December 2006

Low-Cost Treatments for

Horizontal Curve Safety

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NOTICE

This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no 

liability for the use of 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.

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Technical Documentation Page

1. Report No.

FHWA-SA-07-002

2. Government 

Accession No.

3. Recipient’s Catalog No.

 

4. Title and Subtitle

Low-Cost Treatments for Horizontal Curve Safety

5. Report Date

December 2006

6. Performing Organization 

Code

7. Author(s) 

McGee, Hugh W. and Hanscom, Fred R.

8. Performing Organization 

Report No.

9. Performing Organization Name and Address. 

Vanasse Hangen Brustlin, Inc (VHB) Transportation Research Corporation

8300 Boone Blvd., Ste. 700 11671 Audubon Trail

Vienna, VA 22182-2626 Markham, VA 22643 

 

10. Work Unit No.

11. Contract or Grant No.

DTFH61-05-D-00024 

12. Sponsoring Agency Name and Address

U.S. Department of Transportation

Federal Highway Administration (FHWA)

Office of Safety 

400 Seventh Street, SW, HSSD

Washington, DC 20590

13. Type of Report and Period 

Covered 

September 2005- December 

2006

14. Sponsoring Agency Code

FHWA/HSSD

15. Supplementary Notes. 

Contracting Officer’s Technical Representative: Larry J. Brown

Contracting Officer’s Task Order Manager: Debra Chappell

Technical editing was performed Vicki Glenn and production was performed by Michelle Scism, both from VHB.

16. Abstract. Nearly 25 percent of fatal crashes occur at or near a horizontal curve. Hence, addressing the safety 

problem at horizontal curves is one of the 22 emphasis areas of the Strategic Highway Safety Plan prepared by 

AASHTO. Also, crashes at horizontal curves are a big component of the road departure crash problem, which is one 

of FHWA’s three focus areas. This publication was prepared to provide practical information on low-cost treatments 

that can be applied at horizontal curves to address identified or potential safety problems. The publication concisely 

describes the treatment; shows examples; suggests when the treatment might be applicable; provides design features; 

and where available, provides information on the potential safety effectiveness and costs. The treatments include:

•	 Basic traffic signs and markings found in the MUTCD

•	 Enhanced traffic control devices

•	 Additional traffic control devices not found in the MUTCD

•	 Rumble strips

•	 Minor roadway improvements

•	 Innovative and experimental treatments

The publication concludes with a description of maintenance activities that should be conducted to keep the treatments 

effective.

17. Key Words. Horizontal curve, safety treatments, traffic 

control devices, pavement skid resistance, shoulders, rumble 

strips

18. Distribution Statement

No restrictions. 

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of 

this page)

Unclassified

21. No. of 

Pages 60 

22. Price

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

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TABLE OF CONTENTS

CHAPTER 1. INTRODUCTION..................................................................................................... 1

SAFETY PROBLEMS AT HORIZONTAL CURVES............................................................................. 1

PUBLICATION PURPOSE AND SCOPE............................................................................................... 2

ABOUT THE MUTCD............................................................................................................................ 2

INFORMATION IN THIS Publication............................................................................................ 3

Publication ORGANIZATION......................................................................................................... 4

CHAPTER 2. BASIC TREATMENTS FOR HORIZONTAL CURVES...................................... 5

CENTERLINE.......................................................................................................................................... 6

EDGE LINE............................................................................................................................................. 7

HORIZONTAL ALIGNMENT SIGNS.................................................................................................... 9

ADVISORY SPEED PLAQUE................................................................................................................ 11

ONE-DIRECTION LARGE ARROW SIGN........................................................................................... 12

COMBINATION HORIZONTAL ALIGNMENT / ADVISORY SPEED SIGN..................................... 13

CURVE SPEED SIGN............................................................................................................................. 13

CHEVRON ALIGNMENT SIGN............................................................................................................ 14

DELINEATORS....................................................................................................................................... 17

CHAPTER 3. ENHANCED BASIC TREATMENTS..................................................................... 19

LARGER DEVICES................................................................................................................................ 19

DOUBLING-UP OF DEVICES............................................................................................................... 20

HIGH RETROREFLECTIVE INTENSITY AND FLUORESCENT YELLOW SHEETING................ 20

FLASHING BEACONS .......................................................................................................................... 21

PROFILE THERMOPLASTIC MARKINGS.......................................................................................... 21

RAISED PAVEMENT MARKERS.......................................................................................................... 22

CHAPTER 4. OTHER TRAFFIC CONTROL DEVICE TREATMENTS.................................. 25

REFLECTIVE BARRIER DELINEATION............................................................................................ 25

ROADSIDE OBJECT DELINEATION................................................................................................... 27

DYNAMIC CURVE WARNING SYSTEM............................................................................................. 28

SPEED LIMIT ADVISORY MARKING IN LANE................................................................................ 31

CHAPTER 5. RUMBLE STRIPS..................................................................................................... 33

DESCRIPTION........................................................................................................................................ 33

CENTERLINE RUMBLE STRIP............................................................................................................ 34

SHOULDER RUMBLE STRIP................................................................................................................ 35

ROADWAY RUMBLE STRIP................................................................................................................. 37

CHAPTER 6. MINOR ROADWAY IMPROVEMENTS................................................................ 41

PAVED SHOULDER TREATMENT....................................................................................................... 41

SHOULDER DROP-OFF ELIMINATION.............................................................................................. 42

SHOULDER WIDENING........................................................................................................................ 44

SKID RESISTIVE PAVEMENT SURFACE TREATMENT.................................................................... 45

CHAPTER 7. INNOVATIVE AND EXPERIMENTAL TREATMENTS..................................... 49

OPTICAL SPEED BARS......................................................................................................................... 49

PENNDOT CURVE ADVANCE MARKING.......................................................................................... 51

CHAPTER 8. MAINTENANCE TREATMENTS........................................................................... 55

REFERENCES................................................................................................................................... 57

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1 Office of Safety

Chapter 1. Introduction

CHAPTER 1. INTRODUCTION

SAFETY PROBLEMS AT HORIZONTAL CURVES

In 1998, the American Association of State Highway and Transportation Officials 

(AASHTO) approved its Strategic Highway Safety Plan, which sets a goal of reducing 

annual highway fatalities by 5,000 to 7,000. To help implement the plan, the National 

Cooperative Highway Research Program (NCHRP) developed a series of guides State and 

local agencies can use to identify ways to reduce injuries and fatalities in targeted areas. 

One target or emphasis area is the problem of crashes at horizontal curves. 

A Guide for Reducing Collisions on Horizontal Curves, which is 

referred to throughout this publication as the Guide, illustrates the 

problem. The Guide reports that nearly 25 percent of people who 

die each year on the Nation’s roadways are killed in vehicle crashes 

at curves. About 75 percent of all fatal crashes occur in rural areas, 

and more than 70 percent are on two-lane secondary highways, many 

of which are local roads. Furthermore, the average crash rate for 

horizontal curves is about three times that of other highway segments. 

And, 76 percent of the curve-related fatal crashes involve single 

vehicles leaving the roadway and striking trees, utility poles, rocks, 

or other fixed objects or overturning. Another 11 percent are head-on 

crashes, the result of one vehicle drifting into the opposing lane when 

a driver tries to cut the curve or redirect the vehicle after having run 

onto the shoulder.

It is because of these dramatic statistics that the Federal Highway Administration (FHWA) 

has identified Roadway Departure as one of its three program emphasis areas—the other 

two are Intersection Safety and Pedestrian Safety. One aspect of the Roadway Departure 

initiative is to develop a series of practical information publications designed for local road 

agencies. This publication is a result of, and supports Roadway Departure program goals.

Publication PURPOSE AND SCOPE

The Guide identified 20 strategies as alternative countermeasures—or treatments—to 

address the specific safety problem at horizontal curves. These strategies share one of two 

objectives: 

Reduce the likelihood of a vehicle leaving its lane and either crossing the 

roadway centerline or leaving the roadway at a horizontal curve.

1 Key references are italicized and listed at the end of the publication; an internet link to the reference is 

provided if known.

1.

One of several guides 

that provide strategies to 

address safety problems.

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Federal Highway Administration 2

Low-Cost Treatments for Horizontal Curve Safety

Minimize the damaging consequences of a vehicle leaving the roadway at a 

horizontal curve. 

Although the Guide provides information about each strategy, transportation professionals 

felt that a document providing practical information on where, when, or how to apply a 

safety treatment or design feature—a resource that includes cost and examples—would be 

useful to local road agencies. This publication was prepared for this purpose. 

There are numerous strategies or treatments agencies can apply to a single horizontal curve 

or a winding road section to address a safety problem. This publication includes only those 

engineering treatments that are relatively low cost, as compared to reconstructing the curve 

or road section to improve the geometric design features, such as degree and length of 

curve, superelevation, cross section, and shoulders. 

The information presented here is concise. To fully cover all the aspects of an individual 

treatment would require a much larger document that would likely be used less. Rather, 

this publication provides information specifically relating to local roads and the agencies 

that manage them. It will help transportation agencies and their crews understand the 

alternative treatments and how to select and apply them. Where appropriate, and when 

information was available, this publication provides the following for each treatment:

Description—what it is.

Application Guideline—when to install.

Design—what design elements or materials to use.

Effectiveness—how a treatment can improve safety.

Cost—what it will cost.

Additional sources and contacts.

ABOUT THE MUTCD

Throughout this publication, you will see references to the 

MUTCD. Shorthand for the Manual on Uniform Traffic Control 

Devices, the MUTCD defines the standards for all traffic control 

devices (signs, signals, and pavement markings) road managers 

install and maintain to help regulate, warn, and guide drivers 

safely on the Nation’s roadways and streets. The MUTCD is 

published by the FHWA. All States are required to adopt either 

the Federal MUTCD or a State MUTCD that is in substantial 

conformance to the Federal MUTCD. Some States adopt the 

Federal MUTCD with a State Supplement. State laws regarding 

traffic control devices should be consulted.

2.

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MUTCD provides 

standards and guidance 

for application of traffic 

control devices.

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3 Office of Safety

Chapter 1. Introduction

The MUTCD also defines conditions about what, where, and how a device is to be 

placed or installed. In different chapters of this publication you may see a treatment 

and the designation that the MUTCD states it shall be used. Shall means something is 

a standard—a practice or device that is specifically required or mandated­—or explicitly 

prohibited. The MUTCD may designate other treatments as guidance, which tells the road 

manager that a practice or device is recommended and should be used in typical situations, 

with modifications allowed for a specific location if an engineering study or engineering 

judgment indicates the deviation to be appropriate. Finally, the MUTCD provides for 

options, which are presented as may statements. 

To learn more about the MUTCD, visit http://mutcd.fhwa.dot.gov. The site is very easy to 

use and the Frequently Asked Questions (FAQ) section is very helpful.

INFORMATION IN THIS Publication

First, a few comments about the publication’s contents:

The treatments discussed are intended to improve the safety of a single curve 

or a winding section. It is assumed that the agency has identified the location 

as an existing or potential safety problem. All transportation agencies should 

have a program for identifying such locations. If it does not, the Guide can help 

agencies develop such a program.

Some traffic control devices or applications described in this publication do not 

comply with the MUTCD and are considered “experimental.” Any road agency 

wanting to use a noncompliant device on a public road must request and receive 

FHWA approval for testing. The MUTCD refers to this as experimentation. 

MUTCD Section 1A.10 outlines the procedure for experimentation.

Where evaluation information is available, the publication includes estimates of 

the effectiveness of the treatment in reducing crashes. However, agencies should 

not expect to obtain these crash reduction values at a specific location, as the 

actual observed effectiveness of a treatment will vary from site to site.

Several treatments discussed in this publication are signs or other devices placed 

on supports or posts, which makes them a hazard. The MUTCD states that 

roadside sign supports in the clear zone shall be breakaway, yielding, or shielded 

with a longitudinal barrier or crash cushion. For information on breakaway sign 

supports and the definition of clear zone at http://safety.fhwa.dot.gov/roadway_

dept/road_hardware/signsupports.htm.

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Federal Highway Administration 4

Low-Cost Treatments for Horizontal Curve Safety

Publication ORGANIZATION

The balance of this publication organizes and presents information in the following 

chapters:

Chapter 2 - Basic Treatments—Devices found in the MUTCD.

Chapter 3 - Enhanced Basic Treatments—Devices considered enhancements to the 

 basic treatments.

Chapter 4 - Other Traffic Control Device Treatments—Treatments not specifically 

 mentioned in the MUTCD, but which are currently used by State and 

 local agencies.

Chapter 5 - Rumble Strips—Overview of rumble strip applications with 

 centerlines, edge lines, shoulders and transversely across the lane.

Chapter 6 - Minor Roadway Improvements—Relatively low-cost treatments that 

 require minor improvements to the roadway or shoulder.

Chapter 7 - Innovative and Experimental Devices—New and innovative devices 

 and other treatments that have not been applied extensively.

Chapter 8 - Maintenance—Simple, effective maintenance practices.

The FHWA encourages readers to use the information presented in this publication 

to evaluate problems and identify appropriate treatments for problem curve sections. 

Applying these treatments should help agencies reduce roadway departure crashes and 

resulting injuries and fatalities. The FHWA also welcomes feedback on experiences with 

these or other treatments agencies use to solve a safety problem at a horizontal curve. Send 

us comments and treatment results through the Office of Safety website at 

http://safety.fhwa.dot.gov.

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5 Office of Safety

Chapter 2. Basic Treatments for Horizontal Curves

CHAPTER 2. BASIC TREATMENTS 

FOR HORIZONTAL CURVES

There are several traffic control devices road agencies can and, in many situations, should 

consider installing at a horizontal curve, especially curves that data or experience identify 

as a safety problem location. These devices are considered “basic” treatments that are 

found in the MUTCD. They include:

Centerline 

Edge line

Horizontal Alignment signs: Turn (W1-1), Curve (W1-2), Reverse Turn (W1-

3), Reverse Curve (W1-4), Winding Road (W1-5), Hairpin Curve (W1-11), or 

Loop (W1-15) as an advance warning sign depending on the geometry of the 

curve(s) 

Advisory Speed Plaque (W13-1) (with any of the Horizontal Alignment signs)

One-Direction Large Arrow (W1-6) sign

Combination Horizontal Alignment/Advisory Speed (W1-1a or W1-2a) sign

Curve Speed (W13-5) sign

Chevron Alignment (W1-8) sign

Delineators

Agencies should base selection of one or more of the devices on an engineering study or 

engineering judgment. Factors to consider include:

The difference in the posted speed limit and the 85th percentile speed (or a 16-

degree ball-bank reading2).

Geometric features of the curve to include its length, radius, shoulders and 

roadside features.

Sight distance to and around the curve.

Unexpected geometric features within the curve, such as an intersection or 

change in curve radius.

Traffic volume. 

Many curves require nothing more than the standard centerline and edge line (if paved) 

or one of the basic horizontal alignment warning signs. The decision to add one or more 

of the other devices listed will be influenced by the factors noted above. Additional 

consideration is necessary when the curve has been identified as a safety concern or 

recognized safety problem. The Guide mentioned in the previous chapter presents 

additional guidance on identifying safety problems.

The following discussion provides guidelines for using each device. All example signs and 

markings are from the MUTCD.

2 See Traffic Control Devices Handbook for use of a ball bank indicator.

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6

Low-Cost Treatments for Horizontal Curve Safety

Federal Highway Administration 

CENTERLINE

The centerline pavement marking is the minimal treatment to apply to a curve section, 

assuming the road is paved and has sufficient width and volume, as noted in the table 

below. A marked centerline helps drivers keep their vehicles on the correct side of the 

road and further delineates the roadway alignment. For any section of two-way, two-lane 

roadway, where passing is allowed in both directions, the basic centerline marking is a 

broken (dashed) yellow line. On some curves, the horizontal curvature, vertical curvature, 

or other conditions reduce the passing sight distance for 

one or both directions of travel below the minimum values 

given in Part 3 of the MUTCD. Use a solid yellow line to 

advise motorists of the no-passing regulation where the 

restriction exists for each direction of travel. For segments 

where passing is prohibited in both directions, use a solid 

yellow line for both directions, which provides a double 

yellow centerline. The MUTCD permits use of a centerline 

to specific locations, such as around a curve, so it need not 

be for the entire roadway section, unless a centerline is 

required for the entire roadway. 

Application Guideline

The table below summarizes MUTCD requirements for centerline marking of two-way 

roads.

MUTCD Requirements for Centerline Markings on Paved Two-Way Streets.

Area type Road Class Lanes ADT Travel 

Width (ft)

REQUIRED

Urban Collectors Arterials 2 6000 + 20+

All All 3+

RECOMMENDED

Urban Collectors Arterials 2 4000+ 20+

Rural Collectors Arterials 2 3000+ 18+

MAY CONSIDER Any Any 2 Any 16+

The MUTCD also states that “Engineering judgment should be used in determining whether 

to place centerline markings on travel ways less than 16 ft wide because of the potential for 

traffic encroaching on the pavement edges, traffic being affected by parked vehicles, and 

traffic encroaching into the opposing lane.” Therefore, when an agency identifies a curved 

section as a potential safety problem, and the road segment does not have a centerline, this 

should be the first, minimal treatment applied. When the curve carries a low traffic volume 

(fewer than 200 vehicles per day) and/or the pavement is less than 16 ft wide, consider 

Centerline for No Passing on 

horizontal curve.

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7 Office of Safety

Chapter 2. Basic Treatments for Horizontal Curves

using post delineators, chevrons, or curve warning signs, even though the centerline is not 

deemed appropriate. 

Centerline Materials

Road agencies commonly use a variety of paint-based materials and thermoplastic for the 

centerline markings. The specific material to be used depends on what an agency normally 

uses for its pavement marking applications. In general, thermoplastic markings are more 

cost-effective as they last much longer than paint materials. However, their higher initial 

costs may rule out their use on low-volume rural roads.

Other materials agencies can use for centerlines include a variety of raised pavement 

markers and “profile” thermoplastics. Chapter 3 discusses these supplemental devices. 

Also, chapter 5 discusses the use of rumble strips as a supplement to centerline pavement 

markings.

Centerline Width

The standard width for each stripe of a centerline stripe is 4 in to 6 in, with the 4-in line 

more common. A wide line is simply twice the width of a standard line. There is no 

known safety benefit to having a wider centerline other than it provides a larger visual 

marking for motorists. When a double line is called for, such as when passing is not 

allowed, the normal practice in most jurisdictions is to separate the two lines by a measure 

approximately equal to the width of a single line—4 in to 6 in. 

Effectiveness

A 1996 Kentucky study estimated that including centerline markings reduced crashes by 

35 percent. However, this study considered entire roadway sections, not individual curves. 

Other studies report much smaller benefits. These varying results suggest that agencies 

can expect safety benefits from installing centerline markings but that the magnitude of the 

benefits will vary based upon roadway and traffic characteristics.

Cost

The cost to apply pavement markings varies, depending on several factors. For example, 

the material used—paint or thermoplastic; does the agency’s own crew or an outside 

contractor perform the work; and is a single curve or several locations to be treated? 

Individual agencies are aware of their costs for applying pavement markings.

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8

Low-Cost Treatments for Horizontal Curve Safety

Federal Highway Administration 

EDGE LINE

Edge line pavement markings define or delineate the edge of a roadway. It provides a 

visual reference to guide motorists and helps prevent them from drifting onto the shoulder 

and roadside area. For horizontal curves, regardless 

of the road type, edge line markings are a solid 

white line at the right edge of the travel lane (i.e., 

not including any shoulder). With the centerline, or 

adjacent lane line for a multilane road, it defines the 

travel lane for the road user. As with the centerline, 

edge lines can be applied just prior to and within the 

curved section, although it typically is applied to an 

entire section of roadway when used.

Application Guidelines

The MUTCD requires (standard) edge lines only for “. . . rural arterials with a traveled 

way of 20 ft or more in width and an average daily traffic (ADT) of 6,000 vehicles per day 

or greater.” They are recommended (guidance) for “. . . rural arterials and collectors 

with a traveled way of 20 ft or more in width and an ADT of 3,000 vehicles per day” or any 

other paved road where an engineering study identifies a need for edge line markings. 

It is not necessary to have a centerline on the road to add an edge line, but be cautious. On 

a narrow two-way road, white lines applied on both edges, without a yellow centerline 

marking, may signal to the motorist that the road is one-way. 

Edge Line Materials

The same materials discussed for the centerlines apply to edge lines. The exception is 

raised pavement markers (RPMs), which the MUTCD prohibits either as a substitute or 

supplement to the edge line. The rationale is that under wet night conditions when only 

the RPMs are visible, edge line RPMs can confuse drivers who could misinterpret them as 

marking the lane line. Also, RPMs on an edge line can cause bicyclists using the shoulder 

to lose control. Some States have used the profile marking design, which is discussed in 

the next chapter. 

Edge Line Width

As with other longitudinal pavement markings, the standard edge line width is 4 in to 

6 in, with the 4-in line used most. Six-inch edge lines are common on freeways and can be 

found on some lower class roads as well. Using a “wide” edge line (8 in to 12 in) on just 

the curved section, while not common, has been used to “emphasize” the curve section and 

provide a visually stronger guide for the motorist. However, do not use a wide edge line 

Centerline and edge line for two-lane road.

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9 Office of Safety

Chapter 2. Basic Treatments for Horizontal Curves

on roadways with a pavement width less than 20 ft, as it could cause motorists to move too 

far left into the opposing lane.

Roadway with 4-in edge line Roadway with 8-in edge line

A wide (8-in) edge line provides a stronger visual guide.

Effectiveness

A New York study found that standard edge lines on curvy two-lane roads reduced total 

crashes by 5 percent and fixed object crashes by 17 percent. Also, the Low Cost Local 

Road Safety Solutions report documents the safety benefit of adding an edge line and 

provides additional information on low-cost strategies for curves and other roadway 

locations. While these evaluations were conducted on sections of roads, agencies should 

assume that a safety benefit would be realized for individual curves. As for wider edge 

lines, the same report cites several studies citing a safety benefit from wider edge lines. 

The report also cites motorists’ preference, especially elderly drivers.

Cost

Comments regarding the cost of centerlines apply equally to edge 

line treatment.

HORIZONTAL ALIGNMENT SIGNS

The MUTCD prescribes several Horizontal Alignment signs to give 

drivers advance warning of a horizontal curve. For a single curve 

section, there are four signs:

• Turn (W1-1) 

• Curve (W1-2)

• Hairpin Curve (W1-11)

• 270-degree Loop (W1-15) 

The MUTCD prescribes 

Horizontal Alignment Signs 

for advanced warning of a 

single curve section.

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Low-Cost Treatments for Horizontal Curve Safety

Federal Highway Administration 

For sections with a more than one curve in close proximity, there are 

three advance warning signs:

• Reverse Turn (W1-3)

• Reverse Curve (W1-4)

• Winding Road (W1-5)

Application Guidelines

Not all horizontal curves require a Horizontal Alignment sign. 

Curves that have: (1) gentle to moderate curvature for which a speed 

advisory is not necessary, (2) adequate sight distance through the 

curve, and (3) adequate pavement markings and/or raised pavement 

markings and delineators, likely do not require even the Curve 

(W1-2) sign. As the MUTCD states, use the Curve sign where there 

is an advisory speed of greater than 30 mi/h and the Turn sign when 

the advisory speed is 30 mi/h or less. However, this is subject to 

engineering judgment that considers the traffic volume, type of road, 

and other factors. Use the Hairpin Curve sign when the curve is 135 

degrees or more. The Loop sign is usually limited to 270-degree 

loops found on cloverleaf interchange ramps; it is not addressed in 

this publication.

The two sequential curves signs (left turning followed by right 

turning or vice versa) are:

• Reverse Turn (W1-3) 

• Reverse Curve (W1-4)

The guidance is the same for selecting a Turn or Curve sign and agencies should 

base their decision on the advisory speed, as with the single Turn and Curve 

signs.

For road segments with three or more alignment changes in opposite directions in 

relatively close proximity, the Winding Road (W1-5) sign is appropriate. 

Design

Depending on the geometry of the curve or sets of curves, place the appropriate 

sign the distance in advance of the point of curvature, as shown in the table that 

follows. This table is based on Table 2C-4 of the MUTCD. For example, if the 

posted speed of the road is 50 mi/h and the curve has a posted advisory speed of 

30 mi/h, then agencies would place the sign 100 ft before the point of curvature. 

Two options for two 

sequential curves.

Winding 

Road sign for 

three or more 

alignment 

changes 

in close 

proximity.

Advance warning signs for 

sections with more than one 

curve in close proximity.

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11 Office of Safety

Chapter 2. Basic Treatments for Horizontal Curves

Guidelines for Advance Placement of Curve Warning Signs.

Posted or 85th-Percentile 

Speed (mi/h)

Advance Placement Distance (Feet) for

Advisory speed of the curve (mi/h) of

10 20 30 40 50 60 70

20 n/a1 -- -- -- -- -- --

25 n/a1 n/a1 -- -- -- -- --

30 n/a1 n/a1 -- -- -- -- --

35 n/a1 n/a1 n/a1 -- -- -- --

40 n/a1 n/a1 n/a1 -- -- -- --

45 125 n/a1 n/a1 n/a1 -- -- --

50 200 150 100 n/a1 -- -- --

55 275 225 175 100 n/a1 -- --

60 350 300 250 175 n/a1 -- --

65 425 400 350 275 175 n/a1 --

70 525 500 425 350 250 150 --

75 625 600 525 450 350 250 100

1

 No suggested distances are provided for these speeds, as the placement location depends on site 

conditions and other signing to provide an adequate advance warning for the driver.

This sign, and others discussed in this publication, 

should be placed in rural areas as illustrated at 

right. 

Materials

Traffic signs of all types use retroreflective sheeting 

to ensure they are visible to drivers at night or in 

periods of low light. Over the last several years, 

agencies have transitioned from using engineering 

grade (Type I), to high-intensity grade (Type III), 

and even microprismatic sheeting (Type V). Each 

higher grade provides a brighter and longer lasting sign—but with increasing unit costs for 

each.

Effectiveness

A 1968 evaluation of curve warning signs reported that installing warning signs can 

reduce crashes by 18 percent. No recent research identifies or evaluates this device, but a 

beneficial safety effect seems likely with their proper use. 

Offset and height placement for signs on 

rural roads.

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Federal Highway Administration 

ADVISORY SPEED PLAQUE

An Advisory Speed plaque (W13-1) is simply a sign placed below a Horizontal 

Alignment sign (discussed above) to advise motorists of the safe speed through 

the curve(s). It is not the legal speed limit. 

Application Guideline

The MUTCD requires an engineering study to determine if an advisory 

speed is necessary for the condition. The MUTCD further states that “. . . 

the advisory speed may be the 85th-percentile speed of free-flowing 

traffic, the speed corresponding to a 16-degree ball bank indicator 

reading, or the speed otherwise determined by an engineering study 

because of unusual circumstances.” The Traffic Control Devices 

Handbook provides further guidance on when to use an Advisory 

Speed Plaque. It also is suggests using the plaque whenever the 

difference between the advisory speed and the posted speed is 6 mi/h 

or greater. 

ONE-DIRECTION LARGE ARROW SIGN 

The One-Direction Large Arrow sign (W1-6) is used to define a change in horizontal 

alignment. Usually only one of these signs is used for a horizontal curve. It is typically 

placed on the outside of the curve directly in line with the approaching 

tangent section. Nothing in the MUTCD limits using multiple signs along 

the curve, but in this case, it would be more reasonable to use a series of 

Chevron (W1-8) signs. Install this Large Arrow sign only on the outside 

of a turn or curve in line with, and at approximately a right angle to 

approaching traffic. One-Direction 

Large Arrow Sign.

The Traffic Control 

Devices Handbook 

provides additional 

guidance on devices found 

in the MUTCD.

Photo source: Missouri DOT

Curve Horizontal Alignment sign with an Advisory Speed Plaque 

warns the driver of an approaching curve over the crest of a hill.

MUTCD 

Advisory Speed 

Plaque.

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13 Office of Safety

Chapter 2. Basic Treatments for Horizontal Curves

Application Guideline

MUTCD guidance regarding the application of this sign is 

to install either the One-Direction Large Arrow sign or the 

Chevron Alignment sign when the Hairpin Curve sign or the 

Loop sign is installed. Based on standard practice, this sign is 

limited to sharper curves (turns). It should not be used when 

there is no advisory speed plaque.

COMBINATION HORIZONTAL ALIGNMENT / ADVISORY SPEED SIGN

Agencies can combine the Turn (W1-1) sign or the Curve 

(W1-2) sign with the Advisory Speed (W13-1) plaque to 

form a combination Turn/Advisory Speed (W1-1a) sign or a 

combination Curve/Advisory Speed (W1-2a) sign. Use it as a 

supplement to (not a replacement for) the advance Horizontal 

Alignment sign and Advisory Speed plaque, and place it at the 

beginning of the turn or curve. The sign is intended to remind 

motorists of the need to slow down as they begin to negotiate 

the alignment change. 

Application Guideline

The MUTCD contains no guidance as to when to use these signs, so it is up to an agency’s 

“engineering judgment.” It is probably best not use it when the distance from the advance 

horizontal alignment sign and the point of curvature is 200 ft or less because the two signs 

would be too close together. Where this situation occurs, use the Curve Speed sign, which 

is discussed next.

CURVE SPEED SIGN

The Curve Speed (W13-5) sign, serves a similar function as the Combination 

Horizontal Alignment/Advisory Speed sign. It is used to remind motorists of 

the advisory speed and, if appropriate, where the recommended (advisory) speed 

changes with the road curvature, such as for a compound or spiral curve design. 

Place the sign within the curve either on the inside or outside of the curve to 

increase its visibility. Although a special situation may call for both the Curve 

Speed sign and the Combination Horizontal Alignment/Advisory Speed sign, use 

one or the other, not both.

One direction Large Arrow Sign 

placed within curve.

Combination Horizontal 

Alignment / Advisory Speed Signs.

Curve Speed 

Sign.

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CHEVRON ALIGNMENT SIGN

While designated as a warning sign in the MUTCD, the Chevron Alignment (W1-8) sign 

is intended to emphasize and guide drivers through a change in horizontal alignment. 

Because of their pattern and size and that several of the signs are in view of the 

motorist, they define the direction and sharpness of the curve the best of all the 

traffic control devices. When the chevron sign is used, agencies will also need 

one of the advance curve warning signs previously discussed. 

 

 Before installation of Chevrons. After installation of Chevrons.

 The installation of Chevrons provides guidance for the approaching curve.

Application Guidelines

Other than to state that the Chevron Alignment sign may be used as an alternative to 

supplement to standard delineators or to the One-Direction Large Arrow (W1-6) sign, 

the MUTCD provides no specific guidance as to when agencies should use the Chevron 

Alignment sign. The Traffic Control Devices Handbook suggests installing Chevrons when 

the difference between the advisory speed and posted speed is 25 mi/h or greater.

Design

Install a series of these signs on the outside of turn or curve, positioned in line with 

approaching traffic at approximately a right angle to a driver’s line of sight. On two-lane, 

two-way roads, use two-sided Chevron signs properly aimed to guide traffic traveling both 

directions. 

The MUTCD does not specify spacing of the signs; however, it does recommend that 

spacing allow the motorist to view at least two signs until the change in alignment 

eliminates the need for the signs. The following figure illustrates a layout of these devices 

on a curve. Position Chevron signs 5 ft above the surface in rural areas, and 7 ft in urban 

areas. Additionally, the following pages provide examples of spacing charts developed by 

two State DOTs.

Chevron 

Alignment 

Sign

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15 Office of Safety

Chapter 2. Basic Treatments for Horizontal Curves

State recommended spacing charts for Chevrons

Layout of Chevrons.

Advisory 

Speed Limit 

(mi/h)

Chevron 

Spacing (ft)

15 40

20 80

25 80

30 80

35 120

40 120

45 160

50 160

55 160

60 200

65 200

NOTE: The above spacing 

distances apply to points within 

the curve. Approach and departure spacing distances are twice 

those shown above.

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Recommended Chevron Spacing

Degree 

of Curve

Radius 

(ft) 

Spacing (ft)

Curve Turn Tangent

10000 400 200 200

1 304 152 200

5000 282 141 200

3000 218 109 196

2 212 106 191

2500 198 99 178

2000 176 88 158

3 172 86 155

1800 168 84 151

1600 156 78 140

4 148 74 133

1400 148 74 133

1200 136 68 122

5 132 66 119

1000 124 62 112

900 116 58 104

7 110 55 99

800 110 55 99

700 102 51 92

9 96 48 86

600 94 47 85

500 84 42 76

12 82 41 74

400 74 37 67

15 72 36 65

350 70 35 63

18 66 33 59

300 64 32 58

21 60 30 54

250 56 28 50

25 56 28 50

200 48 24 43

30 48 24 43

150 40 20 36

40 38 19 34

100 28 14 25

Chapter 2

Federal Highway Administration 16

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Effectiveness

Although the effectiveness of Chevron signs in reducing crashes has not been established, 

the signs have been shown to reduce vehicle encroachments onto the centerline in curves 

where the degree of curvature is more than 7 degrees.

Cost

The cost to apply Chevrons around a curve will vary with the number of signs installed. A 

typical installation of about 10 signs would cost approximately $500.

DELINEATORS

A commonly used device for showing the curve alignment to the motorist is the 

delineator—a retroreflective device mounted above the roadway surface and along the 

side of the roadway in a series to show roadway alignment. A delineator is considered 

a guidance device rather than a warning device and is most effective at night and during 

adverse weather when pavement markings are not visible.

The delineator device is typically 

either circular or rectangular with a 

3-in minimum dimension. They are 

usually mounted on posts (which can 

have retroreflective material as well) 4 ft 

above the pavement. They can be placed 

on barriers if they are present—if there 

are none, then delineators are placed 

on lightweight breakaway posts. As 

shown in the photo, the delineators are 

rectangular reflectors placed on a flexible 

post. The application shown is on a 

ramp, but they are also used for curved 

sections of roadways.

The MUTCD requires the color of the 

delineators to match the color of the

adjacent edge line. For example on a curve on a two-way road, the edge lines on both 

sides of the road are white, so if delineators are used on the left side of the road they must 

be white. Delineators on the right side are also white.

Low-Cost Treatments for Horizontal Curve Safety

17 Office of Safety

Post delineators installed on a ramp.

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Application Guideline

The MUTCD does not provide any guidance as to when to install this device. However, 

given their relatively low cost, they should be considered for curves where any of the 

advance curve warning signs are used. For curves greater than 7-degree, the Chevron sign, 

discussed next, might be more cost-effective. 

Design

Delineators are placed on the right shoulder 

and, therefore, the reflectors are white (clear) 

to match the white edge line. Adjust spacing 

of delineators on approaches to and throughout 

the horizontal curves so that several delineators 

are always visible to the motorist. Follow the 

approximate spacing shown in the table on the 

next page.

Effectiveness

The Low Cost Local Road Safety Solutions

publication cites a 1966 Ohio Department of 

Transportation Study which found that postmounted delineators on rural two-lane curves 

reduced run-off-road crashes by 15 percent.

Cost

The cost of post delineators applied to a single curve will vary depending upon the number 

used and the material for the post and retroreflector.

Further Information

For further information on post delineators, see Roadway Delineation Practices Handbook

(http://www.fhwa.dot.gov/tfhrc/safety/pubs/93001/intro.htm).

Radius Of Curve (ft) Approximate Spacing 

On Curve (ft)

50 20

115 25

180 35

250 40

300 50

400 55

500 65

600 70

700 75

800 80

900 85

1000 90

Approximate Spacing for Delineators on 

Horizontal Curves.

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Chapter 3. Enhanced Basic Treatments

CHAPTER 3. ENHANCED BASIC TREATMENTS

Most basic devices described in chapter 2 can be improved in different ways so that they 

can be seen farther away. The sooner a motorist is able to see a device and recognize its 

meaning, the more time there is to respond. This chapter discusses the following seven 

treatments have proved effective in getting motorists’ attention:

Larger devices

Doubling up of devices

High retroreflective intensity and fluorescent yellow sheeting

Flashing beacons

Profile Thermoplastic Markings

Raised Pavement Markers

LARGER DEVICES

The table below shows the sizes of the signs described in the previous chapter for 

conventional roads. The MUTCD prescribes the use of the “conventional road” sizes 

for typical situations. The minimum size is not recommended. However, the MUTCD 

allows their use on low-speed roadways where the reduced letter size remains adequate for 

the warning or where physical conditions prevent using a larger size. The MUTCD also 

states that “. . . oversized and larger signs may be used for those special applications where 

speed, volume, or other factors result in conditions where increased emphasis, improved 

recognition, or increased legibility would be desirable.” A horizontal curve identified as a 

safety problem would likely meet this application guideline. Thus, consider larger sizes, 

typically in 6-in increments for both dimensions.

Warning Sign Sizes (in).

Description Minimum Conventional 

Road Expressway Freeway Shape Sign Series

Diamond

W1-1, W1-2, W1-3, 

W1-4, W1-5, W1-11, 

W1-15, W1-1a, W1-2a

24 x 24 30 x 30 36 x 36 48 x 48

Rectangular

W1-6 36 x 18 48 x 24 -- --

W1-8 12 x 18 18 x 24 30 x 36 36 x 48

W13-5 24 x 30 24 x 30 36 x 48 48 x 60

Source: MUTCD, Table 2C-2.

•

•

•

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DOUBLING-UP OF DEVICES

“Doubling-up” simply refers to situations where 

agencies install a second, identical sign on the 

left side of the roadway. Agencies can do this for 

the basic signs discussed in the previous chapter. 

Doubling-up increases the opportunity for the 

motorist to see the sign, and more importantly, 

respond to the message. Doubling-up is a 

candidate treatment when visibility of the single 

right-hand side sign is less than desirable.

HIGH RETROREFLECTIVE INTENSITY AND FLUORESCENT YELLOW 

SHEETING

Another way to make signs more visible or more noticeable to motorists is to use highintensity retroreflective sheeting and fluorescent-yellow sheeting. As noted in chapter 

2, the retroreflective sheeting for signs is available in different intensity grades. Signs 

made with high-intensity sheeting can help motorists see them from a longer distance at 

nighttime visibility. For more information on types of retroreflective sheeting, visit: http://

safety.fhwa.dot.gov/roadway_dept/retro/index.htm

Fluorescent yellow increases visibility of warning 

signs and certain other signs, such as the Chevron 

sign. Again, the higher intensity sheeting makes 

the sign more visible to motorists who can 

recognize and respond to it earlier. This visual 

advantage works day and night.

Initial research based on eye-tracking data 

indicates that upgrading conventional yellow 

chevrons to fluorescent yellow, while not 

affecting speed or lane placement, improves 

driver perception of the signs. This improved 

driver performance effect suggests a potential 

safety effect. 

The estimated cost of an 18-in by 24-in Chevron sign with Type III sheeting is about $335. 

This estimate was based on a unit price of $1.20/ft2

 for sheeting. Applying an estimated 

cost of $4.00/ft2 of fluorescent color microprismatic sheeting brings the total sign cost to 

$343, a cost increase of only 2.4 percent.

Doubling-up of the sign proved effective at this 

site because tree limbs partially blocked the right 

side sign.

Source: Texas Transportation Institute

Stimulus photo illustrating enhanced chevron 

visibility. 

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Chapter 3. Enhanced Basic Treatments

FLASHING BEACONS

Using flashing beacons with a warning sign is another way to gain motorists’ attention. 

The beacons are typically used with one 

of the advance Horizontal Alignment 

signs for a horizontal curve. There are 

no published guidelines for when they 

are appropriate, but reasonable guidelines 

are to limit them to locations where 

other treatments have not solved a safety 

problem. One factor limiting their use 

is the availability of an accessible power 

source, although agencies can use reliable 

solar power panel systems as well.

The beacons used for this treatment are the 

circular yellow sections from a standard 

traffic signal. Agencies can install this 

with one or more beacons, but the photo 

on the right shows a typical arrangement. The beacons can be flashed either alternately or 

simultaneously. To prevent the flashing light from masking the sign message, locate the 

beacon signal housing at least 12 in outside of the nearest edge of the sign. 

The safety effectiveness of this particular treatment is yet to be established, but a 1970s 

study evaluated the effects of signing to warn drivers of wet weather skidding hazards at 

horizontal curves. The study concluded that agencies could significantly reduce vehicle 

speed by adding flashing beacons on the curve warning sign.

PROFILE THERMOPLASTIC MARKINGS

Agencies apply thermoplastic markings to create a profile marking, which also produces 

a rumble effect and enhances visibility of the marking. A few agencies have used this 

treatment with good results, but there is no firm evaluation. As snow plowing can destroy 

this marking, its use is sometimes limited to warmer climate locations. 

The California Department of Transportation (Caltrans) has used two types—raised and 

inverted profile patterns, as pictured on the next page and in the accompanying design 

specification.

Typical arrangement of signs and flashing beacons.

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A raised profile where a thicker layer of thermoplastic is applied on 20-in centers along the stripe. 

An inverted profile where depressions are made every 1 in along the stripe. 

RAISED PAVEMENT MARKERS

In many situations, agencies will install raised pavement markers (RPMs) to supplement or 

substitute for pavement markings. There are a variety of types and they can be reflective 

or nonreflective. For geographic areas where snow is common, the reflective device is 

encased in an iron casting or recessed below the pavement surface in a grooved section 

to prevent damage by snow plows. Agencies typically apply the markers within a long 

(Source: Caltrans)

Raised profile thermoplastic marker.

(Source: Caltrans)

Inverted profile thermoplastic marker.

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Chapter 3. Enhanced Basic Treatments

roadway section; the advantage is that they provide a longer visual range of delineation for 

motorists, especially at night (if reflective) and during wet conditions. RPMs also work 

well when applied to a single curve or curvy section of roadway. The RPMs also provide 

an auditory warning to the motorist who travels on them. 

For more information about using raised pavement markers, visit the MUTCD (http://

mutcd.fhwa.dot.gov/HTM/2003r1/part3/part3a.htm) and in the Roadway Delineation 

Practices Handbook (http://www.fhwa.dot.gov/tfhrc/safety/pubs/93001/intro.htm). 

While studies of the operational effects have shown RPMs can reduce 

the variation in lane placement and move vehicles away from the 

centerline, studies of crash changes have produced mixed results. 

They show a safety benefit on roadways with gentle curvature (less 

than 3.5 degrees) and relatively high volumes (greater than 5,000 

veh/day), and safety disbenefits for roadways with sharper curvature 

(greater than 3.5 degrees) under all volume conditions. It has been 

hypothesized that the disbenefit results from the higher speeds because 

motorists feel safer with the RPMs providing alignment information 

even under wet nighttime conditions.

A comprehensive guide 

on all types of delineation 

devices.

Standard Raised 

Pavement Marker 

(yellow for centerline).

Snowplowable Raised 

Pavement Marker.

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CHAPTER 4. OTHER TRAFFIC CONTROL DEVICE 

TREATMENTS

REFLECTIVE BARRIER DELINEATION

Description

Retroreflective material, such as reflectors or panels of retroreflective sheeting, can be 

a highly effective treatment for delineating curves, especially at nighttime. Strips of 

reflective sheeting shaped to provide linear reflectorization are applied to either concrete 

barriers or metal guardrail to alert drivers of approaching curves. The example shown 

below consists of 34-in-long, 4-in-wide to 6-in-wide panels. The color of the delineation 

should be the same as traditional delineation—the same color as the adjacent edge lines. 

On a two-lane, two-way road, this means that the delineation would be white on both sides 

of the road.

Reflective sheeting shaped to provide linear reflectorization.

The photographs below show the resulting linear reflectorization effect on construction 

zone barriers. Agencies can achieve similar effective delineation on rural curves by 

applying the reflective panels to existing metal guardrails.

 

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Application Guideline

There are no published guidelines for when this or similar treatments are appropriate. 

Obviously, an existing guardrail or concrete barrier must be in place before any material is 

applied. When the existing barrier is frequently hit consider adding new or supplemental 

reflectorization.

Design

Space reflective sheeting panels 18 to 36 in apart, running parallel to traffic. Attach the 

panels to concrete barriers by anchoring them into drilled holes supplemented with a 

caulking compound. To apply on metal guardrail, 

adhesive is used to attach the sheeting panels, 

housed in brackets, to the guardrail. 

The photos below illustrate reflector application on 

metal guardrail. Individual reflectors can be spaced 

according to the criteria noted for post delineators, 

discussed earlier. For curved sections of guardrail, 

it is important to adjust angles of individual 

reflectors (i.e., bend mounting brackets) to ensure 

their alignment is perpendicular to the angle of 

oncoming headlights.

 

Installation of reflectors on W-beam guardrail.

Effectiveness

Oregon DOT (ODOT) conducted an example application of reflective barrier treatments 

(shown above) known as the 3M Linear Delineation System. Results reveal that the 

Continuous and intermittent application of 

retroreflective sheeting on metal guardrail.

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retroreflective panels provide a good alternative to traditional concrete barrier delineation 

methods (such as reflective barrier markers). ODOT realized at the end of construction 

that the panels could be removed from the barrier for reuse on future projects. The success 

of the panels has led ODOT to consider future implementation when crash histories show 

the need for additional safety measures. However, installing the panels proved to be 

more challenging then envisioned, primarily because it is a time-intensive process. The 

evaluation also cited maintenance concerns about keeping the panels clean from dirt and 

road grime to maintain an optimal retroreflectivity level.

Cost

Individual reflectors for guardrail application cost approximately $3.00 each. Strips of 

reflective sheeting, known as the guardrail Linear Delineation System, are available at 

approximately $2.33/lineal ft of 4-in-wide white material. 

ROADSIDE OBJECT DELINEATION

Description

Trees, utility poles, and other obstructions 

located in the roadside area close to the nearest 

travel lane (such as within the designated clear 

zone) can be run-off-road road crash hazards. 

Installing an object marker, reflectorized tape, 

or other simple delineation device is a lowcost procedure to reduce the associated crash 

potential. 

Application Guideline

Depending on the roadside obstruction location and the severity of the hazard, agencies 

should first consider removing objects that have a history of being struck. Agencies will 

also discover the more difficult challenge of overcoming public resistance to removing 

trees. Other options to consider, such as modifying the roadside to shield the object from 

being struck, are flattening or grading sideslopes, regrading ditch sections, or improving 

shoulders. When cost rules out these alternatives, delineation is the preferred option. 

Regardless of the option selected, agencies must first install acceptable delineation on the 

curve, as previously discussed. 

Design

Once the decision is made to delineate a tree or other object, install a Type 2 or Type 

3 object marker (see MUTCD Sections 3C.01 and 3C.04) according to the following 

principles:

Simply adding a reflective tape to a pole or tree that 

cannot be removed may prevent a crash.

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When marking objects 8 ft or less from the shoulder or curb, mount the object 

marker so the bottom of the marker is at least 4 ft above the surface of the 

nearest traffic lane. 

When marking objects more than 8 ft from the shoulder or curb, mount the 

reflective marker so that the bottom of the marker is at least 4 ft above the 

ground. 

Use a band of reflective tape to supplement or substitute for the object marker. 

The band should be at least 6 in wide.

There is no required color for the retroreflective tape, but most agencies prefer 

yellow in most cases. However, in instances when there are few daytime 

crashes, and , especially in aesthetically sensitive areas, agencies can install 

brown retroreflective tape.

Effectiveness

The Pennsylvania Department of Transportation (PennDOT) has an experimental program 

targeting delineation to potentially hazardous objects on road segments with high run-offroad utility pole- and tree-related crash frequencies (particularly at night). The program 

is proving effective when it is not feasible to remove or relocate an object because of 

budget constraints or the object is on private property. PennDOT marks the tree or utility 

pole with a round of reflective tape. One round is used on each tree and each utility pole. 

Two rounds are used for poles at intersections. No evaluation has yet documented the 

effectiveness of this practice.

Contact and Further Information

A.J. Zeigler, Guide to Management of Roadside Trees, Report FHWA-IP-86-17, Michigan 

Department of Transportation, Lansing, MI 48909, December 1986.

Pennsylvania DOT Bureau of Highway Safety, Phone 717-705-1706.

Highway Safety and Trees: The Delicate Balance (FHWA-SA-06-13) is available as a 

DVD; email requests to Report.Center@dot.gov 

DYNAMIC CURVE WARNING SYSTEM

Description

Agencies can enhance curve warning systems by using supplemental beacons and/or 

messages that activate when a motorist approaches the curve at a high speed. A typical 

dynamic curve warning system combines a speed measuring device (such as loop detectors 

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or radar) with flashing beacon and a variable message sign. The system is designed to 

slow high-speed vehicles as they approach and enter a horizontal curve. It works by 

measuring the speeds of approaching vehicles and providing messages to speeding drivers 

to slow down to an advisory speed. Agencies can develop these systems using off-the-shelf 

technology. The advantage of this treatment is that the device has a much greater effect 

on high-speed vehicles than a static curve warning sign. A variety of these systems are 

deployed in the U.S., as the three examples below demonstrate.

 

 

 Speed Actuated Sign – Augusta, ME. Flashing Beacon on Warning Sign.

 (Courtesy of Caltrans) Dynamic Warning System on I-80 in California. 

Application Guideline

Because even the least expensive system is much more costly than static signs, agencies 

should limit their application to locations with high crash rates, especially those involving 

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fatalities and injuries, and where other less expensive devices have failed to solve the 

problem. The only specific guideline found for when to use such a device is from a study 

of a similar device, which recommended that “. . . a vehicle-actuated curve speed control 

may be required if there have been 10 or more reported accidents in a 24-month period, or 

7 or more reported accidents in a 12-month period. The accidents should occur along the 

section of road including the curve and a distance of 1000 ft downstream.” 

One dynamic system application that does not require major reconstruction involves a radar 

speed detection device coupled with warning signs and activated flashing beacons. The 

Texas system, illustrated in the photos below, advises drivers detected driving more than 

5 mph over the 25-mph curve advisory speed limit to reduce their speed. A radar detector 

measures speeds and displays them using a speed display sign stating: “YOUR SPEED 

IS . . .”. A W1-1 warning sign is located 625 ft in advance of the curve, and the overhead 

sign is located in the point of curvature. The radar is set to start processing the speed data 

about 300 ft before a vehicle reaches the overhead sign. 

 

 

Dynamic curve speed warning system in Camp County, Texas.

Cost

The cost of these systems varies, depending upon the specific design. The cost of the Texas 

system presented above, which consists of radar speed detection and associated activation 

of flashing beacon, was approximately $18,000 for equipment and installation. The system 

deployed on a California interstate was $61,000 (in year 2001; the cost included traffic 

control).

Effectiveness

Dynamic curve warning systems can effectively reduce vehicle speeds in horizontal curves, 

especially during wet-pavement conditions. The accident effectiveness will likely vary by 

location. For the system displayed on a California interstate, Caltrans reported a 44 percent 

reduction in accidents in the first year and a 39 percent in the second year, compared to the 

year before installation.

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Contacts and Further Information

For the California installation—Robert Peterson, Chief Traffic Safety Branch, District 3, 

Caltrans, Robert_Peterson@dot.ca.gov

SPEED LIMIT ADVISORY MARKING IN LANE

Description

Pavement markings in advance of horizontal curves provide highly conspicuous, 

supplementary warning information and the potential to increase safety. In particular, 

advisory speed warnings provide essential information directly related to drivers’ safe 

negotiation of curves. Pavement markings are especially important for reducing speeds at 

curve locations where signs have proved ineffective.

An example speed-limit advisory pavement 

marking is illustrated at right. In this 

example, the markings display is CURVE—

55—MPH. PennDOT has also experimented 

with the use of the pavement arrow (such 

as MUTCD figure 3B-21), as discussed in 

chapter 7. That arrow is currently reserved 

for use in the designating lane-use at 

intersections, so other uses such as for curves 

requires FHWA experimentation approval. 

Application Guideline

There is no established guideline for when to use this treatment. It 

is probably more appropriate for higher speed roads and those where 

speed studies indicate excessive speeding.

Design

The MUTCD and the Standard Highway Signs (SHS) manual 

present specifications for designing and placing speed limit advisory 

pavement markings. MUTCD section 3B.19 has examples of 

elongated pavement marking letters and words suitable for reading 

by approaching motorists. Visit http://mutcd.fhwa.dot.gov/kno2003r1.htm. The SHS includes layouts for applying these markings, 

including pavement markings. Visit http://mutcd.fhwa.dot.gov/sershs_millennium.htm. 

Speed-limit advisory pavement marking.

The SHS provides 

specifications for sign 

sizes and message layout.

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The advance distance at which such markings are applied depends on both the approach 

speed and design speed of the curve. Agencies should base advance placement distances 

on specific approach and curve speeds, which should be the same as advance distances 

prescribed for warning signs, as provided in chapter 3.

Effectiveness

A Texas evaluation of advisory speed pavement markings based its findings on speed 

reductions at curves compared with an upstream speed control point. While there were 

inconsistent results between data collection sites, the study concluded that the markings 

were “worthy of further exploration.”

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Chapter 5. Rumble Strips

CHAPTER 5. RUMBLE STRIPS

DESCRIPTION

A rumble strip can be formed in the pavement surface by placing either grooves into the 

surface or strips of material above the surface according to a prescribed spacing pattern. A 

vehicle passing over the rumble strips produces noise and vibration and alerts the driver to 

a potentially hazardous situation. Agencies can install rumble strips on horizontal curves 

longitudinally with the centerline; with the edge line or on the shoulder; or transversally 

across the full lane in the advance of the curve. 

Each of these three will alert drivers to different hazards and each achieves a different 

objective. Use the centerline rumble strip (CLRS) to alert drivers who drive into the 

opposing traffic lane and thereby avoid head-on or sideswipe-opposite direction crashes. 

Use the shoulder (or edge line) rumble strip (SRS) to alert motorists who drive onto the 

shoulder and beyond and thereby avoid run-off-road crashes. Use the roadway (transverse) 

rumble strip (RRS) to alert drivers approaching the curve of the potential need to reduce 

speed or at least be alert while driving through the curve. Because of the similarities in the 

applications, they are discussed together.

 

Illustrations of rumble strip for centerline (left), shoulder (middle), and across roadway (right).

There are four types of rumble strips:

Milled rumble strips are formed with a machine that cuts a smooth groove in a 

roadway of either new or existing asphalt or cement concrete. 

Rolled rumble strips are pressed into freshly laid asphalt pavement by a roller 

with steel pipes welded to a drum.

Formed rumble strips are added to fresh concrete shoulder with a corrugated 

form that is pressed onto the surface just after concrete placement and finishing 

operations.

Raised rumble strips can be formed by applying asphalt material ( 1⁄4 to 1⁄2 in 

high) as raised bars on the surface. 

1.

2.

3.

4.

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The milled or rolled strips are typically used for the centerline and shoulder application. 

The milled type is preferred because it produces a higher noise level and vibration stimuli 

than do rolled rumble strips. The raised types may be used for the across the pavement 

roadway application. The noise and vibration effect is created by the tires bouncing over 

the raised bars.

centerline rumble strip

Application Guideline

Because motorists frequently cross over the centerline through curved sections, the 

centerline rumble strip (CLRS) is a candidate treatment for horizontal curve sections. 

However, the application of CLRS just along a curve section has not been identified as an 

actual practice. This is likely because the installation cost of would not justify their use 

for a relatively short section. Therefore, when used, agencies should install CLRS on a 

considerable section of roadway. 

Several State highway departments have guidelines for applying CLRS, including:

Crash history indicates a large number of head-on or sideswipe crashes.

Posted speed limit of 50 mi/h or greater. 

ADT threshold of at least 1500.

Pavement width of at least 20 ft.

Pavement type of asphalt in good condition with minimum depth of 2.5 in.

CLRS are not recommended for:

Bridge decks.

In area of intersections with public roads or short distances between access 

points.

Existing concrete pavements with an overlay depth less than 2.5 in.

Roadways where residences are close to the highway—because of noise 

complaints.

Design 

There are variations in design patterns, but the 

most common types are milled, 12 in to 16 in long 

(perpendicular to the centerline), 7 in wide (along 

the centerline), 1⁄2 in deep. The two most common 

patterns are continuous rumble strips 12 in to 24 in 

apart, or alternating, with pairs of rumble strips 12 in 

or 24 in apart, with the pairs being 24 in or 48 in apart, 

respectively. 

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Special machinery is needed to install 

milled rumble strips.

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Chapter 5. Rumble Strips

Effectiveness

Based on a variety of information from States using CLRS, the positive effects far 

outweigh the potentially negative effects. The most significant positive effect is the 

reduction in overall, targeted (cross over), injury and/or fatal crashes reported by States 

that use them. For example, Delaware DOT reported a 90 percent reduction in the headon collision rate after installing CLRS on a two-lane undivided rural highway with a high 

fatality rate. Also, studies show that motorists tend to position their vehicles farther away 

from the centerline, and that CLRS help drivers identify the centerline during adverse 

weather conditions, such as blowing snow. 

The potential negative effects include:

Disruption to motorcyclists and bicyclists.

Roadside noise in residential areas.

Pavement deterioration.

Cost

The installation cost will vary based on a number of factors including the length of section, 

pavement type, pattern, and highway location. Those agencies installing them report costs 

of about $0.40 per linear ft; however, when applied just to a single curve, expect a higher 

cost.

SHOULDER RUMBLE STRIP

Application Guideline

Rumble strips can be applied with the edge line or on the shoulder, if it is paved wide 

enough. There are no published guidelines for when to use this treatment, but it should be 

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Schematic for centerline rumble strips illustrates the 

applied pattern.

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considered if there is a high number of run-off-road (ROR) crashes. As with the CLRS, 

the shoulder rumble strip (SRS) would likely not be used for a single curve because of high 

installation cost. The guidelines suggested for CLRS would be reasonable for SRS use as 

well.

Design

The layout design for the shoulder rumble strip (SRS) varies among those States using 

them. The following table and figure show the layout of a typical milled type SRS.

Standard Dimensions of Milled and Rolled SRS.

Dimension Measurement Milled 

A Repeat Pattern approx. 5.1 in

B Longitudinal Width 7.1 in

C Transverse Width 15.8 in

Standard SRS measurements.

 

A concern for the SRS treatment is its effect on bicyclists. To reduce this potential negative 

effect, agencies can change the design to allow gaps in the pattern where bicyclist could 

travel to turn or change their position. The gap pattern adopted by Alaska is shown on the 

following page.

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Chapter 5. Rumble Strips

Gap pattern adopted by Alaska DOT.

Effectiveness

The safety benefit of SRS is well established, at least for large volume, high speed roads. 

For example, the application of SRS on the New York State Thruway resulted in an 88 

percent reduction on ROR crashes, with a 95 percent reduction in fatalities. Their safety 

effectiveness for lower volume roads might be less dramatic, but may still be cost effective 

for roadway sections with high ROR crash rates.

ROADWAY RUMBLE STRIP

Application Guideline

Roadway (or Transverse) rumble strips (RRS) can be raised bars or grooves placed across 

the travel lane. For horizontal curve applications, RRSs are a warning device to supplement 

signing and alert drivers of the need to reduce speed or at least pay attention as they 

negotiate the curve. 

Based on the available literature, the guidelines for their use on horizontal curves are:

Documented crash problem where conventional treatments, discussed earlier, 

have failed to solve the safety problem.

Road segment with distinct differences between the roadway speed limit and 

the curve advisory speed of at least 20 mi/h.

Proximity of another geometric feature such as a side street with limited sight 

distance; a second, sharper curve a short distance beyond the first curve; or a 

downgrade leading into the curve.

Long tangent section ends with a curve. 

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Low-Cost Treatments for Horizontal Curve Safety

Federal Highway Administration 

 

For this RRS application a set of 5 grooves and painted lines were placed 200 ft before the curve. 

Some of the concerns with the use of RRS include:

Noise complaints by nearby residents—avoid using them in residential areas. 

Motorist use opposing lanes to avoid rumble strips—a potential problem for 

local travelers that is reduced by using a discontinuous pattern design such as 

gaps in the bars or grooves across the pavement.

Maintenance problems—grooved RRS is more durable, lasting longer than 

raised bars, especially in areas with snowplowing.

Motorist worries—keep depth of groove or height of bar to 1⁄2 in or less and use 

a warning sign RUMBLE STRIPS AHEAD.

Bicyclist and motorcyclist concerns—address by installing discontinuous 

pattern design—and warning sign RUMBLE STRIPS AHEAD.

Overuse of rumble strips—leads to “crying wolf” attitude and therefore should 

be used sparingly.

Design

Generally, limit rumble strips to a maximum height or depth of 1⁄2 in to minimize the jarring 

action to vehicles. If thermoplastic materials are used to created raised bars for RRS, the 

material should be white.

Configure rumble strips so that most drivers are not tempted to go around the rumble strip 

by driving onto the shoulder or into an adjacent lane. Agencies can do this by extending 

the rumble strip over part of the shoulder as well as the full width of the traveled way or by 

using a discontinuous rumble strip design.

Effectiveness

There is currently no conclusive evidence of roadway rumble strip effectiveness in 

reducing crashes at curves. They do tend to reduce speed, in most cases, but not to a 

practical level.

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39 Office of Safety

Chapter 5. Rumble Strips

Agency Contacts and Further Information

More detailed information on rumble strips can be obtained from the following sources:

NCHRP Synthesis Report 339, Centerline Rumble Strips, Transportation 

Research Board, 2005.

NCHRP Synthesis Report 191, Use of Rumble Strips to Enhance Safety,

Transportation Research Board, 1993.

Synthesis of Shoulder Rumble Strip Practices and Policies obtained from http://

safety.fhwa.dot.gov/roadway_dept/rumble/synthesis/exec_summary.htm

Technical Advisory T 5040.35 Roadway Shoulder Rumble Strips, Federal 

Highway Administration, Washington, DC, December 20, 2001.

The Effectiveness and Use of Continuous Shoulder Rumble Strips. Perrillo, K. 

Federal Highway Administration, August 1998. See http://safety.fhwa.dot.gov/

roadway_dept/docs/continuousrumble.pdf.

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Chapter 6. Minor Roadway Improvements

Office of Safety

CHAPTER 6. MINOR ROADWAY IMPROVEMENTS

There are four treatments that involve physical changes to the roadway and are relatively 

minor in costs; they are discussed in this chapter.

PAVED SHOULDER TREATMENT

Description

Replace unstable or narrow shoulders with paved shoulders to increase usable width and 

driver safety. Variations in surface color also provide a beneficial safety effect. 

Application Guidelines

While tight budgets may influence an agency’s decision to upgrade to paved shoulders 

on two-lane tangent sections, the resulting cost benefit from paved shoulders and fewer 

crashes on curves deserves consideration.

Design 

Reconstructing shoulders involve removing and 

recompacting a 6-in shoulder base. Agencies paving 

with asphalt can texturize the surface to provide visual, 

audible, and tactile clues to a driver leaving the travel 

lane. To texture the surface, apply a larger, uncoated 

seal coat on the shoulder while using a smaller 

aggregate seal coat on the driving lanes. 

Effectiveness

Texas DOT (TxDOT) considers application of this treatment as operationally effective 

because of the visual, audible, and tactile cues that alert a driver straying onto the shoulder. 

TxDOT believes the treatment reduced the number of single-vehicle ROR crashes. The 

agency also reports a positive public response to the strong visual effect, which it regards 

as useful for nighttime travel.

Cost

The approximate cost of seal-coating a gravel shoulder is $1.00/yd2 (when not resurfacing 

the roadway).

Agency Contacts and Further Information

TxDOT applied the treatment cited above in Cook County. The TxDOT Fort Worth 

District Engineer is Maribel Chavez, P.E. (817-370-6500).

In addition to paving the shoulder, color 

variation between the shoulder and travel 

lanes adds visual delineation.

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Federal Highway Administration 42

Low-Cost Treatments for Horizontal Curve Safety

SHOULDER DROP-OFF ELIMINATION

Description

The vertical height difference between the paved surface and the unpaved shoulder is the 

pavement shoulder drop-off. Drop-offs happen when the unstabilized pavement edges 

erode, which creates the height difference. Horizontal curves are 

particularly prone to drop-offs, especially when vehicles stray on 

to the edge of the travel lane. The illustration at left shows how 

to measure height and depth of drop-offs. 

A drop-off as small as 3 in can create an unsafe condition when 

the vertical angle is 90 degrees. As an example, once a vehicle 

crosses from a paved surface onto an unimproved shoulder, 

the driver’s reaction often is to overcorrect to get back on the 

road. The overreaction can cause the rear wheel to catch on the 

shoulder edge and spin the vehicle around. In many instances, 

drivers attempting to return to the road will veer into the adjacent 

lane, cross into opposing traffic, or leave on the opposite side of 

the roadway.

Application Guidelines

Various shapes and depths of drop-offs can cause more or less of 

a hazard at various operational speeds. The shape of the dropoff (vertical, rounded, or tapered edges), or vertical angle, in 

combination with pavement depth can affect the hazard potential 

of drop-offs. The illustration on the left shows various pavement 

edge depths and associated operational speeds that research 

shows to be acceptable. 

Design

Applying filled and compacted shoulder material to reduce 

the drop-off depth has proved a viable method to create a safe 

condition. A simple, cost-effective solution for agencies is to 

adopt a standard contract specification requiring a 

45-degree angle asphalt fillet along each side of the roadway 

in all resurfacing projects. The asphalt fillet provides a safer 

roadway edge and a stronger interface between the roadway and 

the shoulder. 

Drop-off heights and depths 

in the ‘Intolerable’ range are 

unsafe.

Technique for measuring height 

and depth of drop-offs.

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Chapter 6. Minor Roadway Improvements

Office of Safety

One way to create a 45-degree wedge is to use a steel 

wedge or the Safety Edge MakerTM, a commercial 

device being developed by TransTech Systems, Inc. 

A steel wedge device fabricated by the Georgia DOT 

(GDOT) Maintenance Division, shown at right, is 

mounted with a simple two-bolt connection onto the 

screed end gate. This device has a rounded leading 

edge that is crucial to providing a smooth finished 

compaction to the safety edge. The device can also 

adjust vertically to varying drop-off heights.

Effectiveness

Research has shown that drop-off shapes, such as those created by a safety wedge, are 

a safety device that can reduce the rate of driver overcorrection, head-on, and run-offroad crashes. Thus, the benefit of eliminating the drop-offs is fewer crashes and related 

injuries—and reduced tort liability.

GDOT evaluated the Safety Edge (see http://safety.fhwa.dot.gov/roadway_dept/docs/

sa05003.htm) over a 13-mi roadway section. Results showed the Safety Edge Design can 

be placed on any type of roadway facility as part of the asphalt preventative maintenance 

paving process. Applying the treatment neither affects finished pavement smoothness, nor 

increases erosion of the shoulders.

Cost

Agencies can implement the safety edge on any type of roadway facility as part of the 

asphalt paving process. The Safety Edge implementation cost is approximately less than 

one percent of the hot-mix asphalt material cost. No specific cost information is available 

on the Safety Edge MakerTM.

Agency Contacts and Further Information

For more detailed information, refer to the following sources:

The Safety Edge Pavement Treatment 

http://safety.fhwa.dot.gov/roadway_dept/docs/sa05003.htm

The Road Edge Drop Off: The Problem & Solution

http://www.transtechsys.com/rd/rd_proj_current.htm

Safety Impacts of Pavement Edge Drop-Offs

http://www.aaafoundation.org/pdf/PEDO_report.pdf

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Federal Highway Administration 44

Low-Cost Treatments for Horizontal Curve Safety

Frank Julian 

FHWA, Resource Center, Atlanta

(404) 562-3689

Frank.Julian@fhwa.dot.gov

WIDEN SHOULDER

Description

The shoulder is the portion of the roadway next to the traveled way. This area is designed 

to accommodate stopped vehicles (when sufficiently wide) and to provide side support for 

the roadside in close proximity to the travel 

lane. Shoulders can be graded (level surface) 

or useable (rounding on outside edge), as 

illustrated at left. 

Shoulders are a safety feature because they 

provide space that allows drivers to get out 

of the travel lane and avoid crashes. This 

feature is particularly important in horizontal 

curves where vehicles typically use more of 

the travel lane than in straight sections.

Application Guidelines

Shoulder widths can vary from approximately 2 ft on minor rural roads to 12 ft on major 

roads where the entire shoulder may be stabilized or paved. Agencies should stabilize 

widened shoulders and ensure roadside slopes comply with AASHTO guidelines. As the 

figures below illustrate, agencies can widen shoulders on both the inside or outside. 

 

 

 Widening on inside of curve. Widening on the outside of the curve.

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Cross sections of graded and useable shoulder widths.

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Chapter 6. Minor Roadway Improvements

Office of Safety

Effectiveness

The table below lists estimated reductions in related crashes resulting from widening 

paved or unpaved shoulders. Related crashes, i.e., those affected by shoulder widenings, 

include single vehicle run-off-road and multiple vehicle head-on and sideswipe crashes. 

For example, widening an unpaved shoulder by 4ft (e.g., from 2 ft to 6 ft) would reduce 

related crashes by an estimated 25 percent. Adding 8-ft paved shoulders to a road with 

no shoulders would reduce related crashes by an estimated 49 percent. These estimated 

reductions in related crashes apply only when roadside characteristics (side slope and clear 

zone) are rebuilt to the condition existing before the shoulder was widened. Although the 

table below was developed for rural two-lane roads, and not limited to horizontal curves, 

it is reasonable to expect the major benefit from shoulder widening can also be realized for 

horizontal curves. 

Crash Reductions Related to Shoulder Widening.

Shoulder Widening 

per Side, (ft) 

Reduction in Related Crash Types (%)

Paved Unpaved

2 16 13

4 29 25

6 40 35

8 49 43

SKID-RESISTIVE PAVEMENT SURFACE TREATMENT

Description

Agencies should maintain pavements to ensure adequate friction necessary for vehicle 

braking and maneuvering under both dry and wet conditions. A vehicle will skid during 

braking and maneuvering when frictional demand 

exceeds the friction force that can be developed 

between the tire and the road surface. Horizontal 

curves are particularly prone to these types of crashes, 

especially under wet conditions. On road segments 

where skidding crashes are known to occur, consider 

applying remedial treatments, including specific 

asphalt mixtures (type and gradation of aggregate 

as well as asphalt content), pavement overlays on 

both concrete or asphalt pavements, and pavement 

grooving. Application of skid-resistive pavement surface 

in curve.

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Low-Cost Treatments for Horizontal Curve Safety

Application Guidelines

Target locations where skidding is a recognized problem and apply either skid-resistive 

overlays or pavement grooving treatment. Specifically, select sites where vehicle crashes 

directly result from skidding during wet pavement conditions. 

Pavement Surface Overlay Design

Using aggregate that lacks specific particle gradations creates voids on the surface, which 

promotes better drainage and improves skid resistance. Engineers recommend applying a 

1-in, open-graded asphalt concrete to reduce wet pavement crashes. The 1-in maximum 

gradation improves drainage and skid resistance because it has substantially more voids 

than the 3⁄8-in or 1⁄2-in maximum open-graded asphalt concrete standard mix. 

An agency’s first step is to repair major surface defects (cracks, ruts, etc.) and apply densegraded asphalt concrete. Next, apply a tack coat to the existing surface before placing the 

open-graded material. Alternatively, apply a slurry seal using nonpolishing aggregate. 

A 0.15-ft-thick blanket of the 1-in maximum open-graded asphalt concrete should be 

sufficient to remove water, increase traction, and ultimately reduce the number of crashes.

Pavement Grooving Design

Pavement grooves increase skid resistance by improving drainage characteristics and 

creating a rougher pavement surface. Pavement grooving is a technique for installing 

longitudinal or transverse cuts on the surface to increase skid resistance and reduce the 

number of wet-weather crashes. Grooves cut in the longitudinal direction have proved 

most effective in increasing directional control of the vehicle, while transverse grooving 

is most effective at locations where vehicles make frequent stops. Therefore, applying 

longitudinal grooving is the commonsense choice for improving safety on horizontal 

curves. 

Because asphalt concrete’s uniform aggregate composition is not conducive to drainage, 

grooved pavement application is primarily intended for rigid concrete. Agencies can expect 

a greater accident-reduction result with application at 50 mi/h curves than at 30 or 40 mi/h 

curves because the major benefit of grooving is to reduce hydroplaning. An accepted 

application technique is to use a portable grooving machine equipped with carbide-tipped 

flails to install grooves 3/16 in to 3/8 in wide and 5/32 in to 5/16 in deep, with 8 grooves/ft 

on a random spacing.

Skid-Resistive Overlay Effectiveness

The rural two-lane curve shown in the figure on the following page was treated with 1-in 

graded asphalt concrete to improve skid resistance. The appropriate warning sign also was 

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Chapter 6. Minor Roadway Improvements

Office of Safety

installed. The result was an immediate 

reduced crash rate—from 16 wet 

pavement-related crashes in the 13-

month period before treatment to two 

during the first six months following 

the treatment. Before treatment, 60 

percent of crashes along a 2-mile 

section of treated pavement were wetpavement related. After the treatment, 

wet-pavement accidents accounted for 

only 26 percent. 

The New York State DOT (NYSDOT) implemented a program to identify sites statewide 

with a low skid resistance and treat them with overlays as part of the maintenance program. 

A site is eligible for treatment if its 2-year wet accident proportion is 50 percent higher than 

the average wet crash proportion for roads in the same county. Between 1995 and 1997, 

NYSDOT treated 36 sites on Long Island, which reduced the annually recurring wet road 

crashes by more than 800. These results, and others throughout the State, support earlier 

findings that treating wet-road crash locations can reduce this type of crash by 50 percent 

and reduce total crashes by 20 percent. 

The Florida DOT (FDOT) treated a curved freeway ramp with Tyregrip®, a high-friction 

material illustrated below. This system consists of a highly modified exothermic epoxy 

resin two-part binder top dressed with a calcinated bauxite with a Polish Stone Value of 

70 percent plus. The treatment proved effective at increasing the skid resistance value 

from 35 to 104. While the FDOT application was to a freeway ramp, the material may be 

applicable to a higher volume curve with a higher than normal number of wet pavement 

crashes.

Application of Tyregrip® friction material on a curved ramp.

Rural curve treated with skid-resistive graded asphalt concrete, 

warning signs, and chevrons.

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Low-Cost Treatments for Horizontal Curve Safety

Pavement Grooving Effectiveness

As indicated, grooved pavements can reduce wet-weather crashes. One study of a 

California two-lane road with sharp curves found a 72 percent reduction in wet-pavement 

accidents, but only 7 percent reduction in dry-pavement accidents. There is concern that 

grooving accelerates pavement wear, but it has not been shown to affect either ride quality 

or drainage performance.

Cost

Moderate costs are involved in the application of skid-resistive surface treatments. For 

example a 2-mi section of asphalt overlay cost the California DOT $200,000 in 1996. 

Further Information

Technical Advisory T 5040.36 Surface Texture for Asphalt and Concrete Pavements, 

Federal Highway Administration, Washington, DC, June 17, 2005.

Florida DOT’s experience with Tyregrip, high-friction material for Interstate ramp 

installation, Charles Holzschuher, phone (352) 955-6341.

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Chapter 7. Innovative and Experimental Treatments

Office of Safety

CHAPTER 7. INNOVATIVE AND EXPERIMENTAL 

TREATMENTS

The MUTCD considers the two treatments in this section as “experimental” and does not 

approve them for general use. Review the Introduction section of this publication (ABOUT 

THE MUTCD), which advises agencies to get FHWA approval before installing any 

experimental treatment. Agencies wishing to use either of these treatments, should visit 

http://mutcd.fhwa.dot.gov for information on how to submit a request for experimentation.

OPTICAL SPEED BARS

Description

As shown below, Optical Speed Bars are transverse stripes spaced at gradually decreasing 

distances. The rationale for using them is to increase drivers’ perception of speed and cause 

them to reduce speed. The Optical Speed Bar name comes from this intended visual effect 

on drivers’ speed as they react to the spacing of the painted lines. These white transverse 

stripes are 18 in long and 12 in wide. The preferred material is thermoplastic because of 

the exposure to traffic volume over time. 

Application Guideline

Optical Speed Bars are applied almost 

exclusively to road segments where vehicles 

traveling at highway speeds are required to slow 

for curves or other instances where traffic speeds 

should be reduced. To date, the treatment has 

been restricted to known accident locations or 

situations requiring traffic to significantly reduce 

speed. Agencies should avoid applying Optical 

Speed Bars just to reduce traffic speed because 

overuse could jeopardize the visual effect of the 

treatment. 

Design

Various Optical Speed Bars are designed and spaced to produce a gradual slowing from 

a vehicle’s initial approach speed to the reduced curve speed. As spacing between bars 

gradually narrows, drivers sense they have increased speed and will slow down to keep the 

4-bar/sec spacing. For example, think about an applied layout designed to slow a vehicle 

from 55 mi/h (81 ft/sec) into a 35-mi/h (51 ft/sec) curve. The initial bar separation distance 

Photo: Courtesy of Virginia Department of Transportation

Optical Speed Bars used to reduce vehicle speed.

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Low-Cost Treatments for Horizontal Curve Safety

Federal Highway Administration 

is 81÷4 or 20 ft, and the final separation distance is 51÷4 or 13 ft. The table below shows 

New York Department of Transportation applied spacing between successive bars designed 

to cause drivers to reduce vehicle speed from 65 mi/h to 30 mi/h.

Example Spacing Between Sequential Pairs of Optical Speed Bars.

Bars Spacing 

(ft)

Bars Spacing 

(ft)

Bars Spacing 

(ft)

1-2 24 11-12 19 21-22 15

2-3 23 12-13 19 22-23 15

3-4 23 13-14 19 23-24 15

4-5 23 14-15 18 24-25 14

5-6 22 16-17 18 26-27 13

6-7 22 16-17 18 26-27 13

7-8 21 17-18 17 27-28 13

8-9 21 18-19 16 28-29 12

9-10 21 19-20 16 29-30 12

10-11 20 20-21 16 30-31 12

The total length of the paving-marking segment depends upon the speed difference (from 

the approach and to the lower curve speed) the application is designed to produce. The 

following table suggests approximate lengths. Use these lengths as guidelines. The basis 

for the numbers is the need to produce a comfortable speed reduction and provide drivers 

with a minimum 4 seconds of driving time within the painted marking segment length. 

Guideline for Length (ft) of Speed Bar Segment in Advance of Curve.

Approach Speed, mi/h

45 50 55 60 65 70

Curve Speed, mi/h

15 300 385 470 565 670 785

20 275 350 440 535 640 755

25 235 315 405 500 600 720

30 270 360 450 560 670

35 300 400 500 620

40 335 440 555

45 370 480

50 405

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Chapter 7. Innovative and Experimental Treatments

Office of Safety

Effectiveness

Studies by New York, Mississippi, and Texas show transverse pavement markings can 

effectively reduce mean speeds, 85th percentile speeds, and speed variance. Initial 85th 

percentile speed reductions varied from 0 to 5 mi/h. However, their long-term effectiveness 

is not known. The Virginia Department of Transportation (VDOT) was conducting a 

comprehensive study of Optical Speed Bars as this publication was being prepared; the 

results are expected early 2007. 

Cost

Based on a VDOT 2006 estimate, cost to install the marking on two directions is 

approximately $2,000 (labor and materials). 

Contacts and Further Information

Gene Arnold, Virginia Transportation Research Council, Virginia Department of Highways; 

Phone: (434) 293-1990, E-mail: gene.arnold@vdot.virginia.gov 

PENNDOT CURVE ADVANCE MARKING

Description

PennDOT has experimented with an innovative 

pavement marking designed to alert motorists 

that they are approaching a curve and should slow 

down. Referred to as the “PennDOT Curve Advance 

Marking,” this treatment involves two transverse 

bars, a SLOW legend, and an arrow indicating 

the direction of the upcoming curve. PennDOT 

developed the treatment to address driver behavior at 

locations identified as having a high number or rate 

of curve-related crashes. The device objective is that 

reducing the upper percentile speed will reduce the 

number of run-off-road crashes. 

Application Guideline

The PennDOT Advanced Curve Warning marking is designed for two-lane locations 

having a high number of curve-related crashes. Agencies should avoid any location where 

there is any potential for driver confusion because of intersecting roadways or driveways. 

Agencies applying advanced curve warning at a location should also bring all existing 

PennDOT curve advance marking.

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52

Low-Cost Treatments for Horizontal Curve Safety

Federal Highway Administration 

signs, delineation, and pavement markings to standard, and consider posting an advisory 

speed limit.

As with any treatment used with multiple or compound curves, treat the most hazardous 

curve first. 

Design

See the detailed design layout below. The chart on the following page indicates the distance 

of the marking from the point of curvature based on posted speed and posted warning 

speed. Designers used MUTCD Table 2C-4, 

Column B to calculate distance, L, to the advance 

warning sign. 

Agencies can adjust the curve marking location 

when it is not plainly visible to the driver because 

of vertical geometry or other similar sight distance 

problems.

Effectiveness

This device has been shown to reduce overall 

speeds by 6 to 7 percent with slight reductions in 

the proportion of high-speed traffic in curves. Its 

effect on crash reduction is not yet established.

Contacts and Further Information

William Crawford, Pennsylvania Department of 

Transportation, Phone: (717) 705-1437

wicrawford@state.pa.us

Source: Pennsylvania Department of Transportation

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Chapter 7. Innovative and Experimental Treatments

Office of Safety

Distance from the point of curvature based on posted speed and posted warning speed.

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Chapter 8. Maintenance Treatments

Office of Safety

CHAPTER 8. MAINTENANCE TREATMENTS

The preceding chapters present specific treatments that road agencies can apply to a 

horizontal curve or series of curves to improve safety. The final word on this publication, 

however, relates to the importance of maintaining the roadway and the various traffic 

control devices installed. Regardless of the treatments used, road agencies should plan and 

carry out the following six maintenance activities to ensure continued safe travel.

Restripe pavement markings as they lose their visibility.

Pavement markings using paint-based materials have a relatively short service life—1 to 2 

years. How long an agency’s pavement markings last depends on material type, climate, 

and traffic volume. Markings of thermoplastic material have a substantially longer service 

life. To maintain their effectiveness, pavement markings must be visible, especially at 

nighttime and during conditions of limited visibility. An agency’s regular inspection and 

restriping programs are critical to ensuring pavement markings provide needed visibility for 

motorists. For additional information on pavement marking visibility, visit 

http://safety.fhwa.dot.gov/roadway_dept/retro/pavement/pavement.htm.

Replace faded signs and those with low levels of retroreflectivity.

The various signs discussed in this report are visible at night because they are made with 

retroreflective sheeting material. Few, if any, are illuminated by external lighting. Even 

though the retroreflectivity of the sheeting material has improved to provide brighter 

and longer lasting signs, all signs deteriorate over time. Signs lose their color and 

retroreflectivity and eventually they are no longer visible to motorists from a distance. 

Therefore, it is good agency practice to schedule an annual inspection of all signs 

within its jurisdiction to ensure they are at or above the appropriate retroreflective level. 

For alternative methods for conducting this inspection, see Maintaining Traffic Sign 

Retroreflectivity (2005 Edition) (http://tcd.tamu.edu/Documents/MinRetro/MinRetro.htm). 

Replace any signs found to be ineffective as soon as possible. For additional information 

on sign retroreflectivity, visit http://safety.fhwa.dot.gov/roadway_dept/retro/index.htm.

Cut back foliage to improve the sight distance through the curve and 

increase visibility of traffic control devices.

Agencies can improve safety at a horizontal curve by maintaining the longest possible sight 

distance through the curve and to the various traffic signs. During the growing season, 

grass, weeds, brush, and tree limbs can limit a driver’s view of the road and signs. This is 

why agencies should make an annual inspection of the roadway to identify and correct these 

situations. 

More practical tips for controlling vegetation overgrowth are found in the FHWA report 

Vegetation Control for Safety (http://www.fhwa.dot.gov/tfhrc/safety/pubs/90003/intro.htm).

1.

2.

3.

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Low-Cost Treatments for Horizontal Curve Safety

Federal Highway Administration

Maintain the shoulders and smooth transitions between pavement and 

shoulder.

Unpaved or unstabilized shoulders can erode over time, especially on the inside curve 

shoulder. The result is pavement drop-offs. Chapter 4 focused on ways to correct this 

situation. Periodic maintenance of shoulders can prevent this situation.

Eliminate roadside obstacles, such as culvert headwalls, or provide adequate 

shielding.

Run-off-road crashes occur more frequently along horizontal curve sections. While 

implementing the treatments discussed in this publication should reduce the frequency 

of these type crashes, they will unfortunately still occur. It is important, therefore, to 

minimize roadside obstacles and shield them with an appropriate traffic barrier. Once a 

barrier is in place, road agencies must be ready to repair damage to it, as necessary. More 

complete information on treatment of roadside hazards is available in AASHTO’s Roadside 

Design Guide and in the FHWA publication Road Safety Fundamentals. 

Improve drainage around the curve.

Improper and poor drainage is particularly troublesome on horizontal curves because of 

water flows caused by the pavement superelevation. When water cannot drain properly, 

shoulder deterioration can cause pavement drop off and shoulder loss. Agencies should 

install curbing and improve drain maintenance. For tips on road drainage features 

important to safety, see the FHWA publication Maintenance of Drainage Features for 

Safety.

4.

5.

6.

Clearing undergrowth on the right side would 

significantly improve sight distance through the 

curve and allow motorists to judge more accurately 

the length and sharpness of the curve—and more 

importantly see oncoming traffic.

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REFERENCES

AASHTO Strategic Highway Safety Plan. American Association of State Highway 

and Transportation Officials. Washington, DC, 20001, 2005. (http://safety.

transportation.org/doc/Safety-StrategicHighwaySafetyPlan.pdf)

D. J. Torbic et al. Guidance for Implementation of the AASHTO Strategic Highway 

Safety Plan, Volume 7: A Guide for Reducing Collisions on Horizontal Curves. 

National Cooperative Highway Research Program, Report 500. Transportation 

Research Board, 2004. (http://safety.transportation.org/guides.aspx?cid=32)

Manual on Uniform Traffic Control Devices, 2003 Edition. Federal Highway 

Administration, Washington, DC 2003. (http://mutcd.fhwa.dot.gov)

Low Cost Local Road Safety Solutions. The American Traffic Safety Services 

Association. Fredericksburg, VA, 2006, (www.ATSSA.com)

Standard Highway Signs. 2004 Edition. Federal Highway Administration, 

Washington DC, (http://mutcd.fhwa.dot.gov/ser-shs_millennium.htm)

Traffic Control Devices Handbook. Institute of Transportation Engineers. 

Washington DC, 2001. (www.ite.org)

Roadside Design Guide. American Association of State Highway and 

Transportation Officials. Washington DC, January 1996.

Vegetation Control for Safety, A Guide for Street and Highway Maintenance 

Personnel. Federal Highway Administration, Office of Highway Safety. FHWA-RT90-003. 1990.

Road Safety Fundamentals. Federal Highway Administration. Office of Safety. 

FHWA-SA-05-011, September 2005.

Maintenance of Drainage Features for Safety, A Guide for Street and Highway 

Maintenance Personnel. Federal Highway Administration, Office of Highway 

Safety. FHWA-RT-90-005. 1990.

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2.

3.

4.

5.

6.

7.

8.

9.

10.

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Publication No. FHWA-SA-07-002

U.S. Department of Transportation

Federal Highway Administration (FHWA)

Office of Safety

400 Seventh Street, SW, HSSD

Washington, DC 20590

http://safety.fhwa.dot.gov

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