Low-Cost Treatments for Horizontal Curve Safety 2016

FHWA Safety Program 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Notice 

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

TECHNICAL DOCUMENTATION PAGE 

1. Report No.

FHWA-SA-15-084

2. Government Accession

No.

3. Recipient's Catalog No.

4. Title and Subtitle

Low-Cost Treatments for Horizontal Curve Safety 2016 

5. Report Date

January 2016

6. Performing Organization Code

7.Author(s)

Richard Albin, Victoria Brinkly, Joseph Cheung, Frank Julian, Cathy Satterfield,

William Stein, Eric Donnell, Hugh McGee, Ann Holzem, Matthew Albee,

Jonathan Wood, Fred Hanscom

8. Performing Organization Report No.

9. Performing Organization Name and Address

Pennsylvania Transportation Institute 

Pennsylvania State University 

201 Transportation Research Building 

University Park, PA 16802 

Vanasse Hangen Brustlin, Inc (VHB) 

8300 Boone Blvd., Suite 700 

Vienna, VA 22182-2626 

10. Work Unit No.

11. Contract or Grant No.

DTFH61-12-C-00032

12. Sponsoring Agency Name and Address

Federal Highway Administration Office of Safety 

1200 New Jersey Ave., SE 

Washington, DC 20590 

13. Type of Report and Period

Final Report, April 2014 – January

2016

14. Sponsoring Agency Code

FHWA

15. Supplementary Notes

The contract manager for this report was Joseph Cheung.

16. Abstract

FHWA created the Low-Cost Treatments for Horizontal Curve Safety in 2006 (McGee and Hanscom, 2006). There have 

been many advances in highway safety since that initial 2006 guide. The purpose of this publication is to serve as an update 

to the 2006 Low-Cost Treatments for Horizontal Curve Safety. The primary audience for this publication is local 

transportation agencies. This publication provides information specifically relating to lower volume two-lane roads and the 

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

countermeasures and how to select and apply them. 

17. Key Words: horizontal curves, systemic

approach, low-cost countermeasures, two-lane

roads

18. Distribution Statement

No restrictions.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this

page) Unclassified

21. No. of Pages

 94

22. Price

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

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

TABLE OF CONTENTS 

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

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

PUBLICATION PURPOSE AND SCOPE .....................................................................1 

ABOUT THE MANUAL ON UNIFORM TRAFFIC CONTROL DEVICES (MUTCD)......2 

ABOUT THE HIGHWAY SAFETY MANUAL AND CMF CLEARINGHOUSE ............4 

INFORMATION IN THIS PUBLICATION ...................................................................4 

PUBLICATION ORGANIZATION ...............................................................................4 

CHAPTER 2. THE TWO COMPONENTS OF SAFETY 

IMPROVEMENT: SITE ANALYIS AND THE SYSTEMIC APPROACH .6 

ALL PUBLIC ROADS—ADDRESSING THE LOCAL SYSTEM ................................7 

A CHANGE IN MINDSET ..............................................................................................8 

ADDITIONAL RESOURCES....................................................................................... 10 

CHAPTER 3. MARKINGS ........................................................................... 11 

LONGITUDINAL PAVEMENT MARKINGS............................................................. 11 

OTHER LONGITUDINAL DELINEATION .............................................................. 15 

ADVANCE MARKINGS FOR CURVES ..................................................................... 16 

PAVEMENT MARKING MATERIALS, MAINTENANCE, AND COSTS .............. 20 

CHAPTER 4. SIGNS.................................................................................... 25 

BASIC SIGNING COUNTERMEASURES ................................................................. 25 

SUPPLEMENTAL DEVICES IN A CURVE ................................................................ 30 

ENHANCED SIGNING COUNTERMEASURES ...................................................... 34 

SIGN MAINTENANCE................................................................................................ 40 

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

CHAPTER 5. PAVEMENT COUNTERMEASURES ................................ 41 

SKID RESISTANCE PAVEMENT COUNTERMEASURES ..................................... 41 

SHOULDER COUNTERMEASURES ......................................................................... 45 

CHAPTER 6. ROADSIDE IMPROVEMENTS ........................................... 51 

CHAPTER 7. ADDRESSING INTERSECTIONS IN CURVES ............... 59 

DELINEATION TREATMENTS ................................................................................. 59 

ADDRESSING VISIBILITY ISSUES ............................................................................ 61 

PAVEMENT IMPROVEMENTS .................................................................................. 65 

CHANGING INTERSECTION CONFIGURATION ................................................ 66 

REFERENCES............................................................................................... 70 

GLOSSARY .................................................................................................. 75 

APPENDIX A: LOW-COST SAFETY IMPROVEMENTS IN 

PENNSYLVANIA ........................................................................................ 78 

APPENDIX B: SYSTEMIC IMPROVEMENTS IN MINNESOTA........... 80 

APPENDIX C: APPLICATION OF EDGE LINES IN MISSOURI .......... 83 

APPENDIX D: UPGRADING CURVE SIGNING IN OHIO ................... 85 

APPENDIX E: APPLICATION OF SEQUENTIAL DYNAMIC CURVE 

WARNING SYSTEMS (SDCWS) .............................................................. 87 

APPENDIX F: APPLICATION OF HIGH FRICTION SURFACE 

TREATMENT IN KENTUCKY................................................................... 90 

APPENDIX G: EVERY DAY COUNTS – HIGH FRICTION SURFACE 

TREATMENTS............................................................................................. 92 

APPENDIX H: UTILITY POLE MANAGEMENT IN NEW JERSEY ..... 94 

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

LIST OF TABLES 

Table 1. MUTCD requirements for center line markings on paved two-way streets. .................... 12

Table 2. Table 2C-4 from the MUTCD: Guidelines for Advance Placement of Warning Signs. ... 17

Table 3. Guideline for length (feet) of speed reduction markings segment in advance of curve 

(Katz, 2004)....................................................................................................................................................... 19

Table 4. Horizontal Alignment Sign Selection (MUTCD Table2C-5 Excerpt)................................... 27

Table 5. Typical Spacing of Chevron Alignment Signs (MUTCD Table 2C-6). .................................. 32

Table 6. Percent change in crashes relative to providing a 6-foot shoulder on rural two-lane 

roadway segments (Modified from HSM Table13-7). .............................................................................. 47

LIST OF FIGURES 

Figure 1. Photo. The MUTCD provides standards and guidance for installation, placement, and 

use of traffic control devices. .......................................................................................................................... 3

Figure 2. Photo. The Highway Safety Manual provides road safety fundamentals and management

as well as crash prediction methods. Source: AASHTO. ......................................................................... 4

Figure 3. Map. Severe (K and A) roadway departure crashes at curves over a five year period in 

Minnesota’s southeastern District. The wide dispersion of crashes indicates that a traditional site 

analysis/spot location approach will not sufficiently address this type of severe crash. 

Source: Minnesota Crash Mapping Analysis Tool, MnDOT..............................................7 

Figure 4. Photo. Delineation with chevrons. Source: MnDOT. .............................................................. 9

Figure 5. Photo. Centerline rumble strips.................................................................................................... 9

Figure 6. Photo. Adding narrow paved shoulders (2 feet) to existing top width. Includes rumble 

strips and Safety EdgeSM. Source: MnDOT. .................................................................................................. 9

Figure 7. Photo. Delineation with enhanced (6-inch) edge lines. ............................................................ 9

Figure 8. Photo. Center line for No Passing on horizontal curve. ....................................................... 11

Figure 9. Photo. Edge lines. ............................................................................................................................ 13

Figure 10. Photo. Roadway with 4-inch edge line. ................................................................................... 14

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Figure 11. Photo. Roadway with 8-inch edge line.........................................................14 

Figure 12. Photo. The color of delineators must match the color of the adjacent edge line.

Source: Texas Transportation Institute. ..................................................................................................... 15

Figure 13. Photo. Pavement marking for advanced curve warning

Figure 14. Illustration. Design Guidelines for Pavement Markings to Reduce Speeds at Horizontal 

....................................................... 16

Curves. Source: NCHRP Report 600. ....................................................................................................... 17

Figure 15. Photo. Optical speed bars are designed to reduce vehicle speed. Source: KLS 

Engineering ........................................................................................................................................................ 18

Figure 16. Photo. Retroreflective raised pavement marker (yellow for center line). 

Figure 19a. Illustration. An inverted profile where depressions are made every 1 inch along the 

...................... 20

Figure 17. Photo. Snowplowable Retroreflective raised pavement marker. ...................................... 20

Figure 18. Photo. An example of profiled pavement markings. Source: Caltrans. ............................ 22

stripe............................................................................................................23 

Figure 19b. Illustration. A raised profile where a thicker layer of thermoplastic is applied on 20-

inch centers along the stripe. ........................................................................................................................ 23

Figure 20. Illustration. Figure 2C-1 from the MUTCD. .......................................................................... 26

Figure 21. Photo. Advisory Speed Plaque. .................................................................................................. 29

Figure 22. Illustration. Excerpt from Figure 2C-1 of the MUTCD. ...................................................... 30

Figure 23. Photo. Before, without chevrons.. ........................................................................................... 31

Figure 24. Photo. After, with chevrons. Chevrons provide advanced alignment of a curve. ......... 31

Figure 25. Photo. Chevrons assist the driver in navigating curves. Source: Texas Transportation 

Institute. ............................................................................................................................................................. 32

Figure 26. Illustration. Typical installation location of one-direction large arrow signs on a 

horizontal curve. Source: MoDOT Engineering Policy Guide. .............................................................. 33

Figure 27. Photo. Before signage updates..................................................................34 

Figure 28. Photo. After signage updates....................................................................34 

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Figure 29. Photo. Doubling-up of the sign was used at this site because tree limbs partially 

blocked the right side sign. ............................................................................................................................ 35 

Figure 30. Photo. Stimulus photo illustrating enhanced chevron visibility. Source: Texas 

Transportation Institute.......................................................................................36 

Figure 31. Photo. Typical arrangement of signs and flashing beacons.................................37 

Figure 32. Photo. Speed Actuated Sign, Augusta, ME. ............................................................................. 38

Figure 33. Photo. Flashing Beacon on Warning Sign, Augusta, ME. 

Figure 42. Illustration. A typical cross section of a normal crown with the red and blue lines 

...................................................... 38

Figure 34. Photo. Texas System curve advisory speed limit sign. Camp County, Texas. ............... 39

Figure 35. Photo. Texas System curve advisory speed limit sign. Camp County, Texas. ............... 39

Figure 36. Photo. Example of Sequential Dynamic Curve Warning System...........................39 

Figure 37. Photo. Example of a Chevron sign on the ground. ............................................................... 40

Figure 38. Photo. Low Retroreflectivity sign example. ........................................................................... 40

Figure 39. Photo. Curve warning sign covered by foliage. ..................................................................... 40

Figure 40. Graph. Figure 17 from NCHRP 108, Guide for Pavement Friction. ................................ 42

Figure 41. Photo. HFST on a horizontal curve.......................................................................................... 42 

showing a typical cross section of a superelevated section....................................................................44 

Figure 43. Photo. Widening on the inside and outside of the curve.................................................... 46

Figure 44. Photo. Shoulder paving operation. ........................................................................................... 47

Figure 45. Photo. Pavement with and without the Safety EdgeSM. ........................................................ 48

Figure 46. Photo. Backfilling against newly installed Safety EdgeSM. ...................................................... 48

Figure 47. Photo. Milled Center Line Rumble Stripes. ............................................................................ 50

Figure 48. Photo. Milled Edge Line Rumble Stripes. Source: KYTC. ................................................... 50

Figure 49. Photo. Fixed objects (trees) located within clear zone....................................................... 52

Figure 50. Photo. Removing brush as part of clear zone maintenance. Source: Texas DOT. ....... 53

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Figure 51. Photo. Typical barnroof slope design. Source: Alaska DOT...............................54 

Figure 52. Photo. Barrier along inside and outside of horizontal curve.............................................. 55

Figure 53. Illustration. Figure 4.4 from the FLH Barrier Guide shows length of need (LON) on 

the outside of a horizontal curve hazard. .................................................................................................. 56

Figure 54. Photo. Retroreflective panels in the web of a W-beam. Source: Michigan DOT. ........ 57

Figure 55. Photo. Delineators held in place with post bolts and installed on post. ......................... 57

Figure 56. Photo. Delineators installed on post, nighttime view. Source: Michigan DOT. ............. 57

Figure 57. Photo. Delineated concrete barriers. Source: Michigan DOT. .......................................... 58

Figure 58. Photo. Delineated concrete barriers, nighttime view. Source: Michigan DOT. ............ 58

Figure 59. Photo. Dotted edge line extensions at an intersection within a curve............................ 59 

Figure 60. Photo. Pavement markings narrow the travel lane as the driver approaches the 

intersection.....................................................................................................60 

Figure 61. Illustration. Smooth lane narrowing typical design. .............................................................. 61

Figure 62. Photo. An example of a visual trap exists when a crest vertical curve blocks the view

of the upcoming horizontal curve (top photo). What appears to be a continuation of the road in 

the distance is actually an intersecting roadway in the midst of a curve (bottom photo). ............. 62

Figure 63. Photo. Sight distance is limited due to the intersection being inside the curve. ........... 63

Figure 64. Illustration. Limiting the growth of vegetation is important to maintain appropriate 

sight triangles. ................................................................................................................................................... 63

Figure 65. Photo. A dynamic warning sign alerts drivers in real time of other users in the 

roadway. ............................................................................................................................................................ 64

Figure 66. Illustration. Excerpt showing the design of superelevation from Figure 36-1.E of Illinois 

Figure 69. Illustration. Intersection with a skewed approach with an arrow indicating the driver’s

DOT’s Bureau of Design & Environmental Manual. Source: Illinois DOT. ........................................ 65

Figure 67. Photo. Intersection with paved approach. .............................................................................. 66

Figure 68. Photo. Intersection with aggregate scattered on paved roadway. .................................... 66

line of sight. ....................................................................................................................................................... 66

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Figure 70. Illustration. Comparison of potential conflict points between a traditional intersection 

and roundabout.................................................................................................67

Figure 71. Illustration. Typical skewed intersection within a curve....................................68 

Figure 72. Illustration. Re-aligning a skewed intersection to provide a perpendicular intersection.

............................................................................................................................................................................. 68

Figure 73. Illustration. Reconfiguring the intersection with a roundabout. ........................................ 68

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

ACRONYMS 

AADT Average Annual Daily Traffic 

ADT Average Daily Traffic 

AASHTO American Association of State Highway and Transportation Officials 

Caltrans California Department of Transportation 

CMF Crash Modification Factor 

DOT Department of Transportation 

FARS Fatality Analysis Reporting System 

FHWA Federal Highway Administration 

FLH Federal Lands Highway 

HFST High Friction Surface Treatment 

HSIP Highway Safety Improvement Program 

HSM Highway Safety Manual

ICWS Intersection Conflict Warning System 

IDOT Illinois Department of Transportation 

INDOT Indiana Department of Transportation 

ITE Institute of Transportation Engineers 

KYTC Kentucky Transportation Cabinet 

LED Light-Emitting Diode 

LON Length of Need 

MnDOT Minnesota Department of Transportation 

MoDOT Missouri Department of Transportation 

MUTCD Manual on Uniform Traffic Control Devices 

NCAT National Center for Asphalt Technology 

NCHRP National Cooperative Highway Research Program 

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

NHTSA National Highway Traffic Safety Administration 

NJDOT New Jersey Department of Transportation 

NYSDOT New York State Department of Transportation 

PC Point of Curvature 

PDO Property Damage Only 

PennDOT Pennsylvania Department of Transportation

ROR Run-Off-Road 

RPM Raised Pavement Marker 

RSA Road Safety Audit 

SEAHC Surface Enhancements at Horizontal Curves

SSP Safety Strateg Planic 

SV Superelevation Variance

usRAP United States Road Assessment Program 

xi 
CHAPTER 1. INTRODUCTION 

CHAPTER 1. INTRODUCTION 

SAFETY PROBLEMS AT HORIZONTAL CURVES 

In 2013, there were 5.7 million crashes reported in the United States, including 32,719 fatalities

and more than 2.3 million injuries (NHTSA, 2014). More than half of the 2013 fatalities occurred 

as a result of roadway departure crashes. Vehicles are more likely to leave the travel lane of a 

roadway where the roadway alignment changes direction. These locations are known as 

horizontal curves. 

A comprehensive, four-state study by Glennon et al. (1985) found that the average crash rate for 

horizontal curves on two-lane rural highways is three times higher than on tangent road 

segments. The authors also found that the average single-vehicle run-off-road crash rate was 

four times higher on horizontal curves than on tangent segments. The severity of roadway 

departure crashes on horizontal curves was also higher than roadway departure crashes on 

tangent segments. A more recent study by Hummer et al. (2010) found similar results. An 

analysis of North Carolina crash data found that curve collisions have more than three times the 

fatality rate of collisions on all roads statewide. One study on different combinations of 

horizontal and vertical curve alignments found that crash frequency increases with decreasing 

horizontal curve radius, decreasing horizontal curve length, increasing grade difference, and 

increasing percent grade (Bauer and Harwood, 2014).

A Guide for Reducing Collisions on Horizontal Curves (NCHRP Report 500, Volume 7) further 

illustrates the problem. The NCHRP Report 500, Volume 7, 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 twolane secondary highways, many of which are local roads. Approximately 76 percent of the curverelated 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. 

In 2012, a team comprised of the Federal Highway Administration (FHWA) Office of Safety, 

Office of Safety Research and Development, and Resource Center Safety and Design 

Technical Services developed Safe Roads for a Safer Future – A Joint Safety Strategic Plan (SSP). 

The vision presented in the SSP works “toward zero deaths and serious injuries on the 

Nation's roadways.” This publication provides agencies with information to help them

deploy the appropriate countermeasures on horizontal curves in support of this vision. 

PUBLICATION PURPOSE AND SCOPE 

The NCHRP Report 500, Volume 7, identified several strategies to address the specific safety 

problem at horizontal curves. These strategies meet one of the following two objectives:

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

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

centerline or leaving the roadway at a horizontal curve. 

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

curve.

Although the NCHRP Report 500, Volume 7, provides information about each strategy, 

transportation professionals indicated that a document providing practical information on where, 

when, and how to apply a safety countermeasure or design feature—including examples and 

costs—would be valuable to local road agencies. To respond to this need, the FHWA created 

the Low-Cost Treatments for Horizontal Curve Safety (McGee and Hanscom, 2006). There have 

been many advances in highway safety since that initial 2006 guide. The purpose of this 

publication is to serve as an update to the 2006 Low-Cost Treatments for Horizontal Curve Safety. 

The primary audience for this publication is local transportation agencies. 

An agency can apply a number of strategies or countermeasures to a single horizontal curve or a

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

engineering countermeasures that are relatively low-cost, such as signage and pavement 

markings. More moderate or higher cost treatments including varying degrees of infrastructure 

changes are also provided as appropriate, including superelevation, cross section, and shoulder 

adjustments. 

This publication presents summary information and is not meant to cover all aspects of an 

individual countermeasure in detail. Rather, this publication provides information specifically 

relating to lower volume two-lane roads and the agencies that manage them. It will help 

transportation agencies and their crews understand the available countermeasures and how to 

select and apply them. Where appropriate, and when information was available, this publication 

provides the following for each countermeasure: 

 Description: General description of the countermeasure.

 Design: Identification of which design elements or materials to use.

 Applications: How to apply the countermeasure(s).

 Effectiveness: A countermeasure’s effectiveness in improving safety.

 Relative Cost: Identification of the relative cost, such as low-cost (e.g. signs, pavement

markings), moderate cost, or high cost (e.g., changes to infrastructure).

 

ABOUT THE MANUAL ON UNIFORM TRAFFIC CONTROL DEVICES (MUTCD) 

References to the FHWA’s Manual on Uniform Traffic Control Devices (MUTCD) (FHWA, 2009) 

occur throughout this document (Figure 1). The MUTCD defines the standards for all traffic 

control devices that road managers install and maintain to help regulate, warn, and guide drivers

safely on the Nation’s roadways and streets, such as signs, signals, and pavement markings. All 

States are required to adopt either the Federal MUTCD (FHWA) or a State MUTCD that is in 

substantial conformance to the Federal MUTCD. Some States adopt the Federal MUTCD with a 

State Supplement. An agency should consult State laws regarding traffic control devices. 

2
CHAPTER 1. INTRODUCTION 

The MUTCD also defines conditions about what, where, and how 

to place or install a traffic control device. In different chapters of 

this publication you may see a countermeasure and the designation 

that the MUTCD states “shall be used.” Shall means something is a 

standard—a practice or device that is specifically required or

mandated—or, in the case of “shall not be used,” explicitly 

prohibited. The MUTCD may designate other countermeasures as 

guidance, which indicates 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 the Manual on Uniform 

Traffic Control Devices. 

Figure 1. Photo. The 

MUTCD provides standards 

and guidance for 

installation, placement, and 

use of traffic control devices. 

In addition to the traffic control devices required by the MUTCD 

(per “shall” statements), road agencies should consider installing 

other devices at horizontal curves, especially at curves that data or experience identify as having 

a safety problem. 

Agencies generally apply traffic control devices uniformly based on the sharpness of the curve. 

This uniformity provides drivers with a consistent message on which to base their driving 

expectations. The MUTCD provides specific recommendations and requirements for uniform 

application of these devices. Agencies may apply treatments to a single, problematic curve that 

has a history of crashes, or they may also choose to install countermeasures at curves with 

similar characteristics across the roadway network. These system-wide, preventative measures 

are known as systemic improvements, and are discussed further in Chapter 2. Any additional use 

should be based on the information and recommendations contained in an engineering study or 

an engineer’s judgment. Factors to consider include: 

 The difference between the posted speed limit and advisory speed.

 Geometric features of the curve including its length, radius, shoulders, and roadside

features.

 Available sight distance approaching and within the curve limits.

 Unexpected geometric features within the curve, such as an intersection, change in

grade, change in curve radius, or visual cues that violate driver expectations.

 A sudden change in alignment after many miles of consistently straight roadway.

 Traffic volume.

 Risk characteristics including crash frequency and crash severity.

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

ABOUT THE HIGHWAY SAFETY MANUAL AND CMF CLEARINGHOUSE 

Throughout this publication readers will also see reference to the 

American Association of State Highway and Transportation 

Officials’ (AASHTO) Highway Safety Manual (HSM) (AASHTO, 

2010) and the FHWA Crash Modification Factor (CMF) 

Clearinghouse. The HSM includes technical content related to 

road safety fundamentals, the road safety management process, 

and crash prediction methods for several roadway types. It also 

includes CMFs for many roadway geometric design elements and 

traffic control devices. When applicable, this publication uses 

safety effect estimates from the HSM when describing the benefits 

of a horizontal curve safety countermeasure. 

The CMF Clearinghouse is the largest collection of CMFs for 

geometric design elements and traffic control devices available in 

the United States. The CMF Clearinghouse employs a “star 

rating” system to indicate the quality of the CMF based on factors 

such as the evaluation method, sample size, and standard error. 

The star rating system ranges from “1” (least reliable) to “5” 

(most reliable rating). This publication provides the star rating for 

Figure 2. Photo. The

Highway Safety Manual 

provides road safety 

fundamentals and 

management as well as

crash prediction methods.

Source: AASHTO. each CMF discussed. Readers of this publication are encouraged 

to refer to the CMF Clearinghouse for specific horizontal curve safety issues not covered in this 

publication. 

INFORMATION IN THIS PUBLICATION 

The following considerations should be taken into account when reading and using the 

information contained in this publication: 

 The publication includes estimates of the effectiveness of the countermeasure in

reducing crashes where such evaluation information is available. However, agencies

should not expect to obtain exactly these crash reduction values at a specific location,

as the actual observed effectiveness of a countermeasure will vary from site to site.

 Some countermeasures included in this publication use supports or posts, such as signs,

which make them an obstacle that could be hit. The MUTCD states that roadside sign

supports in the clear zone shall be breakaway, yielding, or shielded with a longitudinal

barrier or crash cushion. Information on breakaway sign supports and the definition of

clear zone can be found at Breakaway Hardware.

PUBLICATION ORGANIZATION 

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

problems and identify appropriate countermeasures for problem curve sections. Applying these 

countermeasures will help agencies reduce roadway departure crashes and resulting injuries and 

fatalities. 

4 
 

CHAPTER 1. INTRODUCTION 

The rest of this publication is organized into the following chapters: 

Chapter 2: The Two Components of Safety Improvement: Site Analysis and 

the Systemic Approach 

Chapter 3: Markings 

Chapter 4: Signs 

Chapter 5: Pavement Countermeasures 

Chapter 6: Roadside Improvements 

Chapter 7: Addressing Intersections in Curves 

References 

Glossary 

Appendices – Case Studies 

Appendix A: Low-Cost Safety Improvements in Pennsylvania

Appendix B: Systemic Improvements in Minnesota 

Appendix C: Application of Edge Lines in Missouri 

Appendix D: Upgrading Curve Signing in Ohio 

Appendix E: Application of Sequential Dynamic Curve Warning Systems 

Appendix F: Application of High Friction Surface Treatment in Kentucky 

Appendix G: Every Day Counts – High Friction Surface Treatments 

Appendix H: Utility Pole Management in New Jersey 

5 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

CHAPTER 2. THE TWO COMPONENTS OF SAFETY 

IMPROVEMENT: SITE ANALYIS AND THE SYSTEMIC APPROACH 

The most effective safety improvement process has two components: 

1. A site analysis component. Data analysis is used to identify locations where a clear

safety problem exists. Treatment of these locations may include higher-cost strategies.

This can be thought of as the reactive component of a safety program.

2. A systemic component. Data

analysis is used to identify risk factors

associated with a particular type of

severe crash and to identify locations

at higher risk. Normally, lower-cost

strategies are then deployed at a

larger number of these high-risk

locations. This can be thought of as

the proactive component of a safety

program.

Systemic Safety Improvement: 

An improvement widely implemented based 

on high-risk roadway features that are 

correlated with high severity crash types.

—Systemic Safety Project Selection Tool, FHWA 

The priority of a safety improvement program should be preventing fatal and serious injury 

crashes. In fact, the purpose of the Federal Highway Safety Improvement Program (HSIP) is 

stated in law as follows: 

The purpose of the highway safety improvement program shall be to achieve a significant 

reduction in traffic fatalities and serious injuries on all public roads, including non-Stateowned public roads and roads on tribal land. 

How a safety program and data analyses are focused—severe crashes versus total crashes—

influences the degree to which a particular safety problem is addressed with the systemic 

approach versus the more traditional site analysis approach. With a program where all crashes 

are used as the performance measure, high crash locations will be more prevalent and treatment 

strategies will tilt more heavily toward addressing high crash locations. In contrast, a program 

that uses severe crashes as the performance measure will use a stronger systemic component as 

severe crash locations are not as concentrated.

This is particularly true for severe roadway departure crashes, which tend to be highly scattered 

across the rural and local roads system (see Figure 3). This does not mean that severe crashes 

are random. They tend to be overrepresented at locations with high risk characteristics, 

horizontal curvature being one of those. 

A safety improvement process should include both components: treating high severe crash 

locations where they exist as well as systemically addressing locations or segments at higher risk. 

Both components will provide optimal results with good data and data analysis. FHWA’s Systemic 

Safety Project Selection Tool (2013) provides analytical techniques for determining a reasonable 

6 
 

 

CHAPTER 2. THE TWO COMPONENTS OF SAFETY IMPROVEMENT: SITE ANALYSIS 

AND THE SYSTEMIC APPROACH 

balance between the implementation of spot safety improvements and systemic safety 

improvements.

Figure 3. Map. Severe (K and A) roadway departure crashes at curves over a five year period in 

Minnesota’s southeastern District. The wide dispersion of crashes indicates that a traditional site 

analysis/spot location approach will not sufficiently address this type of severe crash. 

Source: Minnesota Crash Mapping Analysis Tool, MnDOT. 

ALL PUBLIC ROADS—ADDRESSING THE LOCAL SYSTEM 

In addition to a focus on prevention of fatal and serious injury crashes, the most effective safety 

programs also consider all public roads, including those under local jurisdiction. 

In most States, an examination of crash data 

demonstrates that focusing safety investment 

only on higher-level facilities such as the 

Interstate System and State highways will not 

sufficiently address the severe roadway 

departure crashes most prevalent on 

horizontal curves. 

Importance of the local system: 

From 2010–2012, approximately 39 

percent of fatalities in the United States 

occurred on the local system of roads.

—Fatality Analysis Reporting System

The Fatality Analysis Reporting System (FARS) is a national database of fatal highway crashes in 

the United States. FARS does not break down highway fatalities by State versus local jurisdiction, 

but the “Route Signing” field can provide a useful approximation of the magnitude of the fatal 

crash problem that occurs on the local system of roads. Using this method, FARS data suggests 

that from 2010–2012 approximately 39 percent of fatalities in the United States occurred on the 

local system. There were 39 States with 30 percent or higher fatalities on local roads and 

streets. 

7
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

To most effectively improve safety at horizontal curves, it is important to analyze data on both 

the State and local systems. Spot locations where severe crashes are concentrated are even less

common on the local system, and data analysis of the complete roadway network will add 

further support for including a strong systemic component. 

The Pennsylvania Department of Transportation (PennDOT) experienced a higher

percentage of fatal curve-related crashes on rural roads due to the predominance of 

horizontal curves. To combat this, PennDOT identified priority curves by examining crash 

frequency, crash rate, and crash severity. PennDOT then systemically implemented curve 

improvements, such as oversized fluorescent yellow advanced curve warning signs, 

advanced curve pavement markings, correction of any shoulder drop offs within the curve, 

chevron delineation, and curve widening. A three-year before/after analysis of locations 

where a combination of these countermeasures were implemented between 2000 and 

2008 resulted in the following: 

 17-percent reduction in overall crashes.

 44-percent reduction in fatal crashes.

 40-percent reduction in major injury crashes.

See Appendix A for more information. 

A CHANGE IN MINDSET 

For some agencies, improving safety at spot locations on State highways has been the traditional

approach, making up the bulk of safety improvement projects. Shifting to a systemic approach to 

prevent severe roadway departure crashes at curves along all public roads may require a change 

in mindset.

Determining answers to the following questions through data analysis is an effective first step in

the process:

1. Are there a large number of severe (fatal and serious injury) crashes scattered widely

across the system? If possible, plot them spatially as shown in Figure 3.

2. Are the types of safety improvement projects funded in relative alignment with the

findings of question 1? If mostly high-cost projects at high-crash locations are being

implemented, does this approach align with how severe crashes are located and

dispersed?

3. What is the distribution of severe crashes on the State system as compared to local

system? If there are severe crashes occurring on the local system, are safety funds made

available to local agencies so that severe crashes are reduced on all public roads?

8
CHAPTER 2. THE TWO COMPONENTS OF SAFETY IMPROVEMENT: SITE ANALYSIS 

AND THE SYSTEMIC APPROACH 

In 2010, the Minnesota Department of Transportation (MnDOT) began incorporating a 

systemic approach in all of Minnesota’s counties after many years of exclusively using the 

traditional reactive approach to safety. MnDOT performed a network screening on their 

horizontal curves and found five risk-factors associated with high-crash curves. MnDOT 

addressed these curves with countermeasures such as center line rumble strips, advanced 

signing, 2-foot shoulder paving with Safety EdgeSM and edge line rumble strips, 6-inch edge 

line, and most commonly, chevrons. MnDOT has seen a drop in roadway departure crashes 

because of these efforts. See Appendix B for more information. 

Figures 4 through 7 are examples of low-cost systemic treatments that can be applied to a large 

number of high-risk curve locations. 

Figure 4. Photo. Delineation with 

chevrons. Source: MnDOT. Figure 5. Photo. Centerline rumble strips. 

Figure 6. Photo. Adding narrow paved 

shoulders (2 feet) to existing top width. 

Includes rumble strips and Safety EdgeSM. 

Source: MnDOT. 

Figure 7. Photo. Delineation with enhanced 

(6-inch) edge lines. 

9 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

ADDITIONAL RESOURCES 

More information on the systemic approach to safety is available at the following resources: 

● A Systemic Approach to Safety – Using Risk to Drive Action. FHWA Office of Safety website

on the systemic approach.

● The Systemic Safety Project Selection Tool. FHWA guide that includes a step-by-step process

for conducting systemic safety analysis; analytical techniques for determining a reasonable

balance between the implementation of spot safety improvements and systemic safety

improvements; and, a mechanism for quantifying the benefits of safety improvements

implemented through a systemic approach.

● The United States Road Assessment Program (usRAP) is a validated system of protocols

for rating roadway segments for safety. Using video logs coded in 100-meter segments,

usRAP produces a proactive safety investment plan based on the observed design

features of the road. usRAP’s predictive modeling ensures that highway authorities can

make data-driven safety management decisions—even before deaths and injuries occur,

or in the absence of crash data. FHWA cites specific tools, such as usRAP, as an example

of ways to implement safety analysis approaches, not as an endorsement of usRAP over

others. The Roadway Safety Foundation is the primary supporter of usRAP. Contact the

Roadway Safety Foundation at info@roadwaysafety.org.

10 
CHAPTER 3. MARKINGS 

CHAPTER 3. MARKINGS 

Curve delineation can be critical for a driver to navigate roadways successfully. This is of 

particular importance for nighttime driving, when cues to changes in alignment (such as trees and 

guardrail) may not be readily visible. Pavement markings located within the driver’s focus provide 

continuous information to help drivers position their vehicles in the roadway correctly. As such, 

pavement markings provide a good first option for horizontal curve countermeasures on paved 

roadways of sufficient width. Delineators—a retroreflective device mounted on a post or 

roadside barrier along the side of the roadway—also provide the driver with visual cues to the

roadway alignment, although they are point sources and placed slightly outside the main focus 

area. Placing other markings (especially to reduce speeds) on the pavement in advance of the

curve can also help drivers successfully negotiate curves. 

The following discussion provides information on pavement marking countermeasures, marking 

material options, and maintenance and cost considerations. All example markings are from the 

MUTCD unless otherwise noted. 

LONGITUDINAL PAVEMENT MARKINGS 

Center lines and edge lines are the primary types of longitudinal pavement markings on two-lane 

roads. They delineate the travel lane for the driver, assist in lane placement to avoid collisions 

with other vehicles, and provide a preview of changing roadway alignment. Unless center lines

and edge lines are required for the entire roadway, the MUTCD permits their use at specific 

locations, such as around a curve. However, since markings typically have a shorter life-span than

many other devices, particularly at curves where vehicles cross these lines more frequently, the

life-cycle cost of markings should be considered before installing them only at curves. 

The MUTCD Sections 3A.05 and 3A.06 provide guidance on colors, widths, and patterns for

longitudinal pavement markings. This publication, which focuses on two-lane roads, will highlight 

yellow markings used to delineate the separation of traffic traveling in opposite directions and 

white markings used to delineate the right-hand edge of the roadway. Normal line widths are 4 

to 6 inches and wide lines are at least twice the width of a normal line. 

Center Line 

When considering pavement markings to delineate 

a curve, the center line is usually the first 

countermeasure to apply (Figure 8). A center line 

helps drivers keep their vehicles on the correct 

side of the road (MUTCD Sections 3B.01 and 

3B.02). 

Figure 8. Photo. Center line for No Passing on 

horizontal curve. 

11 
 

 

 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Design 

For any section of two-way, two-lane roadway, where passing is allowed in both directions, the

basic center line marking is a broken, or dashed, yellow line. On some curves where 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, a solid 

yellow line is used to inform motorists of the no-passing regulation where the restriction exists 

for either direction of travel. For segments where passing is prohibited in both directions, a solid

yellow line is used for both directions (i.e., double line pattern), separated by a gap 

approximately equal to the width of a normal line. To be effective at night when approximately 

half the crashes occur, the pavement marking must be retroreflective.

Applications 

Table 1 summarizes MUTCD criteria for center line markings of two-way roads. 

Table 1. MUTCD requirements for center line markings on paved two-way streets. 

Area type Road Class Lanes 

Avg. Daily 

Traffic 

(ADT) 

Travel 

Width 

(feet) 

REQUIRED 

Urban Collectors Arterials 2 6,000 + 20+

All All 3+

RECOMMENDED 

Urban Collectors Arterials 2 4,000+ 20+

Rural Collectors Arterials 2 3,000+ 18+

MAY CONSIDER Any Any 2 Any 16+

Adding center lines at curves, even if it not required or recommended per the MUTCD, is worth

considering as these center lines provide delineation of the change in alignment to the driver.

For example, in some Minnesota counties, severe crashes were over-represented on curves with 

ADTs between 400 and 1000 vehicles per day. This would indicate there may be a benefit to 

striping curves in that range of volumes, even though it is lower than what is required or 

recommended by the MUTCD. Also, the Speed Typology Study (Council et al., 2010) indicated 

most speed-related crash problems on horizontal curves are on collector and local routes. 

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

place center line markings on travel ways less than 16 feet 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.” Placing a center line marking only in the curve may be 

appropriate, but it must follow the design parameters to indicate appropriate passing 

requirements, as the MUTCD states that “a single solid yellow line shall not be used as a center

line marking on a two-way roadway.” 

Therefore, the addition of this pavement marking should be the first countermeasure 

considered, at a minimum, when an agency identifies a curved section of roadway as a potential 

12
CHAPTER 3. MARKINGS 

safety problem and the road segment does not have a center line. When the curve carries a low 

traffic volume (fewer than 400 vehicles per day), the pavement is less than 16 feet wide, or it is 

an unpaved road, consider using post delineators, chevrons, or curve warning signs in lieu of a

center line.

Effectiveness 

A variety of studies exist on center lines, but none of these are specific to the application of 

center lines placed only in horizontal curves. One study suggests that there is a connection 

between improved retroreflectivity of the yellow center line and reduced crashes on rural twolane roads (Carlson et al., 2013). The CMF Clearinghouse lists two reduction factors for the 

installation of center lines on roadway segments (both CMFs are 3 stars), with a 1-percent 

decrease for injury crashes and a 1-percent increase for property damage only (PDO) crashes 

(Elvik and Vaa, 2004). However, significant reductions in serious and minor injury crashes can 

also be realized: 

 45-percent reduction when center lines, edge lines, and delineators are installed on

roadway segments (CMF is 4 stars) (Elvik and Vaa, 2004).

 14-percent reduction when center lines and edge lines are installed at sites with higher

incidences of crashes (CMF is 3 stars) (Al-Masaeid, 1994).

 24-percent reduction when center lines and edge lines are installed on rural highways

(CMF is 4 stars) (Elvik and Vaa, 2004).

Relative Cost:

This countermeasure is low cost. 

Edge Line

The edge line pavement marking defines or 

delineates the edge of a roadway (MUTCD, 

Sections 3B.06 and 3B.07) (Figure 9). It provides a 

visual reference to guide motorists and helps 

reduce drifting onto the shoulder and roadside 

area. Edge lines provide the added benefit of 

guidance away from the glare of oncoming 

headlights. When used with the center line or 

adjacent lane line for a multilane road, it defines

the travel lane for the road user. 

Design 

Edge line markings are a solid white line at the right edge of the travel lane for undivided roads. 

Figure 9. Photo. Edge lines. 

13 
 

 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Applications 

The MUTCD requires edge lines not only for 

freeways and expressways, but also for 

“...rural arterials with a traveled way of 20 

feet or more in width and an ADT of 6,000 

vehicles per day or greater.” Edge lines are 

recommended for “...rural arterials and 

collectors with a traveled way of 20 feet or 

more in width and an ADT of 3,000 vehicles 

per day,” and any other paved roadways where 

an engineering study identifies a need for edge 

The Missouri Department of 

Transportation (MoDOT) has found that 

applying an edge line to two-lane rural 

roads with AADT between 400 and 1,000 

vehicles per day has reduced total crashes 

for all crash types by 15 percent. The study 

also found that the application has reduced 

severe crashes by 19 percent. See 

Appendix C for more information. 

line markings. Edge lines may be used on other roadways, with or without center lines, based on

engineering judgment, but the risk of head-on crashes where center lines are not used must be 

considered. 

As with the center line, edge lines may be applied just prior to and within the curved section, as

delineation of the curve is usually more critical than for tangent sections. Also, the edge line 

width can be increased to provide a better visual perspective of the roadway. Figure 10 and 

Figure 11 show the same stretch of road with a 4-inch edge line and with an 8-inch edge line, 

respectively. As can be seen in these photos, the curve of the road is better delineated in Figure 

11. 

Figure 10. Photo. Roadway with 4-inch edge line. Figure 11. Photo. Roadway with 8-inch edge line. 

Effectiveness 

Several studies indicate that using edge lines results in crash reductions on two-lane roads. The 

CMF Clearinghouse lists five reduction factors from a single study for the installation of edge 

lines on curves, with a 26-percent or 33-percent reduction for all crash types in rural 

applications (CMF is 3 stars). The larger reduction was for 9-foot lanes while the smaller 

included lanes up to 11 feet wide. Specifically for run-off-road crashes, reductions of 11-percent 

were found for urban applications (CMF is 3 stars) and 13-percent for rural applications (CMF is 

2 stars). Furthermore, the CMF Clearinghouse lists a 4-percent reduction for speed-related 

crashes in rural applications (CMF is 2 stars) (Tsyganov, 2009). A 2012 study shows that 5-inch 

or 6-inch line widths provide safety benefits on rural two-lane roads beyond the more typical 4-

inch width (Park et al., 2012). Although the 2012 study’s findings are not specific to horizontal 

curves, the safety benefits realized on tangent segments can also potentially also apply to curves.

14
CHAPTER 3. MARKINGS 

Relative Cost

This countermeasure is low cost. 

OTHER LONGITUDINAL DELINEATION 

Delineators may also be used to communicate the roadway alignment to the driver. Delineators 

can be used on roads that are not paved and play a slightly different role than pavement 

markings since they are placed outside the edge of the traveled way.

Delineators 

A delineator is a retroreflective device mounted

above the roadway surface and along the side of 

the road in a series to indicate roadway 

alignment (Figure 12). Delineators are the only 

longitudinal guidance devices that can be used for 

unpaved roads. For paved or unpaved roadways, 

delineators are most effective at night and during

adverse weather when the roadway is wet or 

covered in snow. 

Design 

The retroreflective portion of the delineator is 

typically either a circular button reflector or a rectangular piece of sheeting with a minimum 

diameter/width of 3 inches. They are usually mounted on flexible or lightweight breakaway posts

4 feet above the pavement, except where barriers are present. For more information on 

delineators on barriers, see Chapter 6. 

Chapter 3F of 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, which means that if delineators are used on the left and/or right side of the 

road they must also be white. 

Applications 

Delineators should be placed 2 to 8 feet outside the outer edge of the shoulder, or if

appropriate, in line with the roadside barrier. The delineators should be placed at a constant 

offset, to appropriately reflect the alignment. An exception to this occurs where an obstruction 

is between the pavement edge and the line of delineators. In this case, the line of delineators 

must be transitioned to be within the innermost edge of the obstruction. Delineators should be 

spaced 200 to 530 feet apart on mainline tangent sections. On approaches to and within curves, 

they should be spaced as recommended in Table 3F-1 of the MUTCD. The goal on curved 

alignments is to have several delineators simultaneously visible to the driver to show the 

direction and sharpness of the curve. 

Figure 12. Photo. The color of delineators must 

match the color of the adjacent edge line. 

Source: Texas Transportation Institute. 

15 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Effectiveness 

There is no published research documenting the safety effects of installing delineators specifically 

on horizontal curves as of yet. One study has shown a 4-percent increase in injury crashes and a 

5-percent increase in property damage crashes when delineators are installed in rural areas

(both CMFs are 3 stars) (Elvik and Vaa, 2004). However, the same study showed that the

combination of center lines, edge lines, and delineators led to a 45-percent reduction of injury

crashes on all roadway types (CMF is 4 stars) (Elvik and Vaa, 2004).

Relative Cost

This countermeasure is low cost. 

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

the material used for the post and reflector. These devices also need to be maintained, but due 

to their location, do not typically require the significant traffic control (such as lane closures) 

that is often required to maintain pavement marking and raised pavement markers (RPMs). 

ADVANCE MARKINGS FOR CURVES 

Pavement markings in advance of horizontal curves provide highly conspicuous, supplementary 

warning information and the potential to increase safety. The two pavement markings options 

discussed in this section are especially important for reducing speeds at curve locations where 

signs have proved ineffective. 

Speed Advisory Marking in Lane 

Advisory speed warnings provide essential 

information directly related to drivers’ safe 

negotiation of curves. The marking 

supplements the curve warning sign with 

advisory speed plaque, by providing the same 

information in the driver’s direct line of sight, 

emphasizing the message to the driver. 

An example speed advisory pavement 

marking is illustrated in Figure 13. In this 

example, the markings display is “CURVE—55—MPH.” An arrow symbol may be substituted for 

the word “CURVE” and provides the driver additional information if the curve is not visible due 

to alignment issues such as a crest vertical curve.

Design 

Refer to Section 3B.20 of the MUTCD for design and application criteria. 

Elongated letters as specified in the Pavement Markings section of Standard Highway Signs and 

Markings are required. The elongation makes the letters easier to read by approaching 

Figure 13. Photo. Pavement marking for advanced 

curve warning. 

16
 

 

 

 

 

 

 

 

Goal 

Reduce 

speeds in 

horizontal 

curves 

Design Guidelines 

Do this: Do not do this: 

Curve arrow "Curve 55 mph"text Transverse lines "Curve Ahead"text 

and"50 mph"text 

CHAPTER 3. MARKINGS 

motorists. Pavement marking word messages are designed with the first word in the message 

placed closest to the approaching driver. NCHRP Report 600 contains design guidelines as to 

which markings are effective in reducing speeds at horizontal curves and which markings are not 

as effective (p. 20-4) (see Figure 14). 

Figure 14. Illustration. Design Guidelines for Pavement Markings to Reduce Speeds at Horizontal Curves. 

Source: NCHRP Report 600. 

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 Table 2C-4 of the MUTCD shown in Table 2. 

Table 2. Table 2C-4 from the MUTCD: Guidelines for Advance Placement of Warning Signs. 

Posted or 

85thPercent 

ile Speed 

Advanced Placement Distance 

Condition A: 

Speed 

reduction and 

lane changing 

in heavy traffic 

Condition B: Deceleration to the listed advisory speed (mph) for the condition 

0 10 20 30 40 50 60 70 

20 mph 225 ft 100 ft N/A ─ ─ ─ ─ ─ ─

25 mph 325 ft 100 ft N/A N/A ─ ─ ─ ─ ─

30 mph 460 ft 100 ft N/A N/A ─ ─ ─ ─ ─

35 mph 565 ft 100 ft N/A N/A N/A ─ ─ ─ ─

40 mph 670 ft 125 ft 100 ft 100 ft N/A ─ ─ ─ ─

45 mph 775 ft 175 ft 125 ft 100 ft 100 ft N/A ─ ─ ─

50 mph 885 ft 250 ft 200 ft 175 ft 125 ft 100 ft ─ ─ ─

55 mph 990 ft 325 ft 275 ft 225 ft 200 ft 125 ft N/A ─ ─

60 mph 1,100 ft 400 ft 350 ft 325 ft 275 ft 200 ft 100 ft ─ ─

65 mph 1,200 ft 475 ft 450 ft 400 ft 350 ft 275 ft 200 ft 100 ft ─

70 mph 1,250 ft 550 ft 525 ft 500 ft 450 ft 375 ft 275 ft 150 ft ─

75 mph 1,350 ft 650 ft 625 ft 600 ft 550 ft 475 ft 375 ft 250 ft 100 ft 

17 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Applications 

This countermeasure does not have an established guideline; however, the MUTCD allows for 

use of word, symbol, or arrow markings to supplement signs as determined by engineering 

judgment. This application is probably more appropriate for higher speed roads where the curve 

advisory speed is significantly lower than the posted speed, curves where crash reports indicate 

speed-related issues, and corridors where speed studies indicate excessive speeding. 

Effectiveness 

Currently, the CMF Clearinghouse does not have a countermeasure listed for speed advisory 

markings in the lane. The NCHRP 600 Human Factors Guidelines for Road Systems found that the 

“Curve-55-MPH” text reduced speeds on a rural road by 4 mph (Chrysler and Schrock, 2005). 

In addition, the NCHRP 600 Human Factors Guidelines for Road Systems found that when the curve

arrow and “50 mph” text were tested on an urban roadway, vehicles significantly reduced their 

speeds by 10-percent at the entrance to the curve. There was also an 11- to 20-percent 

reduction in vehicles exceeding the speed limit (Chrysler and Schrock, 2005). Another study in 

the NCHRP 600 tested the a curve arrow with “SLOW” text on a suburban road and found it 

reduced the percentage of drivers exceeding the speed limit by more than 5 mph during the 

daytime and late night timeframes, but not during the evening (Retting and Farmer, 1998). 

Overall, after testing other combinations of symbols, words, and advisory speeds, the markings 

that provided advisory speeds or an action performed most effectively. 

Relative Cost

This countermeasure is low cost. 

Speed Reduction Markings (also known as Optical Speed Bars) 

Speed reduction markings are transverse stripes 

spaced at gradually decreasing distances (MUTCD, 

Section 3B.22). The rationale for using them is to 

increase drivers’ perception of speed and cause 

them to reduce their speed. As spacing between

bars gradually narrows, drivers sense they have 

increased speed and will slow down to keep the 

same time between each set of bars (Figure 15).

Durable marking materials are preferred because 

of the exposure to traffic volume over time. 

Design 

These white transverse stripes are only allowed 

where longitudinal lines are present on both sides 

of the lane. They must be installed perpendicular to the center, edge, or lane lines. They should 

be no greater than 12 inches wide, and should not extend more than 18 inches into the lane

Figure 15. Photo. Optical speed bars are 

designed to reduce vehicle speed. Source: KLS 

Engineering. 

18
 

CHAPTER 3. MARKINGS 

(FHWA, 2009). The spacing between the transverse markings are progressively reduced from the 

upstream to the downstream end of the marked portion of the lane, to produce a gradual 

slowing from a vehicle’s initial approach speed to the advisory curve speed. The goal is to 

achieve the slowing before the vehicle enters the curve.

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

higher approach speed and the lower curve speed. Table 3 suggests approximate lengths, which 

may be used as guidelines. The basis for the numbers is to produce a comfortable speed 

reduction.

Table 3. Guideline for length (feet) of speed reduction markings segment in advance of curve (Katz, 2004). 

Approach Speed, mph 

45 50 55 60 65 70 

Curve Speed, mph 

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

Applications 

Speed reduction markings should be reserved for unexpected horizontal or vertical curves or 

other feature where drivers need to decelerate in advance of the feature. The countermeasure 

should supplement, and not substitute for, appropriate warning signs and other traffic control 

devices. Agencies should avoid applying speed reduction markings to long roadway sections just 

to reduce traffic speed, because overuse could jeopardize the visual effect of the 

countermeasure. In addition, this countermeasure has been shown to be most effective in

locations with unfamiliar drivers. 

Effectiveness 

Speed reduction markings are currently not listed in the CMF Clearinghouse, but are discussed 

in three relevant studies. The first study, using data from New York, Mississippi, and Texas, 

suggests transverse pavement markings can effectively reduce average speeds, median speeds, 

85th percentile speeds, and speed variance before and after the curve both immediately after 

and in the long term (Katz, 2004). Speed reductions downstream of the curves varied from 0 to 

5 mph. The second study, looked at speeds in advance of, within, and at the end of a 0.37-mile 

segment on a two-lane road with inadequate vertical and horizontal curves (Arnold and Lantz, 

2007). Speeds decreased at all key locations, with statistically significant reductions of 1 to 3 

mph. Speed reductions were greater in the first 90 days after installation. The third study 

analyzed 19 sites in Alabama, Arizona, and Massachusetts, using 2 different markings designs. 

19 
Figure 17. Photo. Snowplowable 

Retroreflective raised pavement marker. 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

The study found minor effects on vehicle operating speeds that were inconsistent (Boodlal, 

2015). 

Relative Cost

This countermeasure is low cost. 

PAVEMENT MARKING MATERIALS, MAINTENANCE, AND COSTS 

Material Considerations 

Road agencies commonly use a variety of paint-based materials, or more durable materials such 

as thermoplastic, for center line and edge line markings. The specific material to be used may 

depend on an agency’s equipment or willingness to contract the work. Transverse markings, 

such as word, symbol, and arrow markings or the speed reduction markings discussed above are 

typically applied with a durable marking due to the amount of wear to which they are subjected 

by their placement within the lane. Additional information regarding various marking materials is 

available in the ITE Traffic Control Devices Handbook, Chapter 9: Pavement Markings and Markers. 

Other materials used by agencies to improve visibility of longitudinal lines include a variety of 

RPMs and “profiled” markings. These markings are particularly beneficial under wet nighttime 

conditions, where the retroreflectivity of the normal, flat markings is obscured. Note that the 

MUTCD recommends against use of RPMs 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. Chapter 5 also 

discusses the use of rumble stripes as a supplement to center line and edge line pavement 

markings.

RPMs 

In many situations, agencies will install RPMs to supplement or substitute for center line or lane 

lines. There are a variety of types, including reflective, non-reflective, and internally illuminated 

versions. Figure 16 and Figure 17 provide examples of RPMs. In geographic areas where snow is 

common, the reflective device is encased in an iron casting and recessed below the pavement 

surface in a grooved section to prevent damage by snow plows, as shown in Figure 17. The RPM 

color must conform to the color of the line for which it is used. RPMs may be used as vehicle 

Figure 16. Photo. Retroreflective raised 

pavement marker (yellow for center line). 

20
CHAPTER 3. MARKINGS 

positioning guides, to supplement or substitute for certain markings, as described in MUTCD 

Sections 3B.11 through 3B.14. 

Agencies typically apply the markers within a long roadway section, which is advantageous as it 

provides a longer visual range of delineation for motorists, especially at night (if retroreflective) 

and during wet conditions. RPMs also work well when applied to a single curve or a winding 

section of roadway. However, on curves the spacing must typically be reduced to adequately to

show the alignment. The RPMs may also provide an auditory warning to the motorist who drives

over them.

Effectiveness 

While studies of the operational effects of RPMs have shown they can reduce the variation in 

lane placement and move vehicles away from the center line, studies of crash effects have 

produced mixed results. A study in the HSM indicates a 24-percent crash reduction on two-lane 

roadways with gentle curvature (less than 3.5 degrees) and relatively high volumes (greater than 

15,000 vehicles per day) (CMF is 4 stars), but increased crashes for roadways with sharper 

curvature (greater than 3.5 degrees) regardless of the volume conditions (Bahar et al., 2004). It 

has been hypothesized that the increased crash frequency results from higher speeds because 

motorists feel safer with the RPMs providing alignment information even under wet nighttime 

conditions. 

The same study also indicates that installing snowplowable, permanent RPMs on rural highway 

horizontal curves with radii less than 1,640 feet leads to an increase in total nighttime crashes of 

3 percent to 43 percent, depending on traffic volumes. For rural horizontal curves with radii 

greater than 1,640 feet, use of snowplowable, permanent raised pavement markers leads to a 

16-percent increase in total nighttime crashes for AADTs less than 5,000 vehicles per day (CMF

is 5 stars), and a 1-percent to 24-percent decrease in total nighttime crashes for AADTs greater

than 5,000 vehicles per day (CMFs are 3 to 4 stars) (Bahar et al., 2004).

Relative Cost 

While more expensive than standard paint and thermoplastic markings, this countermeasure is 

low cost. 

For RPMs, material costs vary significantly depending on whether the device includes the metal 

casing to make it snowplowable. The epoxy used to adhere the devices to the pavement must 

also be figured into the materials cost. However, the labor costs are often the more significant 

factor, particularly if the devices need to be recessed into the pavement. In considering the life 

cycle cost for RPMs, agencies should also factor in inspection and repair of the devices. 

21 
 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Profiled Pavement Markings 

Agencies apply thermoplastic markings to create a 

profile marking, which enhances the visibility of the 

marking, particularly in nighttime conditions, and may 

also produce a slight rumble effect. Figure 18 shows an 

example of a profiled pavement marking. 

A few agencies have used this countermeasure 

successfully, but no research at this time provides 

definitive results. As such, no rating for these markings 

are included in the CMF Clearinghouse. As snow 

plowing can destroy this marking, its use is typically 

limited to non-snow zone locations. The California 

Department of Transportation (Caltrans) has used two types—raised and inverted profile 

patterns, as shown in Figures 19a and 19b. The Oregon Department of Transportation has also 

used profiled markings both on top of the road surface (smooth or patterned) and recessed in 

the pavement surface.

Maintenance Considerations 

Regardless of the marking countermeasures used, road agencies should consider maintenance 

needs when deciding what countermeasure(s) to use and the most cost-effective materials for 

the job. Maintenance activities should be carried out on a regular basis to ensure continued safe 

travel.

Pavement markings need to be restriped as they lose their visibility over time. Specifically, 

pavement markings that are made of paint-based materials have a relatively short service life—

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

installation quality, climate, and traffic volume. Markings of thermoplastic material will last 

approximately twice as long as paint-based materials. To maintain their effectiveness, pavement 

markings must be visible, especially at night and during other conditions of limited visibility. An 

agency’s regular inspection and restriping programs are critical to ensuring pavement markings 

provide needed visibility for motorists. 

Like pavement markings, pavement markers are subject to wear. RPMs should be inspected, and

repaired or replaced regularly. The retroreflective lenses degrade or may be cracked by traffic, 

and adhesion of the marker may be lost. This is particularly important with snowplowable RPMs, 

as there have been reports of isolated incidents of loose RPMs becoming projectiles. The heavy 

casings of these RPMs have been reported to cause some serious injuries and few agencies have 

chosen to no longer use these devices. 

Figure 18. Photo. An example of 

profiled pavement markings. Source: 

Caltrans. 

22 
PROFILE I 

Existing 

Pavement 

0.35" (10 mm) Minimum 20" (500 mm) 0.08" (2 mm) Minimum 

0.50'' (15 mm) Maximum 11 lf 0.12" (3 mm) Maximum 

f 20" (500 mm) 

I PROFILE I 

1 1 1.75" (45 mm) to 2.50" (65 mm) 

4" 1 ~--~---~~---~ 

(100mm) { 1111 ~~ 1111 } i ~-~~-IP-LA_N_I~~---~ 

CHAPTER 3. MARKINGS 

Figure 19a. Illustration. An inverted profile where depressions are made every 1 

inch along the stripe. 

Figure 19b. Illustration. A raised profile where a thicker layer of thermoplastic is 

applied on 20-inch centers along the stripe. 

Cost Considerations 

The cost to apply pavement markings varies, depending on several factors. When applying 

pavement markings, an agency should consider the life-cycle cost. For example, paint has a lower

initial cost than most durable materials, but paint likely needs to be reapplied every year. In 

comparison, a thermoplastic or epoxy material is likely to last for several years but is more 

expensive in initial cost. Life cycle cost needs to include or consider the cost of the materials, 

and the cost of the labor and traffic control needed to perform maintenance or replacement. 

The life of pavement markings varies greatly between agencies that need to plow their roads and 

those who do not. 

Other cost considerations include whether the agency’s own crew or a contractor performs the 

work and the number of horizontal curve locations that need markings. Markings placed in the 

lane tend to wear more quickly than longitudinal markings, and longitudinal markings on curves 

tend to wear more quickly than those on tangent sections. Due to these factors, and the

changing cost of materials, it is not feasible to provide a cost estimate in this publication. 

23
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

However, the MoDOT conducted a benefit/cost evaluation of their pavement markings program, 

which showed an 11-percent reduction in fatal and injury crashes and an 11.2 benefit-cost ratio 

(Potts et al., 2011). 

24
CHAPTER 4. SIGNS 

CHAPTER 4. SIGNS 

There are several signing options that road agencies should consider installing at a horizontal 

curve, especially curves with attributes that data or experience identify as potentially 

problematic. 

Agencies should apply signing devices uniformly, based on the sharpness of the curve. This 

uniformity provides drivers with a consistent message on which to base their expectations. The 

MUTCD provides specific recommendations and requirements for uniform application of many 

of these basic devices. The MUTCD requires that the use of warning signs shall be based on an 

engineering study or engineering judgment. Factors to consider include:

 The difference between the posted speed limit and recommended advisory speed.

 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, change in

grade, change in curve radius, or visual cues that contradict the roadway alignment.

 A sudden change in alignment after many miles of consistently straight roadway.

 Traffic volume.

 Crash data.

Many curves need only the basic horizontal alignment warning signs. The decision to add one or 

more of the other basic or enhanced treatments at a specific curve will be influenced by the 

factors noted above, but should be prefaced by an assessment at the system and corridor level. 

The assessment may reveal unnecessary devices that should be removed, improperly placed 

devices that should be moved, or required or recommended devices that are missing. Providing 

uniformity may be all that is necessary to address an identified safety concern. If the problem is 

not resolved by using a uniform application, then additional devices should be considered.

The following discussion provides a summary of basic and enhanced signage, followed by a 

discussion on maintenance considerations applicable to signs. All example signs are from the 

MUTCD. 

BASIC SIGNING COUNTERMEASURES 

Advance Warning Signs 

Warning signs call attention to unexpected conditions on or adjacent to a roadway. The MUTCD 

prescribes several Horizontal Alignment signs to give drivers advance warning of a horizontal 

curve, as illustrated in Figure 20. 

25
 

Figure 2C-1 . Horizontal Alignment Signs and Plaques 

W1-1a W1•2 W1•20 W1•3 

Wl-5 Wl-8 < Wl-8 Wl-10 Wl-108 W1•10b 

W ·10c W1-10d W1-10e Wl-11 Wl -13 

EXIT RAMP 

0 0 EXIT RAMP 

35 25 35 25 25 MPH M PH MP H MPH MPH 

Wl-15 W13-1P W13-2 W13-3 W13-6 W13-7 

e: Tum rrows nd r rselUmarr may be subs Med lor he wrve rrows and reverse wrve arr on lhe 

W 1-10 sorios s ns whor approp,1a . 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

For a single curve section, use one of these four signs in advance of the curve:

 Turn (W1-1).

 Curve (W1-2).

 Hairpin Curve (W1-11).

 270-degree Loop (W1-15).

For sections with more than one curve in close proximity, use one of these three warning signs 

in advance of the first curve:

 Reverse Turn (W1-3).

 Reverse Curve (W1-4).

 Winding Road (W1-5).

26

Figure 20. Illustration. Figure 2C-1 from the MUTCD. 
Type of Horizontal 

Alignment Sign

Difference Between Speed Limit and Advisory Speed

CHAPTER 4. SIGNS 

Design and Application 

The MUTCD requires the use of a warning sign be based on an engineering study or engineering 

judgment, but the MUTCD also has specific requirements for warning signs based on traffic 

volume and curvature. For freeways, expressways, and roadways with more than 1,000 AADT 

that are functionally classified as arterials or collectors, refer to Table 2C-5 from the 2009 

MUTCD (shown in Table 4) to determine those signs that are required or recommended for use 

based on the difference in posted speed limit and advisory speed. This table provides uniform 

guidelines for placement of not only advance warning signs but also chevrons, which provide a 

consistent message to the driver if applied uniformly. The criteria in Table 2C-5 may also be 

used for local roads and those with less than 1,000 ADT, based on engineering judgment. 

Warning signs sizes should follow MUTCD Tables 2C-2 and 2C-3, and the signs should be 

located per Table 2C-4 (MUTCD, Section 2C). Further information on design and application is 

discussed under the individual devices. 

Table 4. Horizontal Alignment Sign Selection (MUTCD Table 2C-5 Excerpt). 

5 mph 10 mph 15 mph 20 mph 25 mph 

or more 

 Turn (W1-1)

 Curve (W1-2)

 Reverse Turn (W1-3)

 Reverse Curve (W1-4)

 Winding Road (W1-5)

 Combination Horizontal

Alignment / Intersection

(W1-10)

(see MUTCD, Section 2C.07 

to determine which sign to 

use) 

Recommended Required Required Required Required

 Advisory Speed Plaque

(W13-1P) Recommended Required Required Required Required

 Chevrons (W1-8)

 One Direction Large

Arrow (W1-6)

Optional Recommended Required Required Required

Note: “Required” indicates that the sign and/or plaque shall be used, “Recommended” indicates that the sign and/or plaque 

should be used, and “Optional” means that the sign and/or plaque may be used. 

For horizontal curves where a Horizontal Alignment sign is not required or recommended, 

engineering judgment should be applied to determine whether a sign is needed. For instance, a 

roadway with center line and edge line pavement markings, where the alignment change is not 

unexpected and where there is no crash history, may not need a sign. For those curves that do 

need advance warning signs, use the Curve sign unless the advisory speed is 30 mph or less, in

27
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

 

 

 

which case the Turn sign is required. Use the Hairpin Curve sign when the change in horizontal 

alignment is 135 degrees or more. The Loop sign indicates a change of approximately 270 

degrees in direction, such as cloverleaf interchange ramps, and 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) and Reverse Curve (W1-4). These should be used when the tangent distance 

between the two curves is less than 600 feet. The guidance on which one to use 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 may be used.

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 MUTCD Table 2 C-4 presented in 

Chapter 3. This type of sign, and others discussed in this publication, should be located as 

described in MUTCD Section 2A.16.

Materials 

Traffic signs of all types use retroreflective sheeting to ensure they are visible to drivers at night 

or in periods of low light. In the interest of improved visibility and sign life, many agencies have 

transitioned from using engineering grade to high-intensity grade and even prismatic sheeting.

Information regarding various available sheeting types can be found in the Traffic Sign

Retroreflective Sheeting Identification Guide. Higher grades generally can be seen from a further 

distance and typically last longer.

Effectiveness 

The CMF Clearinghouse lists two “advance static curve warning signs” countermeasures, which

report a 30-percent decrease in all minor and serious injury crashes (CMF is 1 star) (Elvik and 

Vaa, 2004) and an 8-percent decrease in all property damage only (PDO) crashes (CMF is 1 star) 

(Elvik and Vaa, 2004). However, both of these countermeasures received a rating of only one 

star, indicating the quality or confidence in the results of the study is not reliable. 

Relative Cost

This countermeasure is low cost. 

Advisory Speed Plaque

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

(discussed in previous section) to advise motorists of the safe speed through the curve(s). It 

does not indicate the legal speed limit. Figure 21 shows an example of an advisory speed plaque. 

28 
CHAPTER 4. SIGNS 

Design and Application 

The MUTCD requires an advisory speed plaque when an engineering study indicates a need. It is

also required for roadways with more than 1,000 ADT whenever the difference between the 

advisory speed and the posted speed is 10 mph or greater and recommended at 5 mph 

difference in speeds according to Table 4. An engineering study is required to determine the

appropriate advisory speed. The MUTCD outlines established practices that are appropriate for 

determining the recommended advisory speed for a horizontal curve that include an 

accelerometer, a design speed equation, or a 16-, 14-, or 12-degree ball bank indicator 

depending on the speed range. 

Figure 21. Photo. Advisory Speed Plaque. 

See FHWA-SA-11-22, Procedure for Setting Advisory Speeds on Curves and the ITE Traffic Control 

Devices Handbook for additional methods of determining the advisory speed. 

Effectiveness 

The CMF Clearinghouse does not include any studies specifically for adding combination 

horizontal alignment/intersection warning signs along two-lane rural highway road segments. 

Relative Cost

This countermeasure is low cost. 

Combination Curve/Intersection Signs 

An intersection near or within a curve adds another potential problem and more information for 

the driver to process. The combination Horizontal Alignment/Intersection sign (W1-10 Series) 

quickly communicates to the driver what to expect in advance.

29 
0 

Wl-10 W 1-1Qa. W1-10b 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

The MUTCD states that turn arrows and reverse turn arrows may be substituted for the curve 

arrows and reverse curve arrows on the W1-10 series signs where appropriate (Figure 22).

Design and Application 

The signs shown in Figure 22 are used 

in lieu of the horizontal alignment signs 

previously listed in Figure 20 and 

should comply with the provisions of 

both curve warning signs and with

intersection warning signs. The symbol 

design should approximate the 

configuration of the intersecting 

roadway(s). However, no more than 

one Cross Road or two Side Road 

symbols should be displayed on any 

one combination Horizontal 

Alignment/Intersection sign.

 

 

 

The design and application of the combination Horizontal Alignment/Intersection sign is 

accordance with the appropriate Turn or Curve sign requirements. 

Effectiveness 

To date, no research has documented the safety effects of installing a combination horizontal 

alignment/intersection sign. 

Relative Cost

This countermeasure is low cost.

SUPPLEMENTAL DEVICES IN A CURVE 

Some curves will still violate driver expectancy with only advance warning signs. In those cases, 

additional traffic control devices used within the curve itself help guide motorists through the 

curve. These include combination curve/speed signs, chevrons, large arrow signs, and 

delineators.

Combination Horizontal Alignment/Advisory Speed Sign

When additional emphasis is needed to reduce speeds, agencies can add a combination 

Turn/Advisory Speed (W1-1a) sign or a combination Curve/Advisory Speed (W1-2a) sign. This

sign is used as a supplement to—not a replacement for—the advance Horizontal Alignment sign 

and Advisory Speed plaque, and is placed at the beginning of the turn or curve (i.e., the point of 

curvature). The sign is intended to remind motorists of the need to slow down as they begin to

negotiate the alignment change.

Figure 22. Illustration. Excerpt from Figure 2C-1 of 

the MUTCD. 

30
CHAPTER 4. SIGNS 

 

Design and Application 

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 to use it when the distance from the advance 

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

would be too close together. The advisory speed on the combination Horizontal 

Alignment/Advisory Speed sign should be based on the advisory speed for the curve using

recommended engineering practices.

Effectiveness 

The HSM Table 13-30 shows a 13-percent decrease for injury crashes and 29-percent decrease 

for non-injury crashes associated with the installation of combination horizontal 

alignment/advisory speed sign (Elvik and Vaa, 2004). The CMF Clearinghouse lists the same 

CMFs with 3-star ratings, suggesting that the combined horizontal alignment/speed advisory sign 

safety effectiveness estimates are moderately reliable. 

Relative Cost

This countermeasure is low cost. 

Chevron Alignment Sign

Chevron Alignment (W1-8) signs emphasize and guide drivers through a change in horizontal 

alignment. Because of their pattern, size, and placement with at least two of the signs 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 also need one of the advance curve 

warning signs previously discussed. Figures 23 and 24 illustrate a before and after installation. 

Figure 23. Photo. Before, without chevrons. Figure 24. Photo. After, with chevrons. 

Chevrons provide advanced alignment of 

a curve. 

31
 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Design and Application 

Except on roads functionally classified as local or with volumes less than 1,000 ADT, the use of 

Chevron Alignment signs are to be in accordance with MUTCD Table 2C-5, which recommends 

use of chevrons where the difference between the advisory speed and posted speed is 10 mph, 

and requires their use when that difference is 15 mph or greater (see Table 4). Use at lower 

speed differences or on other roads is optional, based on engineering judgment. An agency may 

use chevrons instead of or in addition to standard delineators. 

Chevrons are one of very few signs without a border, and are installed at a height of at least 4 

feet above the roadway surface. Install a series of these signs on the outside of a 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 spacing of the chevrons is measured from the point of curvature (PC) and should be as 

shown in the MUTCD Table 2C-6 (Table 5). The spacing is based on the advisory speed and 

radius of the curve. Figure 25 illustrates a layout of these devices on a curve with a 

retroreflective strip on the posts for increased conspicuity.

Table 5. Typical Spacing of Chevron Alignment Signs (MUTCD Table 2C-6). 

Advisory Speed Curve Radius Sign Spacing 

15 mph or less Less than 200 feet 40 feet 

20 to 30 mph 200 to 400 feet 80 feet

35 to 45 mph 401 to 700 feet 120 feet

50 to 60 mph 701 to 1,250 feet 160 feet

More than 60 mph More than 1,250 feet 200 feet

Effectiveness 

The CMF Clearinghouse lists a 4-percent to 

25-percent reduction in crashes when

chevrons are installed on rural highway curves

(CMF is 4 stars) depending on the crash type

(Srinivasan, 2009). There are even greater

reductions when chevron installations are

combined with advance curve warning signs

and/or flashing beacons. In addition, according

to NCHRP Report 559, chevrons have been

shown to reduce vehicle encroachments onto

the center line in curves where the degree of

curvature is more than seven degrees (Lyles

and Taylor, 2006).

Figure 25. Photo. Chevrons assist the driver in 

navigating curves. Source: Texas Transportation 

Institute. 

32
CHAPTER 4. SIGNS 

Relative Cost

This countermeasure is low cost. 

The cost to apply chevrons to a curve will vary with the number of signs installed. It is not 

uncommon for one or more of the chevrons in problem curves to be periodically knocked 

down, so it is advisable for an agency to keep a supply of signs. 

One-Direction Large Arrow Sign

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

alignment, as seen in Figure 26. Usually only one of these signs per direction is used for a 

horizontal curve or turn. 

Nothing in the MUTCD limits using multiple 

signs along the curve, but it may be more 

reasonable to use a series of Chevron 

(W1-8) signs. The Large Arrow sign is 

installed only on the outside of a turn or 

curve in line with, and at approximately a 

right angle to approaching traffic. 

Design and Application 

The Large Arrow sign may be used either 

as a supplement or alternative to Chevron 

signs in accordance with the information 

shown in Table 2C-5. If a Large Arrow sign 

is used with Chevron signs, it would take 

the place of a Chevron and not obstruct or 

be obstructed by a Chevron. Based on 

standard practice, this sign is limited to 

sharper curves (i.e., turns). 

Effectiveness 

Figure 26. Illustration. Typical installation location of 

one-direction large arrow signs on a horizontal curve. 

Source: MoDOT Engineering Policy Guide. 

To date there has been no research on the safety effectiveness of installing a large arrow sign, 

and therefore, it is not listed as a countermeasure by the CMF Clearinghouse. 

Relative Cost 

This countermeasure is low cost. 

33
 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

In 2010, the Ohio Department of Transportation introduced a Horizontal Curve Program 

which focused on upgrading and installing various signage at curves to address problematic 

locations. Individual districts conducted site field reviews, evaluated existing conditions and 

countermeasures onsite, and selected the appropriate signs to be installed at the site. 

Figure 27 shows a curve on a rural, two-lane road before signage updates and Figure 28 

shows the same curve after signage updates through the Horizontal Curve Program. 

Images courtesy of Ohio DOT. See Appendix D for more information. 

Figure 27. Photo. Before signage updates. Figure 28. Photo. After signage updates. 

 

 

1um size is not recommended, but the MUTCD allows their use / I on low-speed roadw: 

~ the reduced letter size remains adequate for the warning or where physical conditi 

nt using a larger size. The MUTCD also states that oversized and larger signs" ... sho 

·or those special applications where speed, volume, or other factors result in conditi 

~ increased emphasis, improved recognition, or increased legibility is needed, as dete 

~ineering judgment or study" (MUTCD, Section 2A. 11 ). A horizontal curve identified 

improvements would likely meet this 

ENHANCED SIGNING COUNTERMEASURES 

Most basic devices described above can be improved in different ways to increase the number of 

drivers who perceive and react to them. The sooner a motorist is able to see a device and 

recognize its meaning, the more time there is to respond. The following enhanced signage 

countermeasures have proven effective in enhancing driver perception.

Larger Devices 

The MUTCD prescribes the use of the “conventional road” sizes for typical situations. The 

minimum size is not recommended, but 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 “...should be 

used for those special applications where speed, volume, or other factors result in conditions 

where increased emphasis, improved recognition, or increased legibility is needed, as determined 

by engineering judgment or study” (MUTCD, Section 2A.11). A horizontal curve identified for 

safety improvements would likely meet this condition. condition. 

34
CHAPTER 4. SIGNS 

Effectiveness 

To date there has been no research documenting the safety effects of installing larger warning 

signs. Therefore, these signs are not listed as a countermeasure by the CMF Clearinghouse. 

Relative Cost

This countermeasure is low cost. 

Doubling-Up Devices 

As seen in Figure 29, “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 this chapter. Doubling-up increases 

the opportunity for the motorist to see the sign,

and respond to the message. Doubling-up is also 

a candidate countermeasure when visibility of 

the single right-hand side sign is less than 

desirable. 

Effectiveness 

To date there has been no research documenting the safety effects of doubling-up curve warning 

signs, and therefore, it is not listed as a countermeasure by the CMF Clearinghouse. 

Relative Cost

This countermeasure is low cost. 

Retroreflective Strip on Sign Post 

A strip of retroreflective material may be used on warning sign supports to draw more attention 

to the sign during nighttime conditions (MUTCD, Section 2A.21). If used, the strip of 

retroreflective material shall be at least 2 inches in width, placed along the full length of the sign 

support to within 2 feet of the roadway surface, and its color shall match the background color 

of the warning sign. 

Effectiveness 

To date there has been no research documenting the safety effects of installing a retroreflective 

strip on the posts of curve warning signs. However, a study from Iowa has shown that installing a 

retroreflective strip on chevron sign posts led to moderate reductions in the mean and 

85th percentile operating speeds along curves (Hallmark et al., 2012). This same study also 

Figure 29. Photo. Doubling-up of the sign was 

used at this site because tree limbs partially 

blocked the right side sign. 

35
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

 

 

indicated that there was a statistically significant reduction in the percentage of vehicles 

exceeding the posted speed limit by more than 10 mph.

Relative Cost

This countermeasure is low cost. 

Highly Retroreflective and Fluorescent Sheeting 

Another way to make signs more visible or more 

noticeable to motorists is to use highly 

retroreflective sheeting and fluorescent sheeting. 

As noted earlier in this chapter, the retroreflective 

sheeting for signs is available in different grades. 

Signs made with prismatic sheeting increase 

visibility from a longer distance at nighttime 

visibility. For more information on types of 

retroreflective sheeting, see Traffic Sign 

Retroreflective Sheeting identification Guide. Use of 

fluorescent yellow increases the visibility of warning 

signs, especially at dawn or dusk, as seen in Figure 

Figure 30. Photo. Stimulus photo illustrating 30. This visual advantage works day and night. 

enhanced chevron visibility. Source: Texas 

Transportation Institute. 

Effectiveness 

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 CMF Clearinghouse lists an 18-percent to 35-percent reduction in various crash types on 

rural highways when new fluorescent curve signs are installed or existing curve signs are 

upgraded to fluorescent sheeting (Srinivasan et al., 2009). To date, no CMFs have been 

developed regarding improved retroreflectivity of signs. 

Relative Cost 

This countermeasure is low cost. 

36
CHAPTER 4. SIGNS

Flashing Beacons

Using flashing beacons with a warning sign is another way 

to gain motorists’ attention, as seen in Figure 31. 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 it is reasonable to limit them to

locations where other countermeasures have not solved 

a safety problem. One factor limiting the use of beacons 

is the availability of an accessible power source, although 

agencies can use reliable solar power panel systems.

The beacons used for this countermeasure are the 

circular yellow sections from a standard traffic signal. 

Agencies can install this with one or more beacons, but Figure 31 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 inches outside of the 

nearest edge of the sign.

Effectiveness 

To date there has been no research documenting the safety effects of installing flashing beacons 

with warning signs, but when flashing beacons are installed in combination with curve warning 

signs and chevrons, the CMF Clearinghouse lists a 37-percent to 76-percent reduction in various 

crashes (CMFs are 3 stars) (Montella, 2009).

Relative Cost 

This countermeasure is moderate cost. 

Figure 31. Photo. Typical arrangement 

of signs and flashing beacons. 

37
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Dynamic Curve Warning System 

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 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 

countermeasure 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 United States, as the 

examples in Figures 32 and 33 demonstrate. 

Figure 32. Photo. Speed Actuated Sign, Augusta, ME. Figure 33. Photo. Flashing Beacon on Warning 

Sign, Augusta, ME. 

Application 

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 fatalities and 

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

One dynamic system application involves a radar speed detection device coupled with warning 

signs and activated flashing beacons. The Texas system, illustrated in Figures 34 and 35, 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 feet 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 feet before a vehicle reaches the overhead sign. 

38 
 

CHAPTER 4. SIGNS 

Effectiveness 

Figure 35. Photo. Texas System curve advisory 

speed limit sign. Camp County, Texas. 

Figure 34. Photo. Texas System curve advisory 

speed limit sign. Camp County, Texas. 

A recently completed project by FHWA evaluated the effectiveness of low-cost, speedactivated dynamic curve warning systems on speeding and safety on horizontal curves in rural 

roadways. The study found a 5-percent (CMF is 5 stars) to 7-percent (CMF is 4 stars) crash 

reduction depending on the crash type and direction of the crash (Hallmark et al., 2015). Other 

studies have shown that they can reduce vehicle speeds in horizontal curves. For example, 

Oregon experienced a 3-mph decrease in speeds 

Myrtle Creek installation on I-5 (Bertini et al., 

2006). Another study found that average 

speeds dropped between 1 and 8.8 mph and

concluded that dynamic curve warning signs

have larger impacts at curves with lower 

advisory speeds and on reducing the number of 

higher speed vehicles (Knapp and Robinson, 

2012). A study published by California DOT 

found that an advanced curve warning system 

on an interstate route in Northern California 

led to over 68 percent of drivers to reduce

their speed (Tribbett et al., 2000). 

at the 

Figure 36. Photo. Example of Sequential 

Dynamic Curve Warning System.

Sequential Dynamic Curve Warning Systems (SDCWS) have been implemented as a 

countermeasure on two-lane rural highway curves as a means to reduce vehicle operating 

speeds and improve curve delineation. SDCWS are horizontal curve chevron signs with 

solar powered flashing lights embedded in the sign, as shown in Figure 36. Four States 

collectively installed 12 SDCWS along horizontal curves on two-lane rural highways. These 

sites were identified based on a high-crash history, as well as vehicle operating speeds that 

exceeded the posted speed limit. The study has found that both operating speeds and crash 

frequency have been reduced by SDCWS installations. See Appendix E for more 

information. 

39
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Relative Cost

The cost of these systems varies, depending upon the specific design and the availability of a 

power source. This countermeasure is moderate cost. 

SIGN MAINTENANCE 

Regardless of whether an agency has basic or enhanced signage 

countermeasures, road agencies should consider maintenance 

requirements when deciding what countermeasure to use, and carry 

out maintenance activities on a regular basis to ensure continued safe 

travel. These maintenance activities include: 

 Replace or repair damaged or knocked down signs.

Signs that are damaged through vandalism, accidents, or

storms, as seen in Figure 37, should be repaired or replaced as

soon as feasible.

 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 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, as seen in Figure 38. Therefore, the

MUTCD requires agencies to use an assessment or

management method designed to maintain sign

retroreflectivity at or above the minimum levels in Table 2A-3.

For alternative methods see Maintaining Traffic Sign

Retroreflectivity (FHWA-SA-07-020). Replace any signs found to

be ineffective as soon as practical.

Figure 37. Photo. Example 

of a Chevron sign on the 

ground. 

 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, as seen in

Figure 39. This is why agencies should make periodic

inspections of the roadway to identify and correct these

situations. For more information, see Chapter 5 of this

publication.

Figure 38. Photo. Low 

Retroreflectivity sign 

example. 

More practical tips for controlling vegetation overgrowth are 

found in FHWA’s Vegetation Control for Safety.

Figure 39. Photo. Curve warning 

sign covered by foliage. 

40 
CHAPTER 5. PAVEMENT COUNTERMEASURES 

 

 

 

 

CHAPTER 5. PAVEMENT COUNTERMEASURES 

Low-cost countermeasures that improve pavement surfaces or involve minor reconstruction in

curve sections are also available. These impr6 ovements will function alone or can be completed 

with the use of signs and pavement markings as outlined in Chapters 3 and 4. This chapter 

describes several pavement countermeasures that could reduce roadway departure crashes. 

These countermeasures can be implemented independently as a safety project or incorporated 

into a pavement preservation or 3R (rehabilitation/restoration/resurfacing) program. While most

of the countermeasures in this guide are only applicable at curves, this chapter includes 

countermeasures that are applicable at both curves and tangent sections. Some of the

countermeasures covered in this chapter are “moderate cost” when implemented alone, 

however, costs may be “low” when incorporated into 3R projects or other planned projects. 

SKID RESISTANCE PAVEMENT COUNTERMEASURES 

Maintaining the appropriate amount of pavement friction is critical for safe driving. Low pavement 

friction allows vehicles to skid and lose control, which has been related to many severe crash 

types. Agencies can address this issue by monitoring the pavement friction values and improving 

them when they fall below a certain level. Two conventional approaches to solve this issue are 

repaving with thin overlays or repaving using microsurfacing. Generally, both methods are 

reserved for long sections of roadway, and both restore the pavement friction number when the 

mixes are designed properly. These measures usually can produce friction numbers in the 40s to 

50s, as measured by skid trailers, depending on the aggregate used, as opposed to a low friction 

value in the 20s and 30s. See NCHRP 108, Guide for Pavement Friction and Evaluation of Pavement 

Safety Performance. 

Locations with higher operating speeds or those with demanding geometric conditions may 

require pavement with higher friction capabilities. Such locations are typically found in horizontal 

curves, steep grades, combination of grades and curves, and the approach to intersections. 

Friction can be improved on curves to address geometric characteristics unsuited to the road’s 

operating speeds. These critical locations where the need for friction is greatest also tend to 

lose friction more quickly than flat tangential sections. These high friction demand locations are

typically short sections that can be addressed by high friction surface treatments (HFST). 

Wet pavement surfaces also reduce pavement friction and can cause skidding. Excessive water

on pavement surfaces can result in hydroplaning, but more often crashes are caused by loss of 

friction due to smaller amounts of water on the road since it takes very little water on the 

surface to reduce friction by 20 to 30 percent, as shown in Figure 40. Marginal pavement friction 

numbers can lead to skidding crashes either from speeding vehicles or wet weather. Wet 

weather is a major contributing factor in roadway departure crashes and most agencies monitor 

locations for wet weather crashes. Having higher pavement friction makes the microtexture 

friction loss due to wet weather less critical but does not address hydroplaning. Cross slope, 

drainage improvements, and macrotexture address hydroplaning but can be equally related to 

the condition of each vehicle’s tires. Additional information on practices to reduce wet weather

skidding crashes can be found in State Practices to Reduce Wet Weather Skidding Crashes. 

41 
50 __.,_ Worn Rilbbed. Tire 

---- ]: ew Ribbed Tirre 

45 -----·---t:... ..... -------------·----- -------""" o, - _smooth Tire ---------· 

C 40 

0 --------~ -~' ;:, .;----------------------------------------

... ■ ■ .... 

------·-- -----·------- ----------·---... --,

' 

----------- --·--- ---. ---- ----- l,,,,, . 

.... ... .... _ _ - · ic. 

30 -----------------------------------------------------------------

25 ~---------------~------

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 

WFT,in 

Pigur,e 17. Effect of water film thickness on pavement friction (Henry 2000). 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Figure 40. Graph. Figure 17 from NCHRP 108, Guide for Pavement Friction. 

A variety of skid resistance surface countermeasures are available. While HFST may have a 

higher unit cost than traditional friction improvement courses, they can often be applied in small

quantities at spot locations for a relatively low project cost. In addition, where cross-section 

problems exist, such as lack of appropriate superelevation, this approach can be a low cost but

effective alternative to address the problem. 

High Friction Surface Treatments (HFST) 

HFST is an evolving technology that has 

demonstrated the ability to dramatically and 

immediately reduce crashes and related 

injuries and fatalities. When friction demand 

exceeds conventional pavement friction 

capability, high-quality aggregate is applied to 

existing or potential high-crash areas to help 

motorists maintain better control in dry and 

wet driving conditions. HFST uses calcined 

bauxite aggregate, which has demonstrated 

the best friction characteristics

(microtexture) and polish resistance. Proper 

gradation needs to be specified to provide 

proper macrotexture. The binder layer is Figure 41. Photo. HFST on a horizontal curve.

usually a polymer material. The darker layer of pavement in Figure 41 is an example application 

of HFST. The National Center for Asphalt Technology’s (NCAT) High Friction Surface Treatment 

Alternative Aggregates Study provides discussion and research results on different aggregate types 

for HFST. 

42 
 

 

CHAPTER 5. PAVEMENT COUNTERMEASURES 

Applications 

HFST can be applied to concrete surfaces or asphalt pavement materials and is available from 

many manufacturers and contractors. While the aggregate is specialized and generally not 

produced locally, it is not proprietary. The binder material is usually proprietary but there is 

ample competition in the industry. Recent innovations in the application method have evolved. 

HFST have been installed with simple hand tools for many years, but this is slow and a welltrained crew is necessary to attain good quality installations. Some of the installers that work 

nationally have recently developed equipment for mechanical applications. The equipment varies

by the contractor. Some have incorporated more sophisticated equipment for monitoring the 

quality of the application. Also, application speed varies with the different equipment and 

aggregate distribution method. The mechanical approach can be more cost effective for large 

quantity projects, projects with lane closure limitations or multiple small projects. Recent 

breakthroughs have provided products that are designed to provide quick curing rates even in

cold temperatures. 

Effectiveness 

HFST has been tried at a wide variety 

of countermeasure sites across the 

country as part of the FHWA’s Surface 

Enhancements at Horizontal Curves 

(SEAHC) program. Preliminary crash 

data indicates benefit-cost ratios were 

as high as 50 to 1. Kentucky alone 

placed this countermeasure on 30 

curves at the beginning of their 

To further illustrate Kentucky’s success with

HFST, the Kentucky Transportation Cabinet saw

tremendous results from one exit ramp on 

Interstate 75 in Fayette County. In the three

years prior to HFST treatment, the ramp had 28 

roadway departure crashes (18 wet crashes and 

10 dry crashes). In the two and half years since 

the HFST installation, crashes have nearly been

eliminated. The ramp has been the scene of a 

single crash, which was a dry crash. See Appendix 

F for more information.

Roadway Departure Safety 

Implementation Plan in 2009 and observed a crash reduction of 70 percent to 75 percent at

these curves. A Texas Transportation Institute study, Using High Friction Surface Treatments to 

Improve Safety at Horizontal Curves, compiled crash reductions from other HFST application 

studies. 

HFST is a durable and effective safety countermeasure for roadway departure crashes, especially 

as a spot application in critical locations. See Appendix G for additional information on the use

of HFST in the United States. 

Relative Cost

This countermeasure is moderate cost. 

Pavement Grooving 

Pavement grooving is a pavement countermeasure technique to apply longitudinal or transverse 

cuts onto the pavement surface to increase or restore pavement friction. This is used on 

concrete pavements and is especially effective in reducing wet-weather crashes by improving the 

43 
slope 

Original (alsc 

ground foreslope) ____. Shoulder 

• • 

Shoulder 

Lane 

Centerline Ditch bottom I Original 

Lane Shoulder Foreslope f Backslope ground 

•- - - --•-----4•μ-Lt•~----~-~--- Shoulder break 

] Depth 

of cut 

--------'-

width Lane width 1 

~ ~ Ditch bottom 

width (variable) 

-- - - - --------t 

Clear zone width 

(to obsL'lcle or 

nontraversable slope) Roadway width 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

drainage characteristics of the pavement. Potential side effects include increase of vehicular noise 

(particularly for transverse grooving), possible reduced driver comfort (particularly for 

longitudinal grooving) and potentially premature wearing of the pavement surface. New grooving 

techniques have been recently marketed to reduce sound for longitudinal grooving.

Effectiveness 

New York State DOT evaluated the pavement grooving treatment and found that wet 

pavement-related crashes were reduced by 55 percent, and the total for both wet and dry

pavement crashes were reduced by 23 percent. Various studies cited in the NCHRP Report 500, 

Volume 7, Strategy 15.2.A8 shows significant crash reductions after applying the pavement 

grooving countermeasure. 

Relative Cost

This countermeasure is moderate cost. 

Superelevation 

Providing superelevation at the curve to help keep vehicles on the road is one of the key 

geometric design elements that affects crashes on a curve. Superelevation is designed for driver 

comfort during the acceleration through the curve, and works with friction between the tires 

and pavement to assist vehicles in maneuvering through curves. Figure 42 provides an illustration 

of a cross section of a superelevated section. Superelevation is occasionally inadequately 

designed, lost over time due to settling and/or overlays, or not included as part of the original 

design consideration due to factors such as traffic volume, constructability, and adjacent land use.

As a result, curves with inadequate superelevation may pose a safety problem. 

Figure 42. Illustration. A typical cross section of a normal crown with the red and blue lines showing a 

typical cross section of a superelevated section. 

44 
CHAPTER 5. PAVEMENT COUNTERMEASURES 

According to the HSM, crash prediction models indicate that inadequate superelevation 

increases curve crashes. Research results indicate that safety can be enhanced when the 

superelevation is improved or restored along curves where the actual superelevation is less than 

the optimal superelevation. However, it should be noted that the increase in driver comfort 

associated with increasing superelevation may increase speeds. 

During routine pavement projects under the pavement preservation program, deficiencies in 

superelevation should be addressed. Other issues related to superelevation that the designer 

should pay attention to during routine pavement projects are the slope break between the edge 

of pavement and the adjacent shoulder. The designer should also have guidance on maximum 

recommended algebraic differences between the traveled way and the shoulder slopes. Refer to 

Chapter 3, Section 3.3 of AASHTO’s publication “A Policy on Geometric Design of Highways and 

Streets” (2011) for more details on superelevation design. 

When restoring superelevation, special attention is required to maintain proper drainage. A 

sufficient grade should be maintained along the superelevation transition to provide sufficient 

drainage where the cross-slope is level or close to level. Care should also be taken to ensure 

reverse curves have appropriate transition distance.

Effectiveness 

The HSM provides a function for calculating CMFs for horizontal curves based on superelevation

variance (SV), which is provided in Chapter 10 for two-lane rural highways. 

Relative Cost

This countermeasure is moderate to high cost, depending on extent of correction and scale of 

project.

 SHOULDER COUNTERMEASURES 

The probability of recovering from a run-off-the-road incident is increased if a vehicle is provided 

with a shoulder, the portion of the roadway outside of the travel lane where a driver can reclaim 

control of the vehicle. The shoulder is also 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). Rounding is 

simply a gradual change in slope from the usable shoulder to the foreslope. 

This area can be further enhanced if the recovery is not impeded by surface irregularities such as 

potholes, edge drop-offs, or ruts. Such irregularities may make a vehicle more difficult to 

control. Shoulder countermeasures that promote safe recovery include shoulder widening, 

shoulder paving, and the installation of the Safety EdgeSM. While each strategy could be covered

separately, the effectiveness is related, and the treatments can often be completed as a 

“package” during roadway resurfacing. These pavement countermeasures within the shoulder 

area enable the vehicle’s recovery to be made in a more controlled fashion and reduce the risk 

of overturning or crossing into the opposing lane. 

45
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Shoulder Widening 

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. By widening the 

shoulders or providing a shoulder where one previously did not exist, drivers have more 

recovery area to regain control in the event of a roadway departure. 

Applications 

Shoulder widths can vary from approximately no shoulder on minor rural roads to 12 feet on 

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

widened shoulders and minimize steepening of roadside slopes. As Figures 43 illustrates, 

agencies can widen shoulders on both the inside and outside. If space is only available to one 

side, widening the outside shoulder will most likely provide the greater benefit. 

Figure 43. Photo. Widening on the inside and outside of the curve. 

Effectiveness 

Table 6 shows the percent change in crashes (including single vehicle run-off-road and multiple 

vehicle head-on and sideswipe crashes) in comparison to a road with 6-foot shoulders. The table 

suggests that roads with shoulder widths less than 6 feet will have more crashes than a road 

with 6-foot shoulders. Conversely, roads with shoulder widths 8 feet or more will have fewer

crashes than a road with 6-foot shoulders. Although the table was developed for rural two-lane 

roads, and is not limited to horizontal curves, it is reasonable to expect the maximum benefit 

from shoulder widening can also be realized for horizontal curves. 

46
 

CHAPTER 5. PAVEMENT COUNTERMEASURES 

Table 6. Percent change in crashes relative to providing a 6-foot shoulder on rural two-lane roadway 

segments (Modified from HSM Table13-7). 

Shoulder Width 

Percent change in crashes in comparison to roads with 6-foot shoulders

Average Annual Daily Traffic (AADT) (vehicles/day) 

< 400 400-2,000 > 2,000

0 ft + 10% Between +10% and +50%, depending on AADT + 50%

2 ft + 7% Between +7% and +30%, depending on AADT + 30%

4 ft + 2% Between +2% and +15%, depending on AADT + 15%

6 ft 0% 0% 0%

8 ft or more - 2% Between -2% and -13%, depending on AADT – 13%

* Crash types: Single vehicle run-off-road, multiple vehicle head-on, opposite direction sideswipe, and same-direction sideswipe.

Relative Cost

This countermeasure is high cost. 

Shoulder Paving

When right-of-way permits, replacing unstable 

or narrow shoulders with paved shoulders 

increases the total usable width of the 

roadway. This improves safety for all road 

users (motorized and non-motorized). With 

this extra paved roadway, vehicles have an 

increased capacity for recovery if they leave

the travel lanes. Paving shoulders can also be 

accompanied by Safety EdgeSM and rumble 

strips. Figure 44 shows an example of a 

shoulder paving operation. 

Applications 

While limited budgets may influence an agency’s decision to upgrade to paved shoulders on twolane tangent sections, the resulting benefit-cost ratio from fewer crashes on curves with paved 

shoulders deserves consideration. In some cases, widening shoulders may be more desirable 

than widening lanes. 

Effectiveness 

The CMF Clearinghouse lists only one CMF for paving the shoulder through a curve. It indicates 

an increase in crashes (CMF is 1 star) (Pitale et al., 2009). Other research results for paving 

shoulders (not exclusively within curves) indicate that paving shoulders reduces crashes. 

Figure 44. Photo. Shoulder paving operation. 

47
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Relative Cost

This countermeasure is high cost. 

Safety EdgeSM

Safety EdgeSM is a paving technique used 

system-wide to improve pavement durability 

and reduce crashes by shaping and 

consolidating the pavement edge into a 30-

degree wedge, as demonstrated in Figure 45. 

The shape of the edge allows controlled 

recovery for drivers returning to the

pavement after straying due to inattention. 

The added durability of the edge reduces the 

tendency of the pavement to ravel, providing a 

consistent pavement width. It should be noted 

that the recommended practice is to bring the 

adjacent shoulder material or roadside

vegetation up even with the pavement surface,

thereby covering the Safety Edge after the

paving is complete (Figure 46). The shape of the Safety EdgeSM is exposed at various times over

the life of the pavement, as this material settles or is dislodged by traffic.

SM

Figure 45. Photo. Pavement with and without the 

Safety EdgeSM.

Figure 46. Photo. Backfilling against newly installed Safety EdgeSM. 

48
 

CHAPTER 5. PAVEMENT COUNTERMEASURES 

The Safety EdgeSM is formed while paving and therefore, is not appropriate as a spot 

countermeasure for curves. However, this countermeasure is particularly helpful at curves 

where the roadway departure crashes it addresses are prevalent. It is the ultimate systemic 

countermeasure, which when applied on every paving project, will provide added safety 

wherever a driver leaves the pavement. The cost is a very minor addition to the cost of the 

paving process under which it is applied. 

Most State DOTs now use Safety EdgeSM as a standard practice and therefore have appropriate 

specifications and drawings for use in contract documents. Additional information is available in 

FHWA’s Safety EdgeSM Design and Construction Guide. 

Effectiveness 

Safety EdgeSM has been proven for many years based on physical tests with vehicles. In addition, a

recent study showed it could reduce total crashes by approximately 6 percent on two-lane 

roads (CMF is 4 stars) (Graham et al., 2011). The FHWA Office of Safety is sponsoring a project 

to estimate an updated CMF for the Safety EdgeSM paving technique on two-lane rural highways. 

Results from this evaluation are anticipated in December 2016. 

Relative Cost

This countermeasure is very low cost. 

Rumble Strips and Rumble Stripes 

Longitudinal rumble strips are milled or raised elements on the pavement intended to alert 

inattentive drivers through vibration and sound that their vehicles have left the travel lane. There

are a number of possible applications that can be used: 

 Shoulder rumble strips are installed on a shoulder near the edge of the travel lane.

They significantly reduce run-off-road crashes.

 Edge line rumble strips are very similar to shoulder rumble strips, but placed at the

edge of the travel lane in line with the edge line pavement marking, and therefore often

called a rumble stripe.

 Center line rumble strips are installed at or near the center line of an undivided

roadway, and may be comprised of either a single or double line of rumbles. They reduce

cross center line crashes such as head-on collisions and some run-off-road left crashes.

 Rumble stripes are either edge line or center line rumble strips where the pavement

marking is placed over the rumble strip. This countermeasure increases nighttime

visibility of the pavement marking.

Because rumble strips apply to a human behavior problem rather than a roadway deficiency, they 

are best applied as a systemic countermeasure. Driver inattentiveness or drowsiness cannot be 

predicted by location; however, the type of system where application will be most effective can 

often be predicted from previous crash experience, using factors such as ADT or roadway 

classification. 

49
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Milled rumble strips have been shown to be more effective than other types of rumble strips at 

creating noise loud enough to alert inattentive and fatigued drivers (NCHRP Report 641, 2009). 

Milled rumble strips can also be installed at any time on new or existing pavements. In regions 

where plowing is not an issue, various types of raised rumble strips may be used as an 

alternative. Figure 47 shows a milled centerline rumble stripe, and Figure 48 shows a milled edge 

line rumble stripe. 

Figure 47. Photo. Milled Center Line Rumble 

Stripes. 

Figure 48. Photo. Milled Edge Line Rumble 

Stripes. Source: KYTC. 

For more information on rumble strip design and installation, see FHWA technical advisories at: 

 T5040.39, Shoulder and Edge Line Rumble Strips.

 T5040.40, Center Line Rumble Strips.

 NCHRP Report 641: Guidance for the Design and Application of Shoulder and Centerline

Rumble Strips.

Effectiveness 

The CMF Clearinghouse contains a large number of CMFs for installing rumble strips on various 

types of roads and conditions. NCHRP Report 641 indicates that installing shoulder rumble strips

on two-lane rural roads result in a 15-percent to 29-percent reduction in crashes, depending on 

the crash type (CMF is 5 stars). The same report also indicates that installing center line rumble 

strips on two-lane rural roads result in a 9-percent to 44-percent reduction in crashes, 

depending on the crash type (CMF is 5 stars). Studies have shown that the crash reductions for 

center line rumble strips in curves and tangents are approximately the same. 

Relative Cost 

This countermeasure is low cost.

50
CHAPTER 6. ROADSIDE IMPROVEMENTS 

CHAPTER 6. ROADSIDE IMPROVEMENTS 

Previous chapters have primarily addressed countermeasures to help keep vehicles on the

roadway. However, even with these countermeasures, many drivers will still leave the roadway 

and encroach onto the roadside, particularly on the outside of horizontal curves. Research by 

Glennon, Neuman, and Leisch (1985) found that roadside character (including roadside slope, 

clear-zone width, and coverage of fixed objects) appeared to be the most dominant contributor 

to the probability that a roadway curve has a high reported crash rate. 

Once a driver leaves the roadway, the focus of safety efforts is to reduce the potential that they 

will encounter a slope or ditch likely to induce a rollover or an obstacle that could result in

injury. Where it is not possible to flatten slopes or remove all obstacles, then the focus is to 

minimize the resulting severity through the use of crash barriers and other safety hardware. 

Chapter 5 discussed countermeasures such as shoulder widening and the use of the Safety 

EdgeSM to reduce crashes caused by edge drop-offs. Chapter 6 will focus on other roadside 

countermeasures that would typically be outside of the pavement, such as the clear zone, and 

will also discuss barrier considerations that are appropriate for curved sections. While some of 

these countermeasures may not be considered “low cost,” focusing these countermeasures on 

curves may be cost effective for the entire roadway safety picture. It is important to keep in 

mind and evaluate the tradeoffs of various safety investments. 

Clear Zone 

A clear roadside that is relatively flat and free of 

trees and other non-breakaway features, makes 

it more likely that a driver will be able to regain 

control. AASHTO defines the clear zone as “the 

unobstructed, traversable area provided beyond 

the edge of the through traveled way for the

recovery of errant vehicles.” The AASHTO 

Roadside Design Guide provides suggested values 

for the Design Clear Zone. 

The suggested clear zone values are based on 

studies that found that 80 percent of vehicles 

that left the road stopped within 30 feet of the travelled way. While the values have been 

adjusted to account for speed, sideslope, and the probability of encroachment (based on traffic 

volume), it can still be assumed that there will be vehicles that go farther than the suggested 

values. 

The New Jersey Department of 

Transportation (NJDOT) has 

implemented a Pole Mitigation Program 

to relocate utility poles with three or

more reoccurring pole collisions. 

NJDOT has also piloted energy 

absorbing poles made of fiberglass that 

collapse on impact and do not break 

away into the roadway. See Appendix H 

for more information.

The AASHTO Roadside Design Guide also provides an adjustment for the outside of horizontal 

curves. This adjustment is a multiplier based on the sharpness of the curve (radius) and the 

design speed. For example, for a curve with a radius of 1,475 feet and a design speed of 55 mph, 

an adjustment factor of 1.3 is suggested which means that the Design Clear Zone should be 

increased by 30 percent. 

51 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Applications 

Clear zones are useful for providing sight distance 

along curves and recovery areas for vehicles that

inadvertently leave the roadway. Thus, agencies 

should be cautious to avoid adding new fixed 

objects such as poles or trees in the clear zone, 

especially within the vicinity of horizontal curves 

(see Figure 49). Clear zones also decrease the 

risk of having animals near the roadway. More 

information on clear zones can be found in the 

Roadside Design Guide (AASHTO, 2011). 

Effectiveness 

The CMF Clearinghouse contains several CMFs 

for increasing the clear zone. While these CMFs are not unique to curves, clear zone 

improvements in curves may have greater affect since crashes are over-represented. For 

example, FARS data indicates that 48 percent of fatal crashes occurring on curves involve trees.

In many cases improving the clear zone may be a low-cost countermeasure if it involves the 

removal of shrubs and trees. 

CMFs in the CMF Clearinghouse indicate that increasing the distance to roadside obstacles from

3.3 to 16.7 feet reduces all crash types and severities by 22 percent (CMF is 5 stars). The CMF

Clearinghouse also indicates that increasing the distance from 16.5 feet to 29.5 feet results in a 

reduction of 44 percent of all crash types and severities (CMF is 5 stars). These reductions are 

not specifically for curved road sections. 

Relative Cost 

The cost for this countermeasure can range from low to high depending upon the amount of 

earthwork and grading needed and the fixed objects that need to be removed or relocated. 

Figure 49. Photo. Fixed objects (trees) located 

within clear zone. 

52 
CHAPTER 6. ROADSIDE IMPROVEMENTS 

Clear Zone Maintenance 

Once a clear zone is established, it needs to be

maintained. Maintaining clear zones free of trees

while they are still saplings is typically less costly 

and controversial than removing them when the 

trees are mature. Agencies should develop a policy

for maintenance of the clear zone. Without an 

established policy, in many cases it may become 

difficult for maintenance forces to keep up with 

clear zone maintenance. Figure 50 shows an 

example of clear zone maintenenace. 

Refer to FHWA-SA-07-018, Vegetation Control for 

Safety A Guide for Local Highway and Street 

Maintenance Personnel, for more information. 

Slope Flattening 

After a vehicle leaves the travelled way and traverses over the shoulder, the steepness of the 

sideslope is a critical factor in their ability to keep the vehicle stable, regain control of the 

vehicle, and avoid obstacles. The ideal roadside, from a vehicle stability standpoint, would be flat 

(slopes of 1V:10H are considered essentially flat). The AASHTO Roadside Design Guide considers 

foreslopes that are 1V:4H or flatter to be traversable and recoverable, meaning that the driver 

could bring the vehicle under control and even stop on these slopes. Slopes that are between

1V:3H and 1V:4H are considered traversable but non-recoverable, meaning in most cases the 

driver will not be able to recover until reaching a flatter slope. Slopes steeper than 1V:3H are 

considered critical slopes, meaning the vehicle could become unstable on these slopes to the 

point that the risk of the vehicle overturning is increased. Depending on the height of the slope, 

a barrier might be considered for critical slopes. 

While it may not be practical to flatten all slopes along a corridor, flattening the slopes on the 

outside of curves may provide a significant benefit. FARS data indicates that 45 percent of 

overturn fatal crashes occur on curves. As a cost-saving measure, agencies can re-purpose 

material excavated from other locations to flatten slopes. 

Side slopes often are steeper on the outside of curves due to superelevation of the curve. 

Caution is recommended when using a “barnroof design” on the outside of horizontal curves. A

typical barnroof design exists where the slope immediately past the shoulder is flattened 

significantly but then breaks into a much steeper slope, generally to keep the slope inside of the 

right-of-way, as seen in Figure 51. While the flatter slopes outside of the shoulder facilitate 

recovery, if a vehicle goes past the slope break, it will probably go to the bottom of the slope. 

Since vehicles can be expected to go farther from the roadway when they leave on the outside 

of a curve, there is a higher probability of encroaching on this steeper slope. In addition, 

crashworthy hardware—such as sign supports—may not function as intended when placed on 

slopes steeper than 1V:6H. 

Figure 50. Photo. Removing brush as part of clear 

zone maintenance. Source: Texas DOT. 

53
 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

When ditches near roads are not traversable, 

the resulting roadside may be a particular safety 

concern for run-off-road crashes. This safety 

concern can be attributed to ditches that can 

trap a wheel and guide the vehicle into a fixed 

object, or cause loss of vehicle stability in the

transition to the backslope. 

Application 

 

The Roadside Design Guide recommends rounding Figure 51. Photo. Typical barnroof slope design.

the bottom of the ditch to make the ditch Source: Alaska DOT.

traversable. When this is not possible, other 

options include installing a barrier and partially filling the ditch with small aggregate. Some limited 

research in Sweden indicated that flattening ditches using aggregate improves the traversability of 

the ditch (Kelkka, 2009). 

Effectiveness 

The CMF Clearinghouse contains several CMFs for slope flattening. These CMFs include 

flattening side slopes from 1V:3H to 1V:4H, which has an expected crash reduction of 42 

percent for injury crashes and 29 percent for PDO crashes (CMF is 5 stars). CMFs for flattening 

sideslopes from 1V:4H to 1V:6H include a 22-percent reduction in injury crashes and a 24-

percent reduction in PDO crashes (CMF is 5 stars). While these CMFs were not developed 

specifically for horizontal curves, it is expected to be greatly beneficial on curves because of the

higher potential for roadway departures at these locations.

Relative Cost

The cost for this countermeasure can be high depending upon the amount of earthwork and 

grading needed and the possibility of right-of-way acquisition. 

Slope Maintenance 

While slopes generally don’t require a lot of maintenance, if there are areas where drainage 

runoff is concentrated, there may be a need to reestablish slopes periodically. Care is needed for 

ditch cleaning activities to reduce the potential for steepening the slopes. 

Roadside Barriers 

As previously noted, roadway departure crashes tend to be over-represented on curves. When

measures such as delineation and signing discussed previously have not been sufficient to reduce 

the number of roadway departure crashes, and it is not feasible to clear obstacles and flatten 

slopes, roadside barriers may be an appropriate treatment. In some cases a barrier, such as the 

one seen in Figure 52, may be appropriate on curves for certain conditions as noted above (e.g., 

54
CHAPTER 6. ROADSIDE IMPROVEMENTS 

side slopes, clear zone) where they may not be deemed suitable on tangents. In either case, the 

use of barriers requires engineering judgment to assess the trade-offs. 

Figure 52. Photo. Barrier along inside and outside of horizontal curve. 

There are three types of barriers that might be appropriate for curved sections: 

 Cable barrier: A flexible barrier made from wire rope supported between frangible

posts.

 Guardrail: A semi-rigid barrier usually either a steel box beam or W-beam. These

deflect less than flexible barriers; so they can be located closer to objects when space is

limited.

 Concrete barrier: A rigid barrier that does not deflect. These are not typically used on

rural two-lane roads.

Applications 

Traffic barriers placed on or in the vicinity of horizontal curves deserve special attention. Most 

barriers, while not specifically designed and tested on curves are used because there are no 

other alternatives. Barriers placed along a curved highway may be hit at higher angles and, 

depending on the superelevation and placement relative to a slope break point, vehicles may hit 

the barrier higher than normal. In many cases where there is a significant degree of curvature, 

the impact speed may be reduced, which can help compensate for some of the placement issues. 

A proprietary precast concrete or steel barrier was tested on a curve with a radius of 100 feet. 

This system, called the Safe-T-Curve Barrier System, was tested in accordance with NCHRP 350, 

TL3 (62 mph at a 25-degree angle) and deflected approximately 27 inches. While this type of a 

system may not be appropriate for most installations, it may be appropriate in locations where 

the barrier is hit frequently. 

55 
e~ RUNo l.rrp 

LENGlli OF NEED i<I~ 

---------

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Effectiveness 

The CMF Clearinghouse contains several CMFs for adding new guardrail along embankments. 

CMFs indicate reductions in run-off-road crashes of 47 percent for injury crashes (CMF is 5 

stars), 44 percent for fatal crashes (CMF is 4 stars), and 7 percent on PDO crashes (CMF is 3

stars). There are no specific CMFs for installing guardrail along horizontal curves. It is important 

to note that adding barriers may increase PDO crashes in some cases, but this should be offset 

by the reduction in severity of all crashes. 

Placement 

When a barrier is to be placed on a curve, the position of the barrier relative to a slope break 

may also affect its performance. If the shoulder is not constructed with the superelevation of the 

travel lanes, there is a potential for a vehicle to be partially airborne if it hits the barrier. This will 

result in the vehicle hitting the barrier higher than normal. The ideal conditions for barrier 

performance would be to have the superelevation continue across the shoulder. If this is not 

practical, using taller barriers may be appropriate. 

Terminating a barrier in 

the vicinity of a 

horizontal curve may 

require special attention. 

The AASHTO Roadside 

Design Guide and the 

Federal Lands Highway 

(FLH) Barrier Guide 

provide some guidance

for determining the 

Length of Need (LON) 

when an obstacle to be 

shielded is in the vicinity 

of a curve. Figure 53 

indicates that the LON 

section of the barrier 

should intersect a

runout path that extends from the farthest point of the obstacle to be shielded to a tangent 

point on the curve. While this may result in less barrier than on a tangent, the theory is that

vehicles that depart the roadway prior to the tangential runout path will not be traveling in the 

direction of the obstacle. 

Relative Cost 

This countermeasure is moderate cost. 

Figure 53. Illustration. Figure 4.4 from the FLH Barrier Guide shows length 

of need (LON) on the outside of a horizontal curve hazard. 

56
CHAPTER 6. ROADSIDE IMPROVEMENTS 

Barrier Maintenance 

Refer to the FHWA-SA-08-002, W-beam Guardrail Repair: A Guide for Highway and Street 

Maintenance Personnel, for general information on the maintenance of barriers. 

Delineation on Barriers 

Barriers that are placed along a highway are usually not visible to the driver at night unless there

is lighting or they are delineated. Delineating a barrier not only gives the indication that a barrier 

is present but also provides the driver with information on the alignment of the roadway. 

Applications 

There are several methods that can

be used to delineate barriers, as 

shown in Figures 54 through 58. For 

W-beam guardrail, delineators can be

attached in the web of the W-beam

with clips held in place by the post

bolts. They can also be installed on

the posts.

Several States have experimented 

with using retroreflective paint, tapes 

or panels in the web of the W-beam. 

Concrete barriers can also be delineated by similar products to those used for W-beam 

guardrails. Metal “butterfly” delineators should not be used where the bolt holds the rail to the 

post. This acts as a washer and may prevent the proper performance of the guardrail in a crash. 

Figure 54. Photo. Retroreflective panels in the web of a Wbeam. Source: Michigan DOT. 

57

Figure 56. Photo. Delineators installed on post, nighttime view. Source: 

Michigan DOT. 

Figure 55. Photo. 

Delineators held in place 

with post bolts and installed 

on post. 
 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Figure 57. Photo. Delineated concrete 

barriers. Source: Michigan DOT. 

Figure 58. Photo. Delineated concrete barriers, nighttime view. 

Source: Michigan DOT. 

Effectiveness 

To date there has been no research on the safety effectiveness of delineation on barriers, and 

therefore, is not listed as a countermeasure by the CMF Clearinghouse. 

Relative Cost 

The countermeasure is low cost.

Maintenance 

Barrier delineation does require maintenance to ensure that it will continue to function. Periodic 

cleaning of the delineation may be needed to remove dirt and road spray. There will also be 

increased costs for repair as these products will have to be replaced. 

58 
CHAPTER 7. ADDRESSING INTERSECTIONS IN CURVES 

 

CHAPTER 7. ADDRESSING INTERSECTIONS IN CURVES 

When an intersecting roadway is located within a curve, it presents a unique safety challenge. 

NCHRP Report 600 Human Factors Guidelines for Road Systems points out that the demands on 

drivers approaching and navigating horizontal curves include visual demands, vehicle control 

demands, and speed selection. The closer a driver is to the curve, the harder it is for the driver 

to effectively assimilate information relating to anything other than navigating the curve. The 

geometry often limits the available sight distance for safe maneuvering and the physical 

constraints of the intersecting roadway often limit the application of signing and other 

delineation. Figure 59 shows an example of an intersection within a curve. 

The AASHTO Policy on Geometric 

Design of Highways and Streets 

recommends that “the alignment 

should be as straight and the 

gradient as flat as practical” at 

intersections to allow for easy 

recognition of the potential 

conflicts. It further states that “an 

intersection on a sharp curve 

should be avoided or designed to 

compensate for potential adverse 

grade and reduced sight distance.” 

However, many agencies have 

existing intersections with less than 

ideal design. 

A study by Indiana Department of Transportation (INDOT) found that curvature was a 

significant factor in the relative safety of intersections where the major road is a four-lane 

divided highway. The same study stated that full curvature and superelevation increased crashes 

by 30 percent in comparison to tangent intersections (Savolainen and Tarko, 2004). 

This chapter discusses treatments unique to the combination of intersections and curves as well 

as modifications that may be appropriate to options discussed in previous chapters to address 

this situation. Similar to Chapter 5, the cost of some of these countermeasures may be less 

expensive if the work is completed as part of larger scheduled projects, such as reconstruction 

or resurfacing, rather than as independent safety projects. 

DELINEATION TREATMENTS 

Adjusting Signs and Markings for the Intersecting Roadway 

Where an intersecting roadway is within the curve, the traditional means of delineating the 

roadway alignment is often interrupted. Center line and edge line markings are typically not 

continued through the intersection. The edge line marking is of particular concern if the

intersecting roadway has a wide throat. The MUTCD allows dotted edge line extensions 

59

Figure 59. Photo. Dotted edge line extensions at an 

intersection within a curve. 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

consisting of 2-foot line segments and 2- to 6-foot gaps through intersections along the mainline, 

as illustrated in Figure 60. In fact, the MUTCD guidance recommends this treatment to help 

guide motorists through the intersection. As discussed in Chapter 3, providing center line or 

edge line markings on the approach and through the curve in corridors where markings are 

otherwise not present channelizes vehicles through the curve. This is particularly beneficial when 

an intersecting roadway is present within the curve. 

Similarly, where chevrons or delineators would typically be used to provide delineation, the 

discontinuance through the intersection may leave a significant portion of the curve lacking 

delineation. Adjusting the location of the remaining chevrons or delineators may be appropriate 

to delineate the maximum curve length. A combination of a curve sign and intersection sign can 

also be used, which is discussed in Chapter 4. Providing a visible stop line on the minor road 

approach may also be helpful, especially where the stop line can be seen from a significant 

distance from the intersection or where crashes indicate stop sign violations. 

Smooth Lane Narrowing 

A combination of treatments used at 

intersections that are particularly beneficial 

where there is curvature on either the 

major or minor road has been dubbed 

“smooth lane narrowing.” As seen in 

Figure 60, the treatment narrows the lane 

width approaching the intersection with a 

combination of markings and rumble 

strips. The narrowing is accomplished by 

gradually tapering out from the center. 

The rumble strips are milled in along both 

the left and right sides of each direction of 

travel, with longitudinal center and edge 

line markings added. The combination of Figure 60. Photo. Pavement markings narrow the travel

rumble strips and markings to narrow the lane as the driver approaches the intersection.

lanes reduces operating speeds on the intersection approach. When a curve is present, the 

preferred design is to narrow the lanes on the approach to the curve. The paved width is not 

changed in this countermeasure, but the narrower lane width continues throughout the entire 

length of the curve. The rumble strips and markings are discontinued at the intersection, as seen

in Figure 61. Using smooth lane narrowing on intersection approaches has been found to reduce 

all crashes by 32 percent and fatal and injury crashes by 34 percent. Additionally, 85th percentile 

speeds were reduced by roughly 5 mph. More information on the design of smooth lane 

narrowing can be found in Crash Impacts of Smooth Lane Narrowing with Rumble Strips at Two-Lane 

Rural Stop Controlled Intersections. 

60 
 

RUMBLE STRIP 

S CTION SECTION SECTION 

C 8 A 

200' 100 

START 

D ME SHOULDER 

RUMB E STRIP RU 

4' 

7 START EB SHOOLOER 'TRANSVERS RKING\ Y LlOW, E O EB SHOULDER 

RUMBLE STRIP 2' WICE, 20' SPAC NG, 4 • ANGLE RU BL£ STRIP 

CHAPTER 7. ADDRESSING INTERSECTIONS IN CURVES 

Figure 61. Illustration. Smooth lane narrowing typical design. 

 

 

 

 

ADDRESSING VISIBILITY ISSUES 

Intersections often pose challenges to drivers who do not always see traffic approaching. If there 

is horizontal or vertical curvature near the intersection, this increases the difficulty. Standard 

intersection practices may need to be adjusted when there are alignment changes, and 

treatments that improve intersection visibility may prove even more beneficial at curves with 

intersections. 

Visual Traps 

A visual trap occurs when the road curves, but visual cues such as breaks in the tree line or the

continuation of power poles lead a driver to think the road continues straight. An example of 

this is illustrated in Figure 62. Frequently, a roadway that intersects the curve is one of the visual

miscues. In such cases, additional emphasis should be placed on warning the driver and 

delineating the curve to overcome the driver expectation of a tangent roadway. Delineators, 

chevrons, or pavement marking signs are treatments appropriate to address this issue. Also, 

advanced markings within the lane may be appropriate. See Chapter 3 and 4 for additional 

information on use of these treatments.

61
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Figure 62. Photo. An example of a visual trap exists when a crest vertical curve blocks the view of the 

upcoming horizontal curve (top photo). What appears to be a continuation of the road in the distance is 

actually an intersecting roadway in the midst of a curve (bottom photo). 

62
CHAPTER 7. ADDRESSING INTERSECTIONS IN CURVES 

 

Intersection Sight Triangles 

In the typical rural curve with an 

intersection, the minor road will be stopcontrolled. Assuming the intersecting

roadway is aligned perpendicular to the 

curve of the main roadway and is at or 

near the center of the curve, the sight 

distance issues on the outside of curve 

are similar or perhaps even better than 

for a tangent roadway section. Providing 

appropriate sight triangles will often be 

adequate. The intersection on the inside 

of the curve, however, is restricted by 

the geometry and requires the driver to 

have more mobility to see over-theshoulder to view oncoming traffic, as Figure 63. Photo. Sight distance is limited due to the

intersection being inside the curve. seen in Figure 63. If the intersection is 

not near the center of the curve, sight 

triangles may cut across the curve and require significantly more clearing, as illustrated in Figure 

64. If the terrain is not flat, it may be necessary to cut into slopes to provide the adequate

minimum intersection sight distance. The use of and location of guardrails on grades should also

be considered as it could interrupt the sight lines for intersections in and near curves.

Figure 64. Illustration. Limiting the growth of vegetation is important 

to maintain appropriate sight triangles. 

Where providing the appropriate intersection sight distance is not feasible, the intersection may 

need to be re-configured. In certain limited cases, an “All-Way” stop-controlled intersection may 

be appropriate. Careful consideration to speeds and traffic volumes are appropriate before 

making the decision to change the intersection to an “All-Way STOP.” Typically a roundabout 

would be preferred to this type of control. 

63 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Intersection Conflict Warning Systems (ICWS) 

Another option for an intersecting roadway with limited sight distance is a dynamic warning sign 

as shown in Figure 65. The ICWS can be designed to either detect vehicles on the minor road 

and indicate their presence to drivers on the main road, or indicate to the driver on the minor 

road when there is oncoming traffic on the mainline. The CMF Clearinghouse lists a 32-percent 

reduction for all crash types when installing a “Vehicles Entering When Flashing” system 

(advanced post-mounted signs on the major road and detection loops on the minor road) at 

stop-controlled intersections (CMF is 4 stars) (Simpson and Troy, 2013). This CMF applies to 

intersection-related crashes, but does not explicitly consider intersections located along 

horizontal curves. More information on this treatment may be found in Stop-Controlled 

Intersection Safety Through Route Activated Warning System. 

Figure 65. Photo. A dynamic warning sign alerts drivers 

in real time of other users in the roadway. 

 

 

 

 

Lighting 

The presence of lighting has been shown in improve safety at intersections. The INDOT study of

intersections with curvature found that crashes tended to be overrepresented during nighttime 

conditions (Savolainen and Tarko, 2004). The effect of lighting is generally limited to nighttime 

crashes, since lighting does not generally improve daytime visibility. The CMF Clearinghouse 

indicates that the presence of intersection lighting is associated with an 11.9-percent reduction 

in total nighttime crashes (CMF is 3 stars) (Donnell et al., 2010). The presence of fixed 

illumination is associated with a 2-percent increase in total daytime crashes on rural roadways 

(CMF is 2 stars) (Bullough et al., 2012), presumably due to the presence of fixed objects near the 

intersection.

64 
PAVEMENT IMPROVEMENTS 

Ma rked Highway, County 

Highway, Town Street, or 

Township Road 

_____ Supe r elevat ed Pavement 

PLAN VIE W 

State 

Highway 

Sid e Roa d 

Gr adeline 

CROSS SECTI ON A-A 

Type of Improvement Maximum Superelevation Rate 

Cateaorv "e" for Intersections on Curve 

"New Construction" at an important 4% Desirable Maximum crossroad 

To remain in place with "Reconstruction" 6% Maximum at an important crossroad 

To remain in place with "Reconstruction" 8% Maximum at a minor crossroad 

Rollover 

Guidelines 

5% Desirable Maximum 

6% Maximum 

7% Desirable Maximum 

8% Maximum 

9% Desirable Maximum 

10% Maximum 

CHAPTER 7. ADDRESSING INTERSECTIONS IN CURVES 

 

Improve Friction 

HFST are particularly beneficial at horizontal curves with an intersection. The treatment, which 

is described in detail in Chapter 5, may need to be extended depending on the location of the

intersection within the curve. In addition, it may be appropriate to extend the treatment to the

minor road if it has a high speed approach or crashes indicate a need. 

Adjust Superelevation for an Intersecting Roadway 

When an intersecting roadway is within the curve, adjustments need to be made to 

superelevation. This is different than the corrections to superelevation that was discussed in 

Chapter 5. State design manuals often provide guidance on how to accomplish this, such as the 

example in Figure 66 provided by Illinois DOT. 

Figure 66. Illustration. Excerpt showing the design of superelevation from Figure 36-1.E of Illinois DOT’s 

Bureau of Design & Environmental Manual. Source: Illinois DOT. 

65
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Pave Intersection Approach on Gravel Roads 

When the road intersecting the curve is unpaved, the result is often either a drop-off at the edge 

of the pavement or aggregate from the unpaved road gets on the paved portion of the curve. 

When there is a drop-off, the resulting issue is described in Chapter 4. Loose aggregate on the 

paved portion of the roadway can result in reduced friction between vehicle tires and the

pavement. A solution that can be used for both of these issues is to pave a portion of the 

approach on the leg of the intersection that is unpaved, as illustrated in Figures 67 and 68. 

Figure 67. Photo. Intersection with paved approach. Figure 68. Photo. Intersection with aggregate 

scattered on paved roadway. 

 

CHANGING INTERSECTION 

CONFIGURATION 

When an intersection is at or near a horizontal 

curve, it is not uncommon for the location or 

the configuration of the intersection to cause 

safety concerns. The issue may be traffic on the 

major road not seeing or recognizing that a 

vehicle ahead is stopped while waiting to turn. 

Or, drivers on either the major or minor road 

may have difficulty seeing each other due to the

alignment, as shown in Figure 69. Low-cost 

solutions cannot always address these stated 

intersection sight distance concerns. When

considering higher cost solutions, it is important 

to address the most severe and more frequent 

crash types. 

Figure 69. Illustration. Intersection with a skewed 

approach with an arrow indicating the driver’s 

line of sight. 

66
With roundabouts, head-on and high-speed 

right angle collisions are virtually eliminated. 

Traditional Intersection Roundabout 

• Potential Vehicle Conflict Point 

CHAPTER 7. ADDRESSING INTERSECTIONS IN CURVES 

Roundabouts 

Roundabouts are one of the most 

efficient ways to reconfigure an 

intersection, particularly when there is 

existing curvature. A roundabout is a 

circular intersection with yield control 

for all legs approaching the 

intersection. As illustrated in Figure 

70, roundabouts typically provide the 

most efficient flow of traffic, reduce

severe crashes, and can often be built 

at the same or lower cost than the 

more traditional options discussed 

below. 

Roundabouts use roadway curvature 

and islands to reduce speeds of 

approaching vehicles. Most importantly, 

Figure 70. Illustration. Comparison of potential conflict

points between a traditional intersection and roundabout.

roundabouts reduce the points of conflict. Drivers need only check for traffic on their left before 

entering the circulating roadway. Crash types within a roundabout tend to be sideswipe and 

rear-end, which are typically less severe than the angle crashes that are more common at a 

traditional intersection. Severity is also typically reduced because speeds are slower at a 

roundabout. Traffic flow is smooth because each approaching vehicle only waits if there is a not 

an opening in within the circular portion of the roadway. 

Where a skewed intersection currently exists—which is common within horizontal curves—

redesigning the intersection with a roundabout allows more flexibility in alignment than the 

standard practice of realigning the minor roadway to make the intersection perpendicular 

(shown in Figures 71, 72, and 73). In addition, the roundabout will typically result in significantly 

greater crash reductions because all turning movements at the intersection are safer due to the 

reduced speeds and conflict points. The HSM states that by converting from a two-way stop 

control mechanism to a roundabout, a location can experience an 82-percent reduction in 

severe (injury/fatal) crashes and a 44-percent reduction in overall crashes. It also indicates that 

by converting a signalized intersection to a roundabout, a location can experience a 78-percent 

reduction in severe (injury/fatal) crashes and a 48-percent reduction in overall crashes 

(AASHTO, 2010). 

67
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Figure 71. Illustration. Typical skewed 

intersection within a curve. 

Figure 72. Illustration. Re-aligning a 

skewed intersection to provide a 

perpendicular intersection. 

Figure 73. Illustration. Reconfiguring the 

intersection with a roundabout. 

68
CHAPTER 7. ADDRESSING INTERSECTIONS IN CURVES 

 

 

 

Add Turn Lanes 

If turning movements at the intersection within a horizontal curve are the primary safety 

concern and a roundabout is not feasible to resolve the issue, adding turn lane for the primary 

turning movement may resolve the problem. Left turn lanes on the major roadway remove 

turning vehicles from the high-speed through lane. If existing widths allows room to change the 

lane configuration with little or no additional pavement widening, restriping to add turn lanes can 

be very cost effective. The CMF Clearinghouse indicates that providing a channelized left-turn 

lane at a three-leg intersection on the major-road approach is associated with a 27-percent 

reduction in all crashes (CMF is 3 stars) (Elvik and Vaa, 2004). The CMF Clearinghouse also

indicates that providing a right-turn lane on one major approach to a stop-controlled 

intersection is associated with a 14-percent reduction in all crashes (CMF is 3 stars) (Harwood 

et al., 2002). While these are not specifically for intersections within curves, it would be 

expected that intersections within curves would experience the same benefits in safety.

69 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

REFERENCES 

Al-Masaeid, Hashem R. and Kumares C. Sinha. Analysis of Accident Reduction Potentials of Pavement 

Markings. Journal of Transportation Engineering, 120(5), 1994. pp. 723-736. 

American Association of State Highway and Transportation Officials. Highway Safety Manual.

Washington, DC: American Association of State Highway and Transportation Officials, 2010. 

American Association of State Highway and Transportation Officials. Roadside Design Guide. 

Washington, DC: American Association of State Highway and Transportation Officials, 2011. 

Arnold, E. D., Jr, and K. E. Lantz, Jr. Evaluation of Best Practices in Traffic Operations and Safety: 

Phase I: Flashing LED Stop Sign and Optical Speed Bars. Richmond, VA, Federal Highway 

Administration, 2007. 

Atkinson, Jennifer E., Brian E. Chandler, Vernon Betkey, Karen Weiss, Karen Dixon, Anna 

Giragosian, Kelly Donoughe, and Cara O’Donnell. Manual for Selecting Safety Improvements on 

High Risk Rural Roads. Report No. FHWA-SA-14-175, Federal Highway Administration, 

Washington, DC, 2014. 

Bahar, Geni, Calvin Mollett, Bhagwant Persaud, Craig Lyon, Alison Smiley, Tom Smahel, and 

Hugh McGee. NCHRP Report 518: Safety Evaluation of Permanent Raised Pavement Markers. 

Transportation Research Board, National Research Council, Washington, DC, 2004. 

Bauer, Karin M. and Douglas W. Harwood. Safety Effects of Horizontal Curve and Grade 

Combinations on Rural Two-Lane Highways. Report FHWA-HRT-13-077, Federal Highway 

Administration, Washington DC, 2014. 

Bertini, Robert L., Chrisopher M. Monsere, Casey Nolan, Peter Bosa, and Tarek A. El-Seoud. 

Field Evaluation of the Myrtle Creek Advanced Curve Warning System. Salem, Oregon, Oregon 

Department of Transportation, 2006. 

Boodlal, Leverson, Eric T. Donnell, Richard J. Porter, Dileep Garimella, Thanh Le, Kevin 

Croshaw, Scott Himes, Philip Kulis, and Jonathan Wood. Factors Influencing Operating Speeds and 

Safety on Rural and Suburban Roads. Report No. FHWA-HRT-15-030, Federal Highway 

Administration, Washington, DC, 2015. 

Brimley, Brad and Paul Carlson. Using High Friction Surface Treatments to Improve Safety at 

Horizontal Curves. Texas Transportation Institute, 2012. 

(http://d2dtl5nnlpfr0r.cloudfront.net/tti.tamu.edu/ documents/TTI-2012-8.pdf). 

Bullough, John D., E.T. Donnell, and M.S. Rea. “To illuminate or nor to illuminate: Roadway 

lighting as it affects traffic safety at intersections.” Accident Analysis and Prevention, Vol. 53, pp. 65-

77, 2012 

70 
REFERENCES 

Campbell, John. L., Monica G. Lichty, James L. Brown, Christian M. Richard, Justin S. Graving, 

Jerry Graham, Mitchell O’Laughlin, Darren Torbic, and Douglas Harwood. NCHRP Report 600: 

Human Factors Guidelines for Road Systems. Transportation Research Board of the National 

Academies, Washington, DC, 2012. 

Carlson, Paul, Eun S. Park, and Dong H. Kang. Investigation of Longitudinal Pavement Marking 

Retroreflectivity and Safety. Transportation Research Record, Volume 2337, pp. 59-66, 2013. 

Chrysler, Susan T., and Steven D. Schrock. Field Evaluation and Driver Comprehension Studies of 

Horizontal Signing. FHWA/TX-05/0-4471-2. Texas Transportation Institute, 2005. 

Council, Forrest M., Martine Reurings, Raghavan Srinivasan, Scott Masten, and Daniel Carter. 

Development of a Speeding-Related Crash Typology. Report No. FHWA-HRT-10-024, Federal 

Highway Administration, Washington, DC, 2010. 

Donnell, Eric T., Richard J. Porter, and Venkataraman N. Shankar. "A Framework for Estimating 

the Safety Effects of Roadway Lighting at Intersections." Safety Science, Vol. 48(10), pp. 1436-1444, 

2010. 

Elvik, Rune and Truls Vaa. Handbook of Road Safety Measures. Oxford, United Kingdom, Elsevier, 

2004. 

Federal Highway Administration. Crash Modification Factors Clearinghouse, Accessible at: 

http://www.cmfclearinghouse.org/. 

Federal Highway Administration. Manual on Uniform Traffic Control Devices, Washington, DC, 

2009. Accessible at: http://mutcd.fhwa.dot.gov/. 

Federal Highway Administration Office of Safety. Toward Zero Deaths, Accessible at: 

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

Federal Highway Administration. Strategic Highway Safety Plan, Washington, DC, 2014. 

Accessible at: http://safety.fhwa.dot.gov/hsip/shsp/. 

Glennon, J.C., T.R. Neuman, and J E. Leisch. Safety and Operational Considerations for Design of 

Rural Highway Curves. Report No. FHWA-RD-86-035, Federal Highway Administration, 

Washington, DC, 1985. 

Graham, J.L., K.R. Richard, M.K. O’Laughlin, and D.W. Harwood. Safety Evaluation of the Safety 

Edge Treatment Report No. FHWA-HRT-11-024, Federal Highway Administration, Washington, 

DC, 2011. 

Hall, J.W., Jr., K.L. Smith, L. Titus-Glover, J.C. Wambold, T.J. Yager, and Z. Rado. Guide for 

Pavement Friction. National Cooperative Highway Research Project 01-43 Final Rep., 2011. 

(http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_w108.pdf). 

71 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Hallmark, Shauna L., Neal Hawkins, and Omar Smadi. Evaluation of Low-Cost Treatments on Rural 

Two-Lane Curves. Report No. IHRB Project TR-579, Midwest Transportation Consortium, Iowa 

Department of Transportation and Iowa Highway Research Board, Ames, IA, 2012. 

Hallmark, Shauna L., Neal Hawkins, and Omar Smadi. Evaluation of Speed Activated Displays on 

Curves. Report No. FHWA-PROJ-07-0020, Federal Highway Administration, Washington, DC, 

2015. 

Harwood, D.W., Bauer, K.M., Potts, I.B., Torbic, D.J., Richard, K.R., Rabbani, E.R., Hauer, E., and 

Elefteriadou, L., Safety Effectiveness of Intersection Left- and Right-Turn Lanes. Report No. FHWARD-02-089, Federal Highway Administration, Washington, DC, 2002. 

Hummer, Joseph E., William Rasdorf, Daniel J. Findley, Charles V. Zegeer, and Carl A. 

Sundstrom. “Curve Crashes: Road and Collision Characteristics and Countermeasures.” Journal 

of Transportation Safety and Security, Southeast Transportation Research Center, 2010. 

Katz, Brian J. Pavement Markings for Speed Reduction. McLean, VA: Turner-Fairbank Highway 

Research Center, 2004. Retrieved from: http://www.pooledfund.org/Document/Download/412. 

Kelkka, Marko. Safety of Roadside Area. Analysis of Full-Scale Crash Tests and Simulations. Finnish 

Road Administration, Central Administration, Helsinki, 2009. 

Knapp, Keith K., and Ferrol Robinson. The Vehicle Speed Impacts of a Dynamic Horizontal Curve 

Warning Sign on Low-Volume Local Roads. Report No. CTS 12-12, Minnesota Department of 

Transportation, 2012. 

Lyles, Richard W., and William C. Taylor. NCHRP Report 559: Communicating Changes in 

Horizontal Alignment. Transportation Research Board of the National Academies, Washington, 

DC, 2006. 

Mahoney, Kevin M., Frank Julian, and Harry W. Taylor, Jr. Good Practices: Incorporating Safety into 

Resurfacing and Restoration Projects. Report No. FHWA-SA-07-001, Federal Highway 

Administration, Washington, DC, 2006. 

McGee, Hugh W., and Fred R. Hanscom. Low-Cost Treatments for Horizontal Curve Safety. Report 

No. FHWA-SA-07-002, Federal Highway Administration, Washington, DC, 2006. 

Montella, A. Safety Evaluation of Curve Delineation Improvements An Empirical Bayes Observational 

Before-After Study. TRB 88th Annual Meeting Compendium of Papers CD-ROM. Washington, 

DC, 2009. 

National Highway Traffic Safety Administration. Traffic Safety Facts 2013, Washington, DC, 2014. 

National Highway Traffic Safety Administration. Fatality Analysis Reporting System, Accessible at: 

http://www.nhtsa.gov/FARS. 

72 
Simpson, C.L., and S.A. Troy. "Evaluation of the Safety Effectiveness of 'Vehicle Entering When 

Flashing' Signs and Actuated Flashers at 74 Stop-Controlled Intersections in North Carolina". 

Presented at the 92nd Annual Meeting of the Transportation Research Board, Paper No. 13-

1159, Washington, DC, 2013. 

REFERENCES 

National Research Council. NCHRP Report 500 Volume 18: A Guide for Reducing Collisions Involving 

Bicycles. Washington, DC: The National Academies Press, 2008. 

Neuman, Timothy R., Ronald Pfefer, Kevin L. Slack, Kelly K. Hardy, Hugh McGee, Leanne 

Prothe, Kimberly Eccles, and Forrest Council. NCHRP Report 500: Guidance for Implementation of 

the AASHTO Strategic Highway Safety Plan. Volume 4: A Guide for Addressing Head-On Collisions. 

Transportation Research Board of the National Academies, Washington, DC, 2003. 

Neuman, Timothy R., Ronald Pfefer, Kevin L. Slack, Kelly K. Hardy, Douglas W. Harwood, Ingrid 

B. Potts, Darren J. Torbic, and Emilia R. Kohlman Rabbani. NCHRP Report 500: Guidance for

Implementation of the AASHTO Strategic Highway Safety Plan. Volume 5: A Guide for Addressing

Unsignalized Intersection Collisions. Transportation Research Board of the National Academies,

Washington, DC, 2003.

Neuman, Timothy R., Ronald Pfefer, Kevin L. Slack, Kelly K. Hardy, Forrest Council, Hugh 

McGee, Leanne Prothe, and Kimberly Eccles. NCHRP Report 500: Guidance for Implementation of 

the AASHTO Strategic Highway Safety Plan. Volume 6: A Guide for Addressing Run-Off-Road Collisions. 

Transportation Research Board of the National Academies, Washington, DC, 2003. 

Park, Eun S., Paul J. Carlson, Richard J. Porter, and Carl K. Andersen. “Safety Effects of Wider 

Edge Lines on Rural, Two-Lane Highways.” Accident Analysis Prevention, pp. 317-325. 2012. 

Pitale, Jaswandi T., Craig Shankwitz, Howard Preston, and Michael Barry. Benefit-Cost Analysis of 

In-Vehicle Technologies and Infrastructure Modifications as a Means to Prevent Crashes Along Curves 

and Shoulders. Minnesota Department of Transportation, 2009. 

Potts, Ingrid B., Douglas W. Harwood, Courtney D. Bokenkroger, Melanie M. Knoshaug. 

Benefit/Cost Evaluation of MoDOT’s Total Striping and Delineation Program: Phase II. Missouri 

Department of Transportation, 2011. 

Preston, Howard, Richard Storm, Jacqueline D. Bennett, Beth Wemple. Systemic Safety Project 

Selection Tool. Federal Highway Administration, FHWA-SA-13-019, Washington, DC, 2013. 

Retting, R., and C. Farmer. “Use of Pavement Markings to Reduce Excessive Traffic Speeds on 

Hazardous Curves.” ITE Journal, pp. 30-36. 1998. 

Savolainen, Peter T. and Andrew P. Tarko. Safety of Intersections on High-Speed Road Segments 

with Superelevation. Report No. FHWA/IN/JTRP-2004/25, Federal Highway Administration, 

Washington, DC, 2004. 

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Srinivasan, Raghavan, Jongdae Baek, Daniel Carter, Bhagwant Persaud, Craig Lyon, Kimberly 

Eccles, Frank Gross, Nancy Lefler. Safety Evaluation of Improved Curve Delineation. Report No. 

FHWA-HRT-09-045, Federal Highway Administration, Washington, DC, 2009. 

Torbic, Darren J., Douglas W. Harwood, David K. Gilmore, Ronald Pfefer, Timothy R. Neuman, 

Kevin L. Slack, and Kelly K. Hardy. NCHRP Report 500: Guidance for Implementation of the AASHTO 

Strategic Highway Safety Plan. Volume 7: A Guide for Reducing Collisions on Horizontal Curves. 

Transportation Research Board of the National Academies, Washington, DC, 2004. 

Torbic, D.J., J.M. Hutton, C.D. Bokenkroger, K.M. Bauer, D.W. Harwood, D.K. Gilmore, J. M. 

Dunn, J. J. Ronchetto, E.T. Donnell, H.J. Sommer III, P.M. Gervey, B. Persaud, and C. Lyon. 

NCHRP Report 641: Guidance for the Design and Application of Shoulder and Centerline Rumble Strips. 

Transportation Research Board of the National Academies, Washington, DC, 2009. Accessible 

at: http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_641.pdf. 

Tribbett, Lani, Patrick McGowan, and John Mouncel. An Evaluation of Dynamic Curve Warning 

Systems in the Sacramento River Canyon: Final Report. California Department of Transportation, 

2000. Retrieved from: 

http://www.itslessons.its.dot.gov/its/benecost.nsf/ID/B2437E80B24675D28525699300696D62?O

penDocument&Query. 

Tsyganov, Alexei R., Nicholas M. Warrenchuk, and Randy B. Machemehl. Driver Performance and 

Safety Effects of Edge Lines on Rural Two-Lane Highways. TRB 88th Annual Meeting Compendium of 

Papers CD-ROM. Washington, DC, 2009. 

Zegeer, Charles V., J. Richard Stewart, Forrest M. Council, Donald W. Reinfurt, and Elizabeth 

Hamilton. Safety Effects of Geometric Improvements on Horizontal Curves. Transportation Research

Record 1356, Transportation Research Board, Washington, DC, pp. 11-16. 1992. 

74
 

GLOSSARY

GLOSSARY

Term Description 

Acrylic material Binder material used in high friction surface treatment that 

holds the aggregate firmly to the pavement.

ADT Average Daily Traffic – the traffic volume of a road 

measured in vehicles per day.

Advisory speed plaque A sign that is placed below a Horizontal Alignment sign to 

advise motorists of the safe speed through the curve.

Ball bank indicator An inclinometer that is used for determining safe curve 

speeds for horizontal curves. 

Cable barrier A flexible barrier made from wire rope supported between 

frangible posts. 

Calcined bauxite aggregate A hard, coarse aggregate used in high friction surface 

treatment.

Clear Zone The unobstructed traversable area provided beyond the 

edge of the through traveled way for the recovery of errant 

vehicles, as defined by AASHTO. 

CMF Crash Modification Factor – a multiplicative factor used to 

compute the expected number of crashes after 

implementing a countermeasure. 

CMF Clearinghouse A website that provides the largest collection of CMFs for 

geometric design elements and traffic control devices

available in the United States. 

Compound curves Two or more tangential, consecutive curves.

Concrete safety shape A rigid barrier that does not deflect.

Curve delineation Treatments that enhance the conspicuity of a curve (e.g., 

wider edge line, higher retroreflectivity of signs, postmounted delineators, chevrons, raised pavement markings). 

Delineators A device mounted above the roadway surface and along the 

side of the road in a series to indicate roadway alignment.

75
 

 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

Epoxy material Binder material used in the application of high friction 

surface treatment that hold the aggregate firmly to the 

pavement. 

FARS Fatality Analysis Reporting System – a nationwide census

providing annual data regarding fatal injuries in motor 

vehicle traffic crashes.

Guardrail A semi-rigid barrier usually either a steel box beam or Wbeam that deflect less than flexible barriers. 

HFST High Friction Surface Treatment – a thin layer of aggregate 

bonded to the pavement surface designed to increase 

friction and compensate for sharp curves.

Milled Milled rumble strips are made by a machine with a rotary 

cutting head, creating a smooth, uniform, and consistent 

groove in the pavement. They cause tire noise and vehicle 

vibration when traversed. 

MUTCD Manual on Uniform Traffic Control Devices – defines the 

standards used by road managers nationwide to install and 

maintain traffic control devices on all public streets, 

highways, bikeways, and private roads open to public traffic. 

Pavement grooving A pavement countermeasure technique to apply 

longitudinal or transverse cuts onto the pavement surface 

to increase or restore pavement friction. 

Pavement raveling Deterioration of the pavement surface caused by aggregate 

particles becoming dislodged. 

Retroreflective A material or device that reflects light back to its source. 

RSA Road Safety Audit – a formal safety performance 

examination of an existing or future road or intersection by 

an independent, multidisciplinary team. 

Safety EdgeSM A paving technique used system-wide to improve pavement 

durability and reduce crashes by shaping and consolidating 

the pavement edge into a 30 degree wedge.

Superelevation The banking of a horizontal curve. 

76
Systemic approach The analysis of crash data on a system-wide basis that 

considers identifying factors that indicate higher risks for 

severe crashes. 

GLOSSARY 

77
 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

APPENDIX A: LOW-COST SAFETY IMPROVEMENTS IN 

PENNSYLVANIA 

BACKGROUND 

A higher percentage of fatal 

curve-related crashes 

occur on rural roads—

particularly on two-lane 

rural roads—due largely in 

part to the predominance

of horizontal curves on 

typical rural roads. In 2013, 

more than 13 percent of 

fatal crashes in 

Pennsylvania occurred due 

to Curve Driver Error 

Crashes, many of which 

involved roadway 

departures. To address 

curve-related crashes, the 

Pennsylvania Department 

of Transportation 

Figure A-1. Photo. An orange flag supplementing the reverse curve

warning sign and speed advisory plaque. Chevrons were also used 

to delineate the curve. 

 

(PennDOT) provided 

guidance to the districts on the implementation of enhanced sign and marking improvements for 

curves that exhibit a higher than expected crash frequency. PennDOT identified priority curves 

by examining crash frequency, crash rate, and crash severity.

PROGRAM DETAILS 

Pavement markings in advance of horizontal curves provide additional warning information, and 

can also be considered at curve locations where signs alone have been shown to be insufficient. 

Systematic implementation of improvements on curves is taking place throughout Pennsylvania 

and include one or more of the following strategies: 

 Oversized fluorescent yellow advance curve warning signs that could be doubled up (i.e.,

both sides of the roadway), with optional flashing yellow LED solar powered beacons.

 Advanced curve pavement markings including a “SLOW” legend or “XX MPH” advisory

speed marking as an alternate. (NOTE: Since the PennDOT program began, the NCHRP

Report 600 has since been released and contains design guidelines as to which markings are

effective in reducing speeds at horizontal curves and which markings are not as effective (p. 20-

4)).

 Correction of any shoulder drop offs within the curve.

 Chevron delineation around the curve.

 Curve widening.

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APPENDIX A. LOW-COST SAFETY IMPROVEMENTS IN PENNSYLVANIA 

PennDOT estimates there have been between 500 to 600 applications of the low-cost safety 

improvements throughout the State, many of which have been State funded. 

RESULTS 

PennDOT Districts have found this combination of treatments, when utilized correctly, 

effectively contributes to combatting curve-related crashes. Data suggests these safety 

treatments have improved safety on specifically identified horizontal curves. Overall, Curve 

Driver Error Crashes have fallen from a five-year average of 6,798 in 2007 to 5,060 in 2012. A 

three-year before/after analysis of locations where a combination of these countermeasures 

were implemented between 2000 and 2008 resulted in the following: 

 Decrease in overall crashes from 1,452 to 1,200 (17-percent reduction).

 Decrease in fatal crashes from 27 to 15 (44-percent reduction).

 Decrease in major injury crashes from 65 to 39 (40-percent reduction).

When the treatment was first introduced, there were concerns of motorcycles slipping on the 

paint (2001); but over time, the treatments have become widely accepted and the general public 

have not voiced any other concerns. 

CONSIDERATIONS 

PennDOT stresses the importance of sound engineering judgment when selecting the 

combination of countermeasures for implementation. Additionally, speed limit compliance, 

geometric features of the curve, sight distance, and traffic volume must be taken into

consideration when implementing this treatment. Finally, high friction surface treatment should 

be considered as another possible countermeasure where wet pavement/curve related crashes

occur at a higher rate. 

As noted, NCHRP Report 600 has since been released and contains design guidelines as to which

markings are effective in reducing speeds at horizontal curves and which markings are not as 

effective.

CONTACT INFORMATION 

The Traffic and Safety Section of PennDOT provided the information for this case study. Visit 

http://www.penndot.gov/ for more information. All images are courtesy of PennDOT. 

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LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

APPENDIX B: SYSTEMIC IMPROVEMENTS IN MINNESOTA 

BACKGROUND 

Horizontal curves only comprise 10 percent of the rural roadway network in Minnesota; yet 

from 2003 to 2011, 20 percent of crashes occurred on curves (when reviewed in five-year 

increments). The crash data revealed that over 25 percent of the fatal and serious injury crashes 

occurred at curves. Moreover, over 30 percent of the fatal and serious injury roadway departure 

crashes occurred at curves. 

In 2007, the Minnesota Department of Transportation (MnDOT) completed and delivered the 

first phase of County Safety Plans and District Plans in an effort to improve curve safety. 

MnDOT started with studies in Olmsted County in Southern Minnesota, and then expanded to

20 other counties and eventually, all 87 

counties. The first phase of the analysis 

examined data from 2003-2007 and revealed 

there were nearly 6,900 curves in the State, 

77 fatal crashes, 150 A injury (serious) 

crashes, 349 B injury (moderate) crashes, 394 

C injury (minor) crashes, and 1,117 Property 

Damage Only (PDO) crashes. It took

approximately three years to complete the 

analysis process and the data range kept 

moving as data became available. The final 

analysis used county data from 2007-2011. Figure B-1. Photo. Example of a visual trap; one of 

From the analysis, MnDOT identified five risk MnDOT’s five risk factors for horizontal curves. 

factors these high-crash curves had in common, including: 

1. Radii, typically between 500 and 1,200 feet.

2. Average Daily Traffic (ADT) volume between 500 and 1,000 vehicles per day, depending

on the region.

3. Intersection in the curve.

4. Presence of a visual trap.

5. Prior crash history (i.e., if the curve has had a severe crash in the study period).

Beginning in 2010, action was taken to address the curves that exhibited three or more of the 

five risk factors with the systemic approach to reduce crashes 

(http://safety.fhwa.dot.gov/systemic). 

PROGRAM DETAILS 

Despite some hesitancy to supplement the traditional reactive approach to safety with a 

proactive approach, the program was launched in all 87 counties and 8 districts in 2010. MnDOT 

has also since recommended treating thousands of curves on state-maintained roads. 

80 
RESULTS 

APPENDIX B: SYSTEMIC IMPROVEMENTS IN MINNESOTA 

County and district traffic engineers are provided a list of high risk curves and recommended 

potential project types to select from. Based on that list, each county and district is responsible 

for submitting candidate projects and, if selected, contract and construction administration on 

their system. 

As part of their systemic program, 

MnDOT has recommended installing 

several countermeasures including edge 

line and center line rumble strips, advanced 

signing, 2-foot shoulder paving adding 

rumble strips and Safety EdgeSM, 6-inch 

edge lines, intersection lighting when there 

is an intersection in a curve, and 

delineators. Chevrons have been the most 

commonly installed countermeasure as 

part of this program. The districts received 

funds from direct capital funds, Highway 

Safety Improvement Program (HSIP), and Figure B-2. Photo. MnDOT installed chevrons as 

part of their Systemic Safety Improvements 

Program. 

Section 164 safety funds. Counties 

received funding from HSIP in addition to 

their own capital program. MnDOT’s 

“sharing” of the Federal HSIP funds with local agencies was critical to the success of the 

program. Without those funds the plans would not have been able to be implemented on the

county/local system.

Minnesota has recorded a drop in roadway departure crashes from 2009 to 2013. Although 

definitive data are not yet available, it is assumed that the systemic safety improvement program

contributed to the decrease in crashes. Detailed information about the performance of individual 

curves will be needed in order to further quantify the program effectiveness. Additionally, the 

general public has provided MnDOT with unsolicited positive feedback in response to the

treatments, especially regarding the chevrons, enhanced (6-inch) edge lines, and lighting. 

CONSIDERATIONS 

MnDOT noted that effective communication with the general public and local agencies has 

played a large part of the program’s success. The approval of the public is a crucial aspect of 

moving a program from idea to reality. MnDOT noted that while the Highway Safety Manual 

(HSM) is an important tool for modeling to show proposed projects’ safety benefits, the general 

public do not generally have the background knowledge to understand the methodology. They 

may even become suspicious that the agency is masking information or motivations behind the

numbers. MnDOT showed consideration for the public by creating programmatic goals and 

objectives that were accessible and easy to understand, which in turn contributed to the public’s 

support of the program.

81 
 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

CONTACT INFORMATION 

Brad Estochen and Derek Leuer from MnDOT provided the information for this case study. 

Visit http://www.dot.state.mn.us/ for additional information. All images are courtesy of MnDOT. 

82 
APPENDIX C: APPLICATION OF EDGE LINES IN MISSOURI 

APPENDIX C: APPLICATION OF EDGE LINES IN MISSOURI 

BACKGROUND 

Horizontal curves are the primary location for roadway departure crashes in Missouri—

accounting for approximately two-thirds of the run-off-road crashes in the State. The Missouri 

Department of Transportation (MoDOT) is responsible for over 33,000 centerline miles of 

roadways; and in an effort to address this problem, the agency proposed using a systemic safety 

approach to add an edge line to many of the two-lane rural roads in 2008. With approximately 

18,000 miles of roadway on the Missouri State system that carry less than 1,000 vehicles daily, 

MoDOT was limited by budgetary restraints to restripe every mile. Therefore, MoDOT needed 

a way to prioritize the roads in need of improvement.

PROGRAM DETAILS 

MoDOT chose to apply an edge line 

stripe to state-maintained roads with an 

Annual Average Daily Traffic (AADT) 

volume between 400 to 1,000 vehicles 

per day. Roads with an AADT greater 

than 1,000 vehicles per day were 

assumed to already have an edge line, 

while roads with an AADT less than 400 

vehicles per day were assumed to only 

have a center line stripe (which is 

sufficient for a low volume road). 

Once MoDOT identified the treatment 

locations, they were able to move the 

process forward by first changing 

internal policy and receiving 

management approval. The next step 

was for individual districts to provide estimated initiation timelines and completion dates for the 

project. One district in particular was ambitious and completed their striping within one year. 

All Missouri districts are now required to restripe every other year but are not required to take 

on additional miles below the 400 vehicles per day AADT threshold. 

RESULTS 

A simple before-after analysis of the locations treated with an edge line stripe showed a total of 

576 crashes from 2006 to 2008—105 of which involved a fatality or severe injury. After edge 

lines were added to these roadways, the two-year after data (2010-2011) showed that total 

crashes decreased 43 percent to 327 crashes, and crashes involving a fatality or severe injury 

decreased 56 percent to 46 crashes. A more sophisticated empirical Bayes analysis found that 

the addition of edge lines reduced total crashes for all crash types by 15 percent. The analysis 

Figure C-1. Photo. Application of edge line at a 

horizontal curve. 

83 
 

LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

also revealed that the treatment reduced severe crashes by 19 percent. MoDOT utilized the 

Systemic Safety Project Selection Tool for the evaluation. 

MoDOT has not received any negative feedback regarding the new edge lines from the general 

public or local agencies. 

CONSIDERATIONS 

MoDOT recommends that agencies use a systemic approach to safety, especially with regards to 

edge lines. Since it is not feasible to stripe and maintain every road in the State, MoDOT 

suggests treating sites with higher volumes as those roads will have a greater probability of a 

crash occurring. The improvement process should be data-driven to ensure justification of 

location prioritization. The local county agencies would benefit from installing edge lines on their

roadway system, even though they lack the data to properly identify the roads that may warrant 

the treatment. 

CONTACT INFORMATION 

John Miller from MoDOT provided the information for this case study. Visit 

http://www.modot.org/ for more information. All images are courtesy of MoDOT. 

84
APPENDIX D: UPGRADING CURVE SIGNING IN OHIO 

 

 

APPENDIX D: UPGRADING CURVE SIGNING IN OHIO 

BACKGROUND 

Over 50 percent of Ohio’s fatalities involve roadway departure—many of which were a result of 

high-speed lane departure crashes on the State’s rural roads. In response to this issue, the Ohio 

Department of Transportation developed a systemic program to address roadway departures at 

curves. In 2010, Ohio DOT introduced a Horizontal Curve Program for state-maintained roads. 

The agency focused their efforts on upgrading and installing various signage at curves to address 

the problem. Ohio DOT chose these countermeasures due to the low-cost and their ability to 

be installed at hundreds of locations by all 12 districts in the State. Ohio oversees 42,250 

interstate, U.S., and State route lane miles and maintains approximately 500,000 signs. The 12 

districts within Ohio DOT are responsible for maintaining State and Federal roadways. The 

program was widely accepted by the districts in Ohio, seeing it as a realistic and achievable step 

to address the high number of fatalities and injuries prevalent along curves. Much of the success

of the program can be attributed to effective communication and coordination between the 

central Ohio DOT office and the districts. 

Figure D-1. Photo. A curve on a rural, two-lane road 

before signage updates through the Horizontal 

Curve Program. 

Figure D-2. Photo. A curve on a rural, two-lane 

road after signage updates through the 

Horizontal Curve Program. 

PROGRAM DETAILS 

To facilitate implementation for the districts, Ohio DOT’s central office in Columbus provided 

each district a list of curve locations ranked by crash frequency. A total of 576 sites were 

selected by using a threshold of 6 or more crashes over a 5-year period on 0.3 mile segments to 

flag problematic locations. Next, individual districts conducted site field reviews, evaluated 

existing conditions and countermeasures onsite, and selected the appropriate signs to be 

installed at the site. The first round of upgrading curve signage began in 2010 and the districts 

used funds from High Risk Rural Roads or Highway Safety Improvement Program. Ohio DOT’s 

systematic program is currently implementing additional curve sign upgrades that were

developed through an FHWA Roadway Departure Plan, a 2-year program will finish in summer 

2015. This effort is funded through other Federal safety dollars that are set aside from Ohio 

DOT’s safety program total each year specifically for systemic improvements. 

85 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

86 

RESULTS 

As of late 2014, Ohio DOT is in the process of analyzing the safety effectiveness of the sign 

upgrades, starting with the locations treated in 2010. The results of the signage upgrade from the 

Horizontal Curve Program have been received positively by the general public and local agencies 

alike. The districts noted that treating problematic curves is easy to implement when the central 

DOT office provides them with the necessary tools (i.e., the list of high-crash curves and sign 

order forms). Also, drivers are pleased that the signs provide proper guidance around curves, 

especially at nighttime. 

CONSIDERATIONS 

A key aspect of the program’s success is the cooperation between the DOT main office and the 

individual State districts. The central DOT office supplies the data, which allows the districts to 

focus time and staff on site visits and implementation of the appropriate solutions. For agencies 

considering a similar program, Ohio DOT emphasized the importance of having a data-driven 

program. Crash data enables Ohio DOT to generate the crash lists and prioritize locations for 

Districts to address. 

CONTACT INFORMATION 

Michael McNeill from Ohio DOT provided the information for this case study. Visit 

http://www.dot.state.oh.us/ for more information. All images are courtesy of Ohio DOT. 

Figure D-3. Photo. A curve on a rural, two-lane 

road before signage updates through the 

Horizontal Curve Program. 

Figure D-4. Photo. A curve on a rural, two-lane 

road after signage updates through the 

Horizontal Curve Program. 
APPENDIX E: APPLICATION OF SDCWS 

87

APPENDIX E: APPLICATION OF SEQUENTIAL DYNAMIC CURVE 

WARNING SYSTEMS (SDCWS) 

BACKGROUND 

Roadway departure crash rates are three times higher at horizontal curve locations relative to 

tangent segments of roadway. Sequential Dynamic Curve Warning Systems (SDCWS) have been 

implemented as a countermeasure on two-lane rural highway curves as a means to reduce 

vehicle operating speeds and improve curve delineation. The anticipated benefit of implementing 

SDCWS is reductions in total and severe crashes. 

COUNTERMEASURE DETAILS 

SDCWS are horizontal curve chevron 

signs with solar powered flashing lights 

embedded in the sign. The flashing 

lights can be simultaneous (i.e., each 

sign is flashing at the same time as the 

other signs); or, more often, there may 

be a pattern associated with the 

flashing lights (i.e., a sequence of lights 

moving toward or away from the 

driver). In the latter case, this is 

typically accomplished by having each 

sign flash at least once per second, 

with each flash lasting at least 100 

milliseconds. Each sign begins flashing

at a time that is offset relative to the adjacent sign, producing a sequential flashing effect. 

The States of Missouri, Texas, Washington, and Wisconsin collectively installed 12 TAPCO 

SDCWS’s along horizontal curves on two-lane rural highways as part of an FHWA Highways for 

Life evaluation. Because there were only 12 SDCWS locations included in the study sample, only

one manufacturer’s product was selected for implementation in the evaluation to ensure 

consistency in system design and application. The TAPCO system was selected as a typical 

representation of SDCWS’s. The study sites were identified based on a high-crash history, as 

well as vehicle operating speeds that exceeded the advisory (if present) or posted speed limit. 

All curves selected for treatment with SDCWS were on a two-lane rural paved road and have 

the following: 

 A posted speed limit of 50 mph or higher.

 Existing chevrons.

 No railroad crossing or major access points within the curve.

 At least 10 non-animal related crashes in the previous 5 years (preferably high-speed

related crashes),

Figure E-1. Photo. SDCWS. 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

88

 No major rehabilitation or changes in alignment in the previous 5 years,

 No rehabilitation or alignment changes planned in the 2 years following installation of the

SDCWS.

All installations of SDCWS at the curves occurred between June and September of 2012. A total 

of 24 similar horizontal curves in the same States were used as a control group, without 

SDCWS. 

Speed data were collected before installation, and 1, 12, 18, and 24 months after installation of 

the SDCWS. Crash data were also compiled for each of the SDCWS and control sites, including 

five years before and two years after implementation. 

RESULTS 

The results showed that vehicle operating speeds were lower at the beginning and midpoint of 

horizontal curves for all periods after the SDCWS were installed. The mean and 85th-percentile 

speeds were 1.1 to 1.7 mph lower in the 1, 12, 18, and 24 month periods after installing the 

SDCWS. The results were generally consistent when comparing speeds at the beginning and the 

midpoint of horizontal curves. The percentage of vehicles exceeding the posted and advisory 

speed limits was also lower after installing the SDCWS, and results were generally consistent 

across all time periods after implementation. The change in the fraction of vehicles exceeding the 

advisory speed by 20 mph or more decreased by an average of 32 percent at the beginning of 

the horizontal curve. Similarly, the change in the fraction of vehicles exceeding the advisory 

speed by 15 mph or more decreased by an average of 30 percent at the beginning of the 

horizontal curve. The fraction of vehicles exceeding the advisory speed by 20 mph or more at 

the midpoint of the curve decreased by 26 percent, while the fraction of vehicles exceeding the 

advisory speed by 15 mph or more declined by 16 percent after SDCWS installation. The results 

of the study suggest that SDCWS have long-term and consistent effect on vehicle operating 

speeds. While the magnitude of the effect was relatively small, there was a pronounced effect on 

those vehicles substantially exceeding the advisory speed. 

With regards to safety, a simple before-after analysis of crash data found that the total number 

of crashes per year declined by 17 to 91 percent at 7 locations after the SDCWS was installed. 

At 2 sites, the total number of crashes per year increased by 7 and 11 percent. At three 

locations, no crashes were reported after the SDCWS were installed, so simple before-after 

safety comparisons could not be made. Research is underway to develop crash modification 

factors for SDCWS. 

CONSIDERATIONS 

The research referenced in this case study identified candidate sites for the SDCWS based on 

high crash histories (at least 5 crashes in previous 5 years) and excessive speeds on the same 

horizontal curves. Excessive vehicle operating speeds were defined as those with either of the 

following conditions: 
APPENDIX E: APPLICATION OF SDCWS 

89

Mean speed exceeded the advisory speed limit by 5 mph or more, or, if an advisory speed was 

not posted, exceeded the posted speed limit by 5 mph or more. 

85th percentile speed exceeded the advisory speed limit by 5 mph or more, or exceeded the 

posted speed limit by 5 mph or more, if an advisory speed was not present. 

A radar device on the sign can detect vehicles 300 feet in advance of the horizontal curve. The 

SDCWS is set to activate only when it detects approaching vehicles exceeding a certain speed 

threshold. The threshold is commonly at or slightly below the advisory speed of the curve. A 

wireless communication system maintains synchronization among the chevron signs within the 

system. 

CONTACT INFORMATION 

Julie Zirlin at the FHWA provided information for this case study. Visit 

https://www.fhwa.dot.gov/accelerating/ for more information. Images are courtesy of FHWA. 

REFERENCES

Smadi, O., N. Hawkins, S. Hallmark, and S. Knickerbocker. Evaluation of the TAPCO Sequential 

Dynamic Curve Warning System. Report No. FHWA-HIF-13-040, Federal Highway 

Administration, Washington, DC, June 2013. 

Smadi, O., N. Hawkins, S. Knickerbocker, S. Hallmark, and A. Pike. Evaluation of the TAPCO 

Sequential Dynamic Curve Warning System. Report No. FHWA-HIF-13-040, Federal Highway 

Administration, Washington, DC, June 2013. 
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

90 

APPENDIX F: APPLICATION OF HIGH FRICTION SURFACE 

TREATMENT IN KENTUCKY 

BACKGROUND 

Between 2004 and 2008, more than 60 percent of all fatal crashes on Kentucky’s roads were 

roadway departure crashes. In an effort to address this, the Kentucky Transportation Cabinet 

(KYTC), in conjunction with the Federal Highway Administration (FHWA), developed the 2009 

Kentucky Roadway Departure Safety Implementation Plan, which established a strategic approach to 

reducing the number of roadway departure crashes on State roads. This statewide program is 

funded through the Highway Safety Improvement Program. As of summer 2014, KYTC has 

applied High Friction Surface Treatments (HFSTs) at more than 100 sites—many at locations 

that present multiple risk factors, such as multiple curves. 

PROGRAM DETAILS 

As part of the Roadway Departure Safety Implementation Plan, KYTC applied HFST to a number 

of state-maintained curves and ramps. HFSTs are a thin layer of specially engineered, durable, 

high friction aggregates placed as a topping on a polymer binder. These aggregate systems have 

long-lasting skid resistance and make the overlay more resistant to wear and polishing. 

Additional details are available at EDC 2012 Initiatives – High Friction Surface Treatment. 

KYTC initially used a “black spot” approach to select treatment sites—selecting sites that had 

experienced the highest number of overall 

crashes. After this initial effort, KYTC 

continued to identify additional locations in 

need of improvement. KYTC regularly 

performs a screening prioritization, which 

scans the entire roadway network to 

identify wet-road crashes for additional 

candidates for HFST. After the site 

evaluation, pavement condition is examined 

for HFST application feasibility. 

In 2014, Kentucky updated the Roadway 

Departure Safety Implementation Plan and 

now uses a Highway Safety Manual (HSM) 

methodology to identify roadway segments 

and ramps as candidates for surface 

treatment. KYTC develops Safety 

Performance Functions and uses an empirical Bayes method to help identify curves/ramps that 

are candidates for surface treatment. More information is available at Introduction of Safety 

Performance Functions. 

Figure F-1. Photo. A truck in the process of applying 

HFST to a curve. 
APPENDIX F: APPLICATION OF HFST IN KENTUCKY 

91

RESULTS 

Almost immediate positive impacts were experienced at the 30 initial sites KYTC applied HFST. 

And multiple sources provided positive feedback on the program, including drivers, governing 

agencies, and policy makers. Data have shown a 70- to 75-percent reduction in crashes at the 

treated sites. Due to this success, several local agencies have expressed interest in implementing 

HFSTs. 

One site in Fayette County—Exit 113 on Interstate 75—is illustrative of this success. In the 3 

years prior to HFST treatment, the ramp had 28 roadway departure crashes (18 wet crashes and 

10 dry crashes). In the two and half years since the HFST installation, crashes have nearly been 

eliminated and the ramp has been the scene of a single crash, which was a dry crash. Ramp 

signage was also updated at the same time as the HFST installation. 

CONSIDERATIONS 

KYTC did not have outside guidance when it first began 

applying HFSTs due the relative newness of the treatment. 

Despite this, KYTC was able to apply numerous surface 

treatments in a short amount of time. KYTC staff suggests 

that other agencies should take the time to complete a 

thorough evaluation of each site when considering HFSTs. 

Also, agencies should implement a hierarchy of 

countermeasures, such as: 

 Consider repaving.

 Apply alternative surface treatment.

 Consider other non-pavement treatments.

 Apply high friction surface treatment, if necessary.

KYTC is pleased with the program and will continue to 

address the safety of curves and ramps with the HSM

methodology. The application of HFST successfully decreased roadway departures and 

consequently, serious injuries and fatalities in Kentucky. With the help of Kentucky’s leadership 

in the Roadway Departure Safety Plan, HFSTs are growing in popularity across the nation. 

CONTACT INFORMATION 

Tracy Lovell from the KYTC provided the information for this case study. Visit 

http://transportation.ky.gov/ for more information. All images are courtesy of the KYTC. 

Figure F-2. Photo. A horizontal 

curve with HFST. 
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92 

APPENDIX G: EVERY DAY COUNTS – HIGH FRICTION SURFACE 

TREATMENTS

The Federal Highway Administration's Every Day Counts 

(EDC) initiative is designed to identify and deploy innovations 

aimed at shortening project delivery, enhancing the safety of 

our roadways, and improving environmental sustainability. 

To ensure that the benefits of using HFST are attained quickly 

by a high percentage of the United States market, the EDC 

initiative has established an aggressive program to rapidly 

accelerate HFST deployment and adoption. As part of the 

subsequent EDC2 initiative, an implementation plan was 

created to serve as a roadmap for rapid, successful implementation of HFST, including technical 

guidance and assistance, benchmarking, marketing and communications, training, and project 

demonstrations that will highlight best practices. These treatments generated widespread 

interest during EDC2, and by the end of the two-year cycle, the number of States using HFSTs

had grown from 14 to 39. As of the end of October 2015 that number has reached 42 states 

(including Puerto Rico, the U.S. Virgin Islands, and Federal Land) and 14 States have made the 

use of HFSTs a standard practice for reducing crashes at critical locations.

FHWA selected calcined bauxite as the aggregate of choice for HFSTs as it is high-quality, 

durable, resistant to polishing, and provides long lasting value as compared to other natively

available aggregates. A recent study by National Center of Asphalt Technology to examine 

pavement surface friction performance of bauxite and seven alternative aggregate sources 

ranked calcined bauxite as the top performing aggregate.

Visit the EDC 2 HFST web site for more information about the EDC2 HFST program and 

resources. 

BENEFITS

 Reduce crashes, injuries, and fatalities. Wisconsin DOT placed HFST on a ramp in

Milwaukee in October 2011 that has experienced 87 crashes in one year and to date

has only two crashes at this location. Additionally, the National Cooperative Highway

Research Program (NCHRP) 617 indicates a crash reduction of 20 percent for all

intersection crashes.

 Benefits outweigh costs. A recent before-and-after study from South Carolina DOT

for a series of curve installations indicates a cost-benefit ratio of about 24 to 1.

 Relatively low in cost compared to geometric improvements. The square-foot

cost of HFST is not cheap, but its durability makes it worth the cost since the

treatments are long-lasting and the life-cycle cost is excellent.

 Durable and long-lasting. HFST provide a durable and long-lasting solution to

pavement locations where insufficient friction is a contributing factor in crashes.
APPENDIX G: EVERY DAY COUNTS – HIGH FRICTION SURFACE TREATMENTS 

93 

 Customizable to specific State and local safety needs. Road owners can use

where most needed, such as two-lane urban or rural roads at horizontal curves, areas

near steep grades, areas at or near lane changes and rural and urban intersections.

 Produce negligible environmental impacts and minimal impact on traffic.

Project lengths are short and the materials set up very quickly so the treatments can

often be applied in hours, requiring minimal impact on traffic as compared to a

conventional pavement overlay project.

Key activities delivered during the EDC 2 cycle: 

http://safety.fhwa.dot.gov/roadway_dept/pavement_friction/high_friction/ 

 Case Studies, Noteworthy Practice and Fact Sheets showcasing HFSTs

 HFST Education Video

 Demonstration of HFST installations at four states.

 AIDs Grant to help State DOTs to mainstream HFSTs in their States

 STIC Grant to help State DOTs to have HFSTs technology sharing with other states

and locals.
LOW-COST TREATMENTS FOR HORIZONTAL CURVE SAFETY 2016 

94 

APPENDIX H: UTILITY POLE MANAGEMENT IN NEW JERSEY 

BACKGROUND 

Objects permanently fixed in the clear zone of a roadway, such as trees and utility poles, may 

present obstacles for vehicles that depart the travel lane. Researchers from Rowan University, in 

Glassboro, New Jersey, identified approximately 260 sites on New Jersey State Highways with 

multiple utility pole crashes from the years 2003 to 2005. Researchers identified these sites by 

ranking poles with three or more recurring pole hits in one location, in accordance with the 

New Jersey Department of Transportation (NJDOT) Roadway Design Manual Section 

8.2.4 under "Utility Poles." The poles were also ranked by the crash hit/severity ratio of five or 

greater.

The Utility Management Unit of NJDOT is responsible for implementing the Pole Mitigation 

Program based on the list created by the Rowan University researchers. The Pole Mitigation 

Program is a formal program to proactively identify and remediate high risk utility pole crash 

sites in an effort to reduce crashes and injuries. Locations of poles that were within limits of 

active projects in design and planning stages were (and are currently) forwarded to the Division 

of Project Management for approval to be included in the mitigation program. NJDOT 

subsequently focused on the top 20 locations that were not a part of any active design projects

from the original list of 260 sites. 

PROGRAM DETAILS 

One aspect of the Pole Mitigation Project is 

to pilot energy absorbing poles at some 

locations. The applicability of the poles is 

limited due to height and electrical 

appurtenance restrictions. NJDOT is using 

special poles made of fiberglass that collapse 

on impact and do not break away into the 

traffic. Initially NJDOT found it difficult to 

secure participation from the major utility 

companies on this project. However, 

upon holding informative discussions with 

the utility companies, NJDOT and the 

companies successfully reached an 

agreement to replacing and installing 

fiberglass poles when possible in accordance 

with all standards and guidelines and as 

specified in the NJDOT Utility 

Accommodation Policy. As a result of this coordination, six fiberglass poles have been installed. 

Figure H-1. Photo. A close-up of the energy 

absorbing utility poles used as a part of NJDOT’s 

pilot project. 
APPENDIX H: UTILITY POLE MANAGEMENT IN NEW JERSEY 

95 

RESULTS 

After installation of the poles, the utility companies 

periodically conduct an inspection and submit a report to 

NJDOT as a part of the agreement. NJDOT can analyze 

the performance data of the poles and can establish policy 

regarding the usage of non-wooden poles. The utility 

companies have also agreed to replace the fiberglass poles 

if one is hit and damaged. NJDOT has made plans for data 

collection to establish a database containing the crash types 

based on geometry of the roadway in addition to other 

information that will be helpful to target sites where 

crashes with poles could be a problem.

CONSIDERATIONS 

NJDOT recognizes the value in safeguarding the motoring 

public and minimizing the risk on the roadway and 

roadside. NJDOT highly recommends other agencies to 

initiate a clear zone management program, such as utility 

pole mitigation, if the resources are available. 

CONTACT INFORMATION 

NJDOT provided the information for this case study. Visit http://www.state.nj.us/transportation/

for more information. All images are courtesy of NJDOT. 

Figure H-2. Photo. NJDOT used 

utility poles made from fiberglass 

that would collapse upon impact 

and not break away into the road. 
For More Information:

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

FHWA, Offi ce of Safety

Joseph Cheung

joseph.cheung@dot.gov

202-366-6994

FHWA-SA-15-084