Once the speed 

limits are appropriately set and the judiciary informed, a program of strict enforcement with a 

low tolerance for speeds exceeding the limits is combined with public information and education 

explaining the purpose of the revised limits and the consequences for violators. Evaluation of 

program effectiveness is a critical element of the demonstrations. 

SPEED LIMIT SOFTWARE

With so many technical and legal factors involved, government agencies may need help in 

determining appropriate speed limits. This type of assistance is available through the web-based 

expert speed zoning software advisor USLIMITS. This software provides an objective and 

consistent process for incorporating the various factors that may be considered in the decision 

making process according to the MUTCD but for which no guidance is provided. The speed 

zone expert system was adapted from similar expert systems used by most Australian state road 

authorities but modified to reflect elements of speed setting philosophy used in the U.S. 

The expert system recommends a speed limit for a section of road based on road function, 

roadside development, operating speeds, road characteristics and other factors required to 

determine appropriate speed limits in speed zones. The system also warns users of issues that 

might require further investigation and engineering judgment. USLIMITS provides a screen 

report and a more detailed print report. USLIMITS will be of particular use to small communities 

and agencies that lack experienced traffic engineers. For experienced traffic engineers, it can 

provide a second opinion and increase confidence in speed zoning decisions. 

For further information on USLIMITS see http://www.uslimits.com

MITIGATION MEASURES

Speed limits are not the only tool that agencies can draw on to manage operating speeds. In 

fact, as discussed previously speed limits usually have a limited effect on operating speeds. 

Roadway geometry and the frequency of enforcement also play a role in driver judgments and 

choices regarding speed. A number of proven and promising speed management practices 

and technologies are available. The suitability of each as an element in an agency’s speed 

management and safety program should be evaluated based on the community, legal and 

transportation contexts. 

Advisory Speed Posting

Speed limits should not be lowered to reflect an isolated restrictive element. This practice 

tends to reduce the credibility of speed limits. When a speed lower than the speed limit is 

appropriate for a particular location, the use of an advisory speed plaque and associated traffic 

control devices should be considered. The MUTCD and other publications, such as state DOT 

procedures and traffic manuals, provide guidance on the use of Advisory Speed signs. 

In the Notice of Proposed Amendments (NPA) to the MUTCD, the FHWA proposes that advisory 

speeds be determined by engineering study. The NPA also indicates that the advisory speed 

should be determined based on free-flowing conditions and , because changes in conditions 
36 

(e.g. roadway geometrics and sight distance) might affect the advisory speed, each location 

should be periodically evaluated when conditions change.The MUTCD also provides guidance 

on advanced placement of signs prior to the horizontal curve as well as supplemental distance 

plaques where there is an area with continuous roadway curves. Figure 19 shows an example of 

a horizontal alignment change (i.e., curve) combined with advisory speed plaque. 

Figure 19. Horizontal curve warning sign with speed advisory plaque.

The NPA proposes the following among established engineering practices to determine the 

recommended advisory speed for a horizontal curve:

•	 An accelerometer that provides a direct determination of side friction factors.

•	 A design speed equation (see Equation 2, page 13).

•	 A ball-bank indicator using the following criteria:

o 16 degrees for speeds of 20 mph or less.

o 14 degrees for speeds of 25 and 30 mph.

o 12 degrees for speeds of 35 mph and higher.

The ball-bank indicator readings are similar to those contained in the AASHTO Green Book (see 

Table 4).

Two other methods of determining a curve advisory speed have been recently outlined in a 

Horizontal Curve Signing Handbook prepared by the Texas Transportation Institute (19). The 

two methods are referred to as the direction method and the compass method. Each is discussed 

briefly below.

The direct method is simple to apply, requiring minimal time and cost. The first step is to record 
37

125 or more free-flow passenger cars in each direction of travel and compute the mean speed 

separately for each direction. The speed measurements should be measured within the middle 

third of the curve. Free-flow vehicles are those with at least three seconds of space between 

leading and following vehicles. The second step is to multiply both average speeds from the first 

step by 0.97, which approximates the average truck-speed. The advisory speed for each direction 

is calculated by adding 1 mph to the average truck speed and then round down to the nearest 5 

mph increment, unless the value ends in a 4 or 9. If the value ends in a 4 or 9, then it should be 

rounded up to the next 5 mph increment. This value should be confirmed by driving the curve at 

the advisory speed.

The compass method requires field data, which are then used as input in a truck speed prediction 

model. The average truck speed from the model is recommended as the advisory speed. To 

be accurate, the method requires that the curve length be at least 200 feet long and the curve 

deflection angle should be at least 15 degrees. In the first step of the procedure, the analyst must 

measure the curve deflection in the direction of travel, heading at the 1/3 point of the curve, ballbank reading of curve superelevation at the 1/3 point of the curve, the length of curve between 

the 1/3 and 2/3 points of the curve, the heading at the 2/3 point of the curve, and the posted 

speed limit or 85th percentile speed on the tangents near the curve. A digital compass, distancemeasuring instrument, and ball-bank indicator are required to make the field measurements. In 

the second step of the procedure, the Texas Curve Advisory Speed (TCAS) software is used to 

compute a “rounded speed advisory.” This speed should be confirmed by driving the curve at the 

recommended advisory speed. 

The intent of the compass method is to improve the process for determining curve advisory 

speeds – the MUTCD indicates that advisory speeds may be based on the 85th-percentile speed 

of free-flowing traffic, the speed determined by an engineering study, or the speed corresponding 

to a 16-degree ball-bank indicator reading (16). A ball-bank indicator is normally attached to a 

vehicle such that its physical presence will not change while driving through a horizontal curve. 

The ball-bank indicator reading is a function of the lateral acceleration and roll rate of a vehicle. 

From this, a speed at which drivers will become uncomfortable while traversing a horizontal 

curve can be determined. The MUTCD ball-bank indicator method assumes a constant friction 

threshold for all speeds, which is not necessarily applicable to highway geometric design. The 

compass method accounts for variable side-friction thresholds for different speeds. 

Specific details concerning the direct and compass methods to set curve advisory speeds can 

be found in Bonneson, et al. (19), or at the following link: [http://tti.tamu.edu/documents/0-

5439-P1.pdf].

Improving Friction on Roadway Surfaces

Friction is needed to drive, brake and corner a vehicle. Forces are transmitted between the 

vehicle and road through friction at the tire-road interface. The characteristics of driving 

maneuvers (e.g., turning radius and speed) influence the frictional demand. The available

friction is a characteristic of roadway material and vehicle tire properties. It is possible but 

undesirable for the demand friction to exceed available friction. Think about a car trying to stop 
38 

on an ice-covered section of road. 

The available friction needed for both turning and stopping decreases with increasing speed. 

However, as vehicle speeds increase, friction demand increases. The maximum design values 

for friction adequately accommodate the design speed over a wide range of conditions, including 

poor conditions (i.e., wet/icy pavement, worn tires, smooth road). However, at some level of 

speed, the available friction will be exceeded. High speeds, alone or in combination with other 

factors, increases the probability that available friction will be exceeded by the demand. Adverse 

environmental conditions (e.g., wet weather) increase the frequency of this condition and one of 

its more common results, skidding. 

The need for friction is a major consideration in the selection and application of materials and 

treatments for all pavements. Many transportation agencies also have programs to increase 

skid-resistance at particularly vulnerable locations, such as curves and intersections. Examples 

of treatments used by agencies include: bituminous surface treatments, chip seals, grooving and 

microtexturing.

Bituminous surface treatments should include aggregate gradations that create voids in the 

material. This is intended to increase surface drainage as well as improve skid resistance. 

Agencies should first repair all major surface defects and then apply a bituminous tack coat. 

Finally, a bituminous surface treatment can then be overlaid onto the roadway surface. Figure 20 

is an example of a bituminous surface treatment. 

Figure 20. Example bituminous surface treatment.

Grooving the surface of a horizontal curve can be completed using a portable milling machine. 

An accepted technique is to use carbide-tipped flails to install grooves 3/16 to 3/8 inch wide and 

5/32 to 5/16 inch deep, with 8 grooves per foot on a random spacing (20). Longitudinal grooves 

have been shown to increase directional control of the vehicle while transverse grooves are most 

effective at locations where vehicles are making stops.

Microtexture is the small-scale roughness that is related to the fine aggregate in a mix (21). 

Microtexture relates to the size of small aggregate and the surface roughness of larger aggregate. 
39

As pavement wears and weathers over time, microtexture can be lost. This can be avoided 

by using high-quality, skid resistant aggregate. However, microtexture loss can be restored 

through pavement overlays, such as open-graded friction courses. Diamond grinding Portland 

Cement Concrete (PCC) increases the microtexture by dislodging sand particles in the mix. The 

longitudinal direction of grinding also increases directional stability of vehicles.

Chip sealing is the process of sealing an asphalt roadway surface with a polymer-modified 

asphaltic emulsion and crushed aggregate (22). The emulsion seals the cracks in the roadway 

surface, keeping moisture out of the surface and base. The crushed aggregate provides for 

increased friction as well as an all weather wearing surface. 

Speed Display Signs

Speed display signs measure the speed of approaching vehicles, typically with radar, and display 

the measured speed. LED (light emitting diodes) is a common display technology. The signs 

may be mounted on trailers to increase portability or on fixed support systems. An example 

dynamic speed display sign is shown below in figure 21. Many of the signs also display the 

applicable speed limit, creating a direct comparison for drivers. Speed display signs were 

originally used in connection with temporary conditions, such as works zones. More recently, 

agencies have deployed and begun to evaluate the effectiveness of these signs in reducing 

speeds at permanent, speed-sensitive locations. One study (23) investigated installations at 

three locations and found various levels of speed-reduction effectiveness. For example, a speed 

display sign installed at the entry of a school zone led to a 9-mph reduction in the average speed. 

Smaller reductions were found at other locations. 

Figure 21. Example dynamic speed display sign.

On the basis of this evaluation, the researchers offered the following insights on how various 

factors affect the effectiveness of speed display signs:
40 

•	 More effective if perception of regular enforcement (and threat of citation) exists at site.

•	 More effective if the sight distance to the treated condition is less than decision sight 

distance.

•	 More effective where only one lane exists per direction.

•	 More effective if used with other information “indicators” of a need to reduce speed 

(school speed limit beacons, signal change in area of speed display sign warning beacons, 

etc.).

•	 More effective if the speed display sign is used to support a regulatory speed limit (as 

opposed to an advisory speed limit).

•	 More effective if the overall information system at the location does not overwhelm the 

speed display sign. 

Speed display signs are available from a number of vendors.

Traffic Calming

Traffic calming is a term used to describe a set of techniques, consisting mostly of physical 

features, to affect vehicle operations on one or more streets to improve the street environment for 

other users (i.e., those not using motorized vehicles). Speed reduction is one of several traffic 

calming objectives. The specific traffic calming measures selected for application at a particular 

location should correspond with the unique conditions of the location and the objectives, since 

not every speed calming technique is appropriate for every roadway. Also, including such 

measures can result in drivers slowing at the speed calming feature, and speeding between them. 

Some of the measures that have been employed to reduce vehicle speeds include:

•	 Speed humps.

•	 Speed tables.

•	 Raised intersections.

•	 Traffic circles.

•	 Modern roundabouts.

•	 Chicanes.

•	 Lateral shifts.

•	 Realigned intersections.

•	 Lane narrowing.

As noted above, traffic calming encompasses many issues other than speed management. This 

short summary, which emphasizes speed considerations, should not serve as the basis for traffic 

calming decisions. The Institute of Transportation Engineers (ITE) and the FHWA have brought 

together the experiences and practices of many individuals and agencies in the informational 

report Traffic Calming: State of the Practice (24). The report is for informational purposes 
41

and does not recommend the preferred or appropriate treatment for a particular location or set 

of conditions. The report includes more detailed information on the installation and reported 

effectiveness of specific measures and devices.

Variable Speed Limits

Variable speed limits are speed limits that change based on road, traffic, or weather conditions. 

At a particular time and place, the applicable speed limit reflects some of the same factors a 

prudent driver also considers. Examples include the effects of reduced visibility and slick road 

conditions (which may increase the required stopping distance). Improving the consistency 

between a responsible driver’s speed selection and the speed limit may help to restore speed limit 

credibility and improve safety.

The experience base with variable speed limits in the U.S. is limited. Quite a few agencies use 

changeable message signs to display speeds that vary with prevailing conditions but nearly all 

of these systems display advisory speeds. The underlying algorithms and display technologies 

may be very well developed but the enforcement and legal sanctions that define speed limits are 

not present. The most familiar U.S. application of variable speed limits is in school zones, where 

the speed limit changes in relation to the school schedule. Although not common, states are 

beginning to experiment with systems that could lead to variable speed limits in work zones.

The movement toward variable speed limits in the U.S. is in its early development phase. The 

legal doctrine and enabling legislation will evolve through legislative action and judicial review 

and interpretation. 

Improve Sight Distance

AASHTO stopping sight distance values were revised in 2001 due to the different parameters 

of the changing vehicle fleet. Since the new recommended stopping sight distance values are 

greater than the old minimum stopping sight distance criteria (although less than the requirement 

for desirable stopping sight distance), this has resulted in many roadways falling outside the 

current range of criteria, even though the stopping sight distance design criteria was met at the 

time. It has been published that “moderate reductions in minimum sight distance do not appear 

to be a safety problem (9).” Stopping sight distance profiles can show a designer where sight 

restrictions occur, and the amount of sight distance that is actually available at that location. 

Figure 22 shows an example sight distance profile that was produced from the IHSDM. 

The first mitigation measure to increase available sight distance is to lengthen crest vertical 

curves. This measure, however, can be quite costly and can have significant impacts to adjacent 

land uses. If severe sight distance limitations exist, improvements can be made even if the 

minimum SSD is not met. Other improvements that can be made are to remove objects that are 

within the sight limited area as well as increase crash avoidance areas through lane widening or 

shoulder widening. 

The available sight distance along horizontal curves and at intersections can be improved by 

controlling vegetation. Vegetation can decrease the sight distance available to traffic control 
42 

devices as well as decrease sight distance on the inside of horizontal curves. Another mitigation 

measure for inadequate sight distance on an intersection leg is to install warning flashers on the 

approach leg. 

Figure 22. Example stopping sight distance profile from the IHSDM.
43

CHAPTER 8. SUMMARY

Following are some of the key points included in this Informational Guide:

•	 As speed increases, crash severity increases. The relationship between the probability of 

a crash and speed is less clear.

•	 The ability to accurately predict speeds on all road and street types does not exist nor is 

there reliable guidance available on how to attain specific operating speed characteristics 

and speed relationships in geometric design. 

•	 Speed limits are not always known during the design process. 

•	 The design process does not include a speed-prediction component. 

•	 Inferred design speeds are sometimes substantially higher than the designated design 

speed.

•	 After a facility is open to traffic, the 85th percentile speed may be higher than the anticipated and designated design speeds, particularly on low and moderate speed roads. 

•	 The posted speed may change after the road has been designed, built and open to traffic.

•	 The design speed is NOT the maximum safe speed. 

•	 Reducing the posted speed without other changes is likely to result in a small reduction in 

operating speeds (i.e., significantly less than the amount of the posted speed change).

•	 Posted speeds should almost always be within 5 mph of the 85th percentile speed.

•	 The posted speed should not be established based on an isolated restrictive feature (e.g., 

sharp curve or limited sight distance) within a segment. The use of an advisory speed 

should be considered at these locations.

•	 Speed management is a comprehensive approach to determining and attaining appropriate speeds through a coordinated effort of driver education, enforcement and engineering 

efforts. 

•	 Additional information is available from:

o The Green Book, MUTCD, and state driver’s manuals.

o The references listed in this Informational Guide.

o FHWA Speed Management web page (http://safety.fhwa.dot.gov/speedmgt/).

o Local Technical Assistance Program (LTAP) and technology transfer centers.
44 
45

REFERENCES

1. American Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets, Washington, DC, 2004.

2. Bowie, N. N., Jr. and M. Walz, “Data Analysis of the Speed-Related Crash Issue,” Auto 

and Traffic Safety, Vol. 2, Winter 1994.

3. U.K. Department of Transportation, Killing Speed and Saving Lives, London, 1987.

4. Davis, G. A., “A Simple Threshold Model Relating Pedestrian Injury Severity to Impact 

Speed in Vehicle/Pedestrian Crashes,” Transportation Research Record 1773, Transportation Research Board, Washington, D.C. 2001, pp. 108-113.

5. Fitzpatrick, K., P. Carlson, M. Brewer, M. Wooldridge, and S.P. Miaou, “Design Speed, 

Operating Speed, and Posted Speed Practices,” NCHRP Report 504, Transportation Research Board, National Research Council, Washington, D.C. 2003.

6. Barnett, J., “Safe Side Friction Factors and Superelevation Design,” Highway Research 

Board, Proc., Vol. 16, 1936. pp. 69-80.

7. Cron, F.W. “Highway Design for Motor Vehicles – A Historical Review. Part 6: Development of a Rational System of Geometrical Design,” Public Roads, Vol. 40, No. 1, 1976. 

pp. 9-18.

8. American Association of State Highway Officials, A Policy on Design of Rural Highways, 

Washington, DC, 1954.

9. Fambro, D., K. Fitzpatrick, and R. Koppa, “Determination of Stopping Sight Distances,” 

NCHRP Report 400, Transportation Research Board, National Research Council, Washington, D.C. 1997.

10. Dewar, R. and P. Olson, “Human Factors in Traffic Safety,” Lawyers & Judges Publishing 

Company, Inc., Tucson, AZ, 2002.

11. Lu, J. “Vehicle Traction Performance on Snowy and Icy Surfaces,” Transportation Research Record 1536, Transportation Research Board, Washington, D.C. 1996, pp. 82-89.

12. Harwood, D., D. Torbic, K. Richard, W. Glauz, and L. Elefteriadou, “Review of Truck 

Characteristics as Factors in Roadway Design.” Transportation Research Board, National 

Research Council, Washington, D.C. 2003.

13. Krammes, R.A., and S. W. Glascock, S.W., “Geometric Inconsistencies and Accident Experience on Two-Lane Rural Highways,” Transportation Research Record 1356, Transportation Research Board, Washington, D.C. 1992, pp. 1-10.
46 

14. Krammes, R. A., R. Q. Brackett, M. A. Shafer, J. L. Ottesen, I. B. Anderson, K. L. Fink, 

K. M. Collins, O. J. Pendleton, and C. J. Messer. Horizontal Alignment Design Consistency for Rural Two-lane Highways. Report No. FHWA-RD-94-034, Washington, DC, 

1995.

15. Collins, K. M. and R. A. Krammes, “Preliminary Validation of a Speed-Profile Model for 

Design Consistency Evaluation,” Transportation Research Record 1523, Transportation 

Research Board, Washington, D.C. 1996, pp. 11-21.

16. Manual on Uniform Traffic Control Devices for Streets and Highways. Federal Highway 

Administration, U.S. Department of Transportation, Washington, DC, 2009.

17. Parker, M. R., Jr. Synthesis of Speed Zoning Practices, Report No. FHWA/RD-85/096, 

Federal Highway Administration, Washington, D.C., 1985.

18. Fitzpatrick, K., L. Elefteriadou, D. Harwood, J. Collins, J. McFadden, I. Anderson, R. 

Krammes, N. Irizarry, K. Parma, K. Bauer and K. Passetti. Speed Prediction for TwoLane Rural Highways. Federal Highway Administration Report FHWA-RD-99-171, 

Washington, D.C., 2000.

19. Bonneson, J., M. Pratt, J. Miles, and P. Carlson, Horizontal Curve Signing Handbook, 

Report No. FHWA/TX-07/0-5439-P1, Available at: [http://tti.tamu.edu/documents/0-

5439-P1.pdf], August 2007.

20. McGee, H.W. and F. Hanscom, Low-Cost Treatments for Horizontal Curve Safety, Report 

No. FHWA-SA-07-002, Washington, DC, December 2006.

21. Kuennen, T., “Creating Friction Where Rubber Meets the Road,” Better Roads, Volume 

73, Number 10. 2003.

22. Davis, L. “Chip Sealing over Fabric in Borrego Springs, California,” Transportation 

Research Circular Number E-C078, October 2005. 

23. Rose, E. R. and G. L. Ullman, Report 0-4475-1, “Evaluation of Dynamic Speed Display 

Signs,” Texas Transportation Institute, College Station, TX, 2003.

24. Traffic Calming: State of the Practice, Institute of Transportation Engineers and Federal 

Highway Administration, August 1999 (accessed May 2007 http://www.ite.org/traffic/

tcstate.htm#tcsop).

25. Mason, J. M. and K. M. Mahoney. “Design Exception Practices.” NCHRP Synthesis 

316, Transportation Research Board, Washington, DC, 2003.
47

 APPENDIX: CALCULATING INFERRED DESIGN SPEED FROM 

HORIZONTAL AND VERTICAL CURVATURE

HORIZONTAL CURVATURE

Given:

Designated Design Speed = 60 mph

Radius = 716.20 ft

Superelevation Rate = 6.6 %

e(max) = 8 %

Determine the inferred design speed based on Green Book criterion for horizontal curvature 

based on a method 2 distribution. The method 2 distribution assumes that all lateral acceleration 

is first used by the friction and the rest is distributed to the superelevation rate using the 

following equation:

where:

e = rate of superelevation, percent;

f = side friction (demand factor);

V = vehicle speed, mph; 

R = radius of curve, ft.

Rearranging the equation to solve for the inferred design speed yields:

An iterative process is used to determine the inferred design speed using the maximum friction 

values, maximum superelevation, and design speed from Exhibit 3-15 of the Green Book (1).

Since the maximum superelevation is not used, it is obvious that a radius that exceeds the 

calculated minimum will be necessary. Thus, beginning with the existing radius, existing 

superelevation, and a design speed of 50 mph, determine if the maximum friction of 0.14 is 

exceeded as follow.

f = 0.1667, which exceeds the maximum friction value of 0.14.
48 

The next iteration is to try a 49 mph design speed with a maximum friction value of 0.142

f = 0.157, which exceeds the maximum friction value of 0.142

The next iteration is to try a 48 mph design speed with a maximum friction value of 0.144

f = 0.148, which exceeds the maximum friction value of 0.144

The next iteration is try a 47 mph design speed with a maximum friction value of 0.146

f = 0.140, which does not exceed the maximum friction value of 0.146

From this point, it can be stated that the maximum one mile-per-hour increment of speed that the 

existing features meet is a 47 mph design speed. Therefore, the inferred design speed for this 

horizontal curve is 47 mph. 

An assumption that is used in this calculation is that available friction for design speed values is 

linear between speed increments. This assumption is true for design speeds that meet and exceed 

40 mph. 

VERTICAL CURVATURE

Method 1

Given: 

A crest vertical curve has the following attributes:

 g1 = +2.6%

 g2 = -3.5%

Length = 800 ft
49

Determine the inferred design speed based on Green Book stopping sight distance criterion for a 

vehicle traveling from left to right. 

Using terminology and criteria in the 2004 Green Book, the following are determined:

A = | -3.5 - 2.6 | = 6.1 %

Using the Green Book recommended values for driver eye height and object height and a given L

(vertical curve length) and A (algebraic difference in grades), the reduced form equations below 

are used to determine S (sight distance):

L = _AS2

_ (for S < L)

 2158

L = 2S – 2158 (for S > L)

 A

The first equation controls (S < L) and S = 532 ft.

The minimum stopping sight distance criteria are provided in the Green Book for design speeds 

in 5 mph increments. The value of 532 ft falls between the values associated with 55 mph (495 

ft) and 60 mph (570 ft). The highest even 5-mph increment for which stopping sight distance is 

met is 55 mph. Interpolating leads to an estimate of 57.5 mph. A more precise estimate can be 

obtained by using the basic sight distance formula (2004 Green Book Equation 3-2) to compute 

the corresponding speed. This approach yields an estimate of 57.7 mph. Grades can also affect 

stopping distance but no adjustment is appropriate in this case. Keeping in mind the various 

assumptions and approximations, an estimate rounded to the nearest one mph is recommended, 

58 mph in this example.

Method 2

The inferred design speed can also be calculated for vertical curves if the available sight distance 

is known. The available sight distance can be measured through field surveys, as-built plan and 

profile sheets, and through existing software packages such as IHSDM.

The following equation can be used to determine the inferred design speed with the available 

sight distance being known:

where:

SSD = Available stopping sight distance

 V = Design speed (or inferred design speed)

 t = perception-reaction time (assumed to be 2.5 seconds)

 a = deceleration rate (assumed to be 11.2 ft/sec2

)
50 

Given:

Crest vertical curve

Minimum SSD = 485 ft

Having put the stopping sight distance equation into a spreadsheet file, the required stopping 

sight distance for 0.5 mph increments can be shown as follows.

Speed 

(mph)

Stopping Sight Distance 

(ft)

45 359.739

45.5 365.920

46 372.148

46.5 378.425

47 384.750

47.5 391.122

48 397.543

48.5 404.011

49 410.528

49.5 417.093

50 423.705

50.5 430.366

51 437.075

51.5 443.831

52 450.636

52.5 457.488

53 464.389

53.5 471.337

54 478.334

54.5 485.378

55 492.471

55.5 499.611

56 506.800

56.5 514.036

57 521.321

57.5 528.653

58 536.034

58.5 543.462

59 550.939

59.5 558.463

60 566.036

As can be seen from this table, the available stopping sight distance does not quite meet the requirements for a 54.5 mph design speed. Rounding to the nearest mph, the inferred design speed 

for this curve is 54 mph. 

This same method can be applied to horizontal curves that have a sight obstruction on the inside 

of the curve. When the available sight distance is calculated or found from field survey, the same 

equation can be applied for determining the inferred design speed.
51