Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

PUBLICATION NO. FHWA-HRT-20-061 SEPTEMBER 2020 

Developing Crash-Modification Factors 

for High-Friction Surface Treatments
FOREWORD

The research documented in this report was conducted as part of the Federal Highway 

Administration’s (FHWA’s) Evaluation of Low-Cost Safety Improvements Pooled Fund Study 

(ELCSI-PFS). FHWA established this PFS in 2005 to research the effectiveness of the safety 

improvements identified by the National Cooperative Highway Research Program’s Report 500 

Series as part of the implementation of the American Association of State Highway and 

Transportation Officials’ Strategic Highway Safety Plan. The ELCSI-PFS research studies 

provide a crash modification factor and benefit–cost economic analysis for each targeted safety 

strategy identified as a priority by the PFS member States.

This report provides high-quality CMFs and B/C ratios for high-friction surface treatments 

(HFSTs) with calcined bauxite aggregate and recommends materials and specifications for 

applications to effectively reduce roadway-departure crashes. This data-driven study used 

before- and after-crash data to quantify crash-reduction benefits and used friction data collected 

by the research team before and after HFST installation to help quantify the impact of increased 

pavement friction on CMFs. The study collected data for HFST installations on curves and 

ramps in Arkansas, Georgia, Kentucky, Louisiana, Pennsylvania, and West Virginia. The results 

of this study indicate using HFSTs with calcined bauxite aggregate reduces roadway-departure 

crashes at curves and ramps, particularly for wet-weather crashes. This report is supplemented by 

FHWA-HRT-20-062, Developing Crash-Modification Factors for High-Friction Surface 

Treatments: Friction Change Report, and will benefit safety engineers and safety planners by 

providing greater insight into applications of HFSTs for improving highway safety. 

Brian P. Cronin, P.E. 

Director, Office of Safety and Operations

Research and Development

Notice

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

(USDOT) in the interest of information exchange. The U.S. Government assumes no liability for 

the use of the information contained in this document. 

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

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document. 

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement. 
TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

FHWA-HRT-20-061

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

4. Title and Subtitle

Developing Crash-Modification Factors for High-Friction

Surface Treatments

5. Report Date

September 2020

6. Performing Organization Code

7. Author(s)

David K. Merritt, Craig A. Lyon, Bhagwant N. Persaud

(ORCID: 0000-0002-8404-9236), and Helga N. Torres

8. Performing Organization Report No.

9. Performing Organization Name and Address

VHB

8300 Boone Boulevard, Suite 700

Vienna, VA 22182

10. Work Unit No.

11. Contract or Grant No.

DTFH61-13-D-0001

12. Sponsoring Agency Name and Address

Office of Safety

Federal Highway Administration

6300 Georgetown Pike

McLean, VA 22101

13. Type of Report and Period Covered

Final Report; December 2015–August 2019

14. Sponsoring Agency Code

HRDS-20

15. Supplementary Notes

This report was prepared for the Federal Highway Administration (FHWA) Office of Safety Research and

Development under Contract DTFH61-13-D-0001. The FHWA program and task manager for this project was

Roya Amjadi (HRDS-20; ORCID: 0000-0001-7672-8485). Andy Mergenmeier (DTS-RC-BAL-1; ORCID: 0000-

0003-4837-6528) and Joseph Cheung (HAS-HSST) contributed to this study.

16. Abstract

Over the past 10 to 15 yr, the use of high-friction surface treatments (HFSTs) as a safety countermeasure has

grown exponentially. HFSTs are pavement-surface treatments that restore or enhance pavement friction at

locations with high friction demand, such as curves, ramps, and intersection approaches. While the crash-reduction

benefits of HFSTs have been observed by many State highway agencies, the availability of crash-modification

factors (CMFs) for HFSTs is limited. This study provides high-quality and robust CMFs and benefit–cost ratios for

HFSTs with calcined bauxite aggregate and recommends materials and specifications (as appropriate) for

applications. This study also notes where and under what conditions to use HFSTs to effectively reduce

roadway-departure crashes. This study was data-driven and used before- and after-crash data to quantify

crash-reduction benefits. The study also used friction data collected by the research team before (when available)

and after HFST installation to help quantify the impact of increased pavement friction on the CMFs. Data were

collected for hundreds of HFST installations in Arkansas, Georgia, Kentucky, Louisiana, Pennsylvania, and

West Virginia on curves and ramps. The results for curves and ramps indicate significant benefits in terms of low

CMFs, particularly for wet-weather crashes. A thorough disaggregate analysis of the before–after evaluation data

for curves suggested a logical and consistent relationship between CMFs and three variables: friction

improvement, annual average daily traffic, and expected crash frequency before treatment. A complementary

evaluation was conducted under this study for friction changes of HFSTs over time; the results are documented in

FHWA-HRT-20-062, Developing Crash-Modification Factors for High-Friction Surface Treatments: Friction

Change Report. Friction testing was performed on several older HFST installations where previous friction data

were collected. All friction testing was performed with the Federal Highway Administration’s highway friction

tester, a continuous fixed-slip measurement device. The friction data collected during this research and

documented in this report were evaluated for friction change before and after HFST installation, friction change of

the HFST and existing pavement over time, and friction change through a curve.

17. Key Words

High-friction surface treatment, crash-modification

factor, benefit–cost ratio, friction testing

18. Distribution Statement

No restrictions. This document is available to the public

through the National Technical Information Service,

Springfield, VA 22161.

http://www.ntis.gov

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

84

22. Price

N/A

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

TABLE OF CONTENTS

EXECUTIVE SUMMARY .......................................................................................................... 1

CHAPTER 1. INTRODUCTION ................................................................................................ 3

Importance of Pavement Safety ............................................................................................. 3

HFSTs....................................................................................................................................... 3

Scope and Objectives .............................................................................................................. 6

CHAPTER 2. LITERATURE REVIEW .................................................................................... 9

Overview of HFSTs ................................................................................................................. 9

HFST Evaluation Studies ..................................................................................................... 10

2015 Washington State: Kwik Bond Polymers® High Friction Surface Treatment 

(Anderson et al. 2015)....................................................................................................... 11

2015 Kentucky: Kentucky Transportation Cabinet’s High Friction Surface 

Treatment and Field Installation Program (Von Quintus and Mergenmeier 2015) .......... 11

2014 Kentucky: Evaluation of High Friction Surface Treatments (Scully 2014) ............ 12

2012 Washington State: Evaluation of Tyregrip® High-Friction Surfacing 

(Anderson et al. 2012)....................................................................................................... 12

2009 Kansas: Evaluation of High Friction Surface Locations in Kansas (Meggers 

2015) ................................................................................................................................. 13

2009 Vermont: Ennis Paint, Inc. Tyregrip® High Friction Surface System (Kipp 

and Sanborn 2014) ............................................................................................................ 14

2008–2010 South Carolina: High Friction Surface Course (HFSC) Before and After 

Study (Riddle 2012) .......................................................................................................... 15

2006 Florida: Evaluation of a High Friction Pavement Surface Treatment 

(Savolainen et al. 2009) .................................................................................................... 15

1999 Wisconsin: Investigative Study of the ItalgripTM System (Bischoff 2008) ............. 16

2012 California: Northern California US 199 – Del Norte County, Case Study 

(Cheung and Julian 2015) ................................................................................................. 16

2015 FHWA: Evaluation of Pavement Safety Performance (Merritt et al. 2015) ............ 18

2014 Texas Transportation Institute: Evaluating the Need for Surface Treatments to 

Reduce Crash Frequency at Horizontal Curves (Pratt et al. 2014) ................................... 18

2016 Virginia Tech Transportation Institute: A New Approach for Managing 

Pavement Friction and Reducing Road Crashes (McCarthy et al. 2016) ......................... 19

2016 Virginia Tech Transportation Institute: Pioneering the Use of Continuous 

Pavement Friction Measurements to Develop New Safety Performance Functions, 

Improve the Accuracy of Crash Count Predictions, and Evaluate Possible 

Treatments for the Roads in Virginia (de León et al. 2016) ............................................. 20

2012 Texas Transportation Institute: Using High Friction Surface Treatments to 

Improve Safety at Horizontal Curves (Brimley and Carlson 2012) ................................. 20

2015 National Center for Asphalt Technology: High Friction Surface Treatment 

Alternative Aggregates Study (Heitzman 2015) ............................................................... 21

2011 Virginia Tech Transportation Institute: Review of High-Friction Surface 

Technologies – Constructability and Field Performance (Flintsch et al. 2011) ............... 21

2015 Oklahoma State University: 3D Laser Imaging based Real Time Pavement 

Surface Evaluation for High Friction Surfacing Treatments (Li et al. 2015) ................... 21
iv

HFST Specifications.............................................................................................................. 22

U.S. Specifications ............................................................................................................ 22

Materials ..................................................................................................................... 22

Installer Qualifications ............................................................................................... 22

Construction Practices................................................................................................ 23

Acceptance Testing ..................................................................................................... 23

International Specifications .............................................................................................. 24

Literature Review Conclusions ............................................................................................ 26

CHAPTER 3. DATA COLLECTION ...................................................................................... 27

Overview of the Data-Collection Process ............................................................................ 27

Identifying HFST Sites ......................................................................................................... 27

Final Site Selection ........................................................................................................... 29

Reasons for Exclusion....................................................................................................... 30

Crash Data ............................................................................................................................. 30

Method .............................................................................................................................. 30

Challenges ......................................................................................................................... 31

Friction Data Collection ....................................................................................................... 31

Method .............................................................................................................................. 32

Testing Protocol ................................................................................................................ 33

Data Reporting .................................................................................................................. 34

Challenges ......................................................................................................................... 34

Roadway and Pavement Data .............................................................................................. 35

Roadway Data ................................................................................................................... 35

Pavement Data .................................................................................................................. 35

Challenges ......................................................................................................................... 36

Reference Sites ...................................................................................................................... 37

Upstream/Downstream ..................................................................................................... 37

Curve Inventory ................................................................................................................ 37

Similar Systemic Criteria .................................................................................................. 38

Summary of Data Collection by State ................................................................................. 38

Arkansas ............................................................................................................................ 38

Crash Data .................................................................................................................. 39

Reference Sites ............................................................................................................ 39

Roadway Data ............................................................................................................. 39

Friction Data .............................................................................................................. 39

Georgia .............................................................................................................................. 40

Crash Data .................................................................................................................. 40

Reference Sites ............................................................................................................ 40

Roadway Data ............................................................................................................. 40

Friction Data .............................................................................................................. 40

Kentucky ........................................................................................................................... 41

Crash Data .................................................................................................................. 41

Reference Sites ............................................................................................................ 41

Roadway Data ............................................................................................................. 41

Friction Data .............................................................................................................. 42
v 

Louisiana ........................................................................................................................... 42

Crash Data .................................................................................................................. 43

Reference Sites ............................................................................................................ 43

Roadway Data ............................................................................................................. 43

Friction Data .............................................................................................................. 43

Pennsylvania ..................................................................................................................... 44

Crash Data .................................................................................................................. 44

Reference Sites ............................................................................................................ 44

Roadway Data ............................................................................................................. 44

Friction Data .............................................................................................................. 45

West Virginia .................................................................................................................... 45

Crash Data .................................................................................................................. 45

Reference Sites ............................................................................................................ 46

Roadway Data ............................................................................................................. 46

Friction Data .............................................................................................................. 46

Summary of Data Used for the Before–After Safety Evaluation ..................................... 46

CHAPTER 4. ANALYSIS .......................................................................................................... 49

Analysis Objectives ............................................................................................................... 49

Analysis Methodology ........................................................................................................... 49

Development of SPFs ............................................................................................................ 50

Specifics of the EB Before–After Evaluation...................................................................... 51

Overall Safety Effects ....................................................................................................... 51

Effects on Different Severity and Impact Types ............................................................... 52

Effects of Design, Traffic, Operational, and Safety Characteristics ................................. 52

SPFs ........................................................................................................................................ 52

Pennsylvania Curves ......................................................................................................... 53

Kentucky Curves ............................................................................................................... 53

West Virginia Curves ........................................................................................................ 54

Arkansas Ramps................................................................................................................ 54

Kentucky Ramps ............................................................................................................... 55

CHAPTER 5. BEFORE–AFTER ANALYSIS RESULTS...................................................... 57

Aggregate Results.................................................................................................................. 57

Aggregate Results for Curves ........................................................................................... 57

Aggregate Results for Ramps ........................................................................................... 59

Disaggregate Results (for Curves) ....................................................................................... 60

Univariate Categorical Analysis Results .......................................................................... 60

Investigation of CMFunctions .......................................................................................... 61

CHAPTER 6. B/C ANALYSIS .................................................................................................. 63

Methodology .......................................................................................................................... 63

CHAPTER 7. DISCUSSION AND CONCLUSIONS ............................................................. 65

Safety Evaluation and Recommendations .......................................................................... 65

Performance Evaluation and Recommendations ............................................................... 66

ACKNOWLEDGMENTS .......................................................................................................... 69

REFERENCES ............................................................................................................................ 71
vi 

LIST OF FIGURES

Figure 1. Photograph. Manual HFST installation. .......................................................................... 4

Figure 2. Photograph. Fully automated HFST installation. ............................................................ 4

Figure 3. Photograph. HFST post installation. ............................................................................... 5

Figure 4. Photograph. Curve with an HFST installed. .................................................................... 5

Figure 5. Map. Implementation of HFSTs as of December 2018. .................................................. 6

Figure 6. Graphic. Comparison of an HFST versus curve realignment (Cheung and 

Julian 2015). ........................................................................................................................ 17

Figure 7. Photograph. FHWA’s HFT. .......................................................................................... 33

Figure 8. Equation. Change in EB before–after evaluation. ......................................................... 51

Figure 9. Equation. Calculation of m. ........................................................................................... 51

Figure 10. Equation. Calculation of w. ......................................................................................... 51

Figure 11. Equation. Calculation of θ. .......................................................................................... 52

Figure 12. Equation. Standard deviation of θ. .............................................................................. 52

Figure 13. Equation. Model form for Pennsylvania curve SPFs. ................................................. 53

Figure 14. Equation. Model form for Kentucky curve SPFs. ....................................................... 53

Figure 15. Equation. Model form for Arkansas ramp SPFs. ........................................................ 54

Figure 16. Equation. Model form for Kentucky ramp SPFs. ........................................................ 55

Figure 17. Equation. Calculation for observed crashes. ............................................................... 61

Figure 18. Equation. Approximate CMF for a site. ...................................................................... 61

Figure 19. Equation. CMF for an individual site based on the site characteristics....................... 61

Figure 20. Equation. Functional form for CMFunctions. ............................................................. 62

Figure 21. Equation. Annualized cost of an HFST. ...................................................................... 63
vii 

LIST OF TABLES

Table 1. Summary of 2015 WSDOT HFST evaluation (interim results) 

(Anderson et al. 2015). ........................................................................................................ 11

Table 2. Summary of 2012 WSDOT HFST evaluation (interim results) 

(Anderson et al. 2012). ........................................................................................................ 13

Table 3. Summary of 2009 KDOT HFST evaluation (Meggers 2015). ....................................... 14

Table 4. Summary of 2009 VTrans HFST evaluation (Kipp and Sanborn 2014). ....................... 15

Table 5. Summary of 2008–2010 SCDOT HFST evaluation (Riddle 2012). ............................... 15

Table 6. Summary of 2006 FDOT HFST evaluation (Savolainen et al. 2009). ........................... 16

Table 7. Summary of 1999 WisDOT HFST evaluation (Bischoff 2008). .................................... 16

Table 8. Summary of 2012 Caltrans HFST application (Cheung and Julian 2015). .................... 17

Table 9. Results for the naïve before–after study based on all sites (Merritt et al. 2015). ........... 18

Table 10. Results for the before–after with C-G study for treatment sites for which 

comparison sites were available (Merritt et al. 2015). ........................................................ 18

Table 11. Road surfacing aggregate PSV requirements based on site characteristics from 

Highways England Design Manual for Roads and Concrete Bridges (Highways 

England 2006). .................................................................................................................... 25

Table 12. Initial sites planned for testing. ..................................................................................... 28

Table 13. Final States and number of sites for data collection. .................................................... 29

Table 14. Summary of friction data for Arkansas sites. ............................................................... 39

Table 15. Summary of friction data for Georgia sites. ................................................................. 41

Table 16. Summary of friction data for Kentucky sites. ............................................................... 42

Table 17. Summary of friction data for Louisiana sites. ............................................................... 43

Table 18. Summary of friction data for Pennsylvania sites. ......................................................... 45

Table 19. Summary of friction data for West Virginia sites. ........................................................ 46

Table 20. Summary of curve treatment data used for safety evaluation. ...................................... 48

Table 21. Summary of ramp treatment data used for safety evaluation. ...................................... 48

Table 22. Summary of curve reference site data. .......................................................................... 48

Table 23. Summary of ramp reference site data. .......................................................................... 48

Table 24. SPF parameter estimates for Pennsylvania curves. ...................................................... 53

Table 25. SPF parameter estimates for Kentucky curves. ............................................................ 54

Table 26. SPF parameter estimates for Arkansas ramps............................................................... 54

Table 27. SPF parameter estimates for Kentucky ramps. ............................................................. 55

Table 28. Aggregate results for Pennsylvania curves. .................................................................. 57

Table 29. Aggregate results for West Virginia curves. ................................................................. 58

Table 30. Aggregate results for Kentucky curves. ........................................................................ 58

Table 31. Aggregate results for curves for Kentucky, Pennsylvania, and West Virginia 

combined. ............................................................................................................................ 58

Table 32. Aggregate results for Arkansas ramps. ......................................................................... 59

Table 33. Aggregate results for Kentucky ramps. ........................................................................ 59

Table 34. Curve CMF results from the univariate categorical analysis. ....................................... 60

Table 35. CMFunction parameter estimates for curves. ............................................................... 62

Table 36. Applicable ranges of variables for curve CMFunctions. .............................................. 62

Table 37. B/C ratios and recommended CMFs. ............................................................................ 66
viii

LIST OF ABBREVIATIONS

AADT annual average daily traffic

AASHTO American Association of State Highway and Transportation Officials

ARDOT Arkansas Department of Transportation 

B/C benefit–cost

BBA HAPAS Board of Agrément Highways Agency Product Approval Scheme 

Caltrans California Department of Transportation 

C-G comparison group

CMF crash-modification factor

CMFunction crash-modification function 

DFT dynamic friction tester

DOT department of transportation 

EB empirical Bayes

ELCSI-PFS Evaluation of Low-Cost Safety Improvements Pooled Fund Study 

FDOT Florida Department of Transportation 

FHWA Federal Highway Administration

FN40R friction number value 

GDOT Georgia Department of Transportation 

GLM generalized linear modeling

GPS global positioning system 

HFST high-friction surface treatment

HFT highway friction tester

HOSSOD head-on plus opposite direction sideswipe 

HSM Highway Safety Manual 

KABC injury and fatal 

KDOT Kansas Department of Transportation

KYTC Kentucky Transportation Cabinet 

LADOTD Louisiana Department of Transportation and Development 

LWST locked-wheel skid tester

MMA methyl methacrylate

MPD mean profile depth

NCAT National Center for Asphalt Technology

PCC portland cement concrete

PDO property damage only 

PennDOT Pennsylvania Department of Transportation 

PSV polished stone value 

QC quality control 

QPL/APL qualified/approved products list 

RTM regression to the mean 

RTSA Road Surface Treatments Association

SCDOT South Carolina Department of Transportation 

SEaHC Surface Enhancements at Horizontal Curves

SN skid number 

SPF safety performance function

STH State Trunk Highway 
ix

TDOT Tennessee Department of Transportation 

VDOT Virginia Department of Transportation

VTrans Vermont Agency of Transportation 

WisDOT Wisconsin Department of Transportation 

WSDOT Washington State Department of Transportation 

WVDOT West Virginia Department of Transportation

1 

EXECUTIVE SUMMARY

This report provides high-quality and robust crash-modification factors (CMFs) and benefit–cost 

(B/C) ratios for high-friction surface treatments (HFSTs) with calcined bauxite aggregate and 

recommends current materials and specifications (as appropriate) for applications. This report

also notes where and under what conditions to use HFSTs to reduce roadway-departure crashes.

The research presented in this report was conducted as part of Federal Highway Administration’s 

Development of Crash-Modification Factors Program, the goal of which is to identify CMF

research needs, develop CMFs that meet the criteria for inclusion in the Highway Safety Manual

(HSM), promote the installation of promising safety improvements as standard practice, and 

facilitate the development of practical guides for implementation.

The research presented in this report builds on research conducted under Phase VI of the 

Evaluation of Low-Cost Safety Improvements Pooled Fund Study (ELCSI-PFS), which, through 

a detailed statistical analysis of crash data on various pavement-surface treatments, showed a 

significant reduction in wet-pavement crashes after treatments were applied (Merritt et al. 2015). 

The use of HFSTs showed substantial crash reduction using both naïve and comparison group 

analyses. Initially, a lack of treatment sites and reference groups prevented a state-of-the-art 

empirical Bayes (EB) analysis. In the years since the ELCSI-PFS was initiated, several State 

highway agencies have deployed HFSTs on a large scale, providing a greater number of 

treatment sites for a more rigorous EB analysis of the crash-reduction benefits of these 

treatments. With significant interest in HFSTs as a safety countermeasure by most State highway 

agencies, the need for reliable CMFs and B/C ratios is great, providing the motivation for this 

definitive study.

The state-of-the-art EB before–after methodology analyzed the effects of HFSTs on various 

crash types, including total, injury, wet-road, run-off-road (ROR), and head-on plus 

opposite-direction sideswipe (HOSSOD) using data collected in Arkansas (ramps), Kentucky 

(curves and ramps), Pennsylvania (curves), and West Virginia (curves). Limited data were also 

collected in Louisiana and Georgia but were not included in the rigorous EB analysis due to a 

lack of reference sites or after-period crash data. 

The EB analysis results for curves indicate significant benefits in terms of low CMFs for each 

State and the three States combined, especially for the primary crash types targeted by HFST 

programs: wet-road, ROR, and HOSSOD. Because of the relative consistency in the results for 

the three States, the CMFs based on the three States combined are recommended for estimating 

the benefits of contemplated treatments and for use in the CMF Clearinghouse (FHWA 2020a). 

The EB analysis results for ramps for Arkansas and Kentucky were highly inconsistent. The 

outlier was wet-weather crashes, for which the benefits in terms of low CMFs were significant. 

The benefits for all crash types were significant for Kentucky, while there were negligible 

benefits for crashes in Arkansas. This anomaly can be explained by the relatively small 

proportion of wet-weather crashes—the main HFST target—in Arkansas compared to Kentucky. 

To achieve the significant benefits of HFSTs, ramps should be identified for treatment based on a 

high proportion of wet-weather crashes.
2 

A thorough disaggregate analysis of the before–after evaluation data was only feasible for 

curves. For this, univariate categorical and multivariable regression analyses were used to 

investigate the effects on the CMFs of a number of variables. The data suggest a logical and 

consistent relationship between CMFs and three variables: friction improvement, annual average 

daily traffic, and expected crash frequency before treatment. These three variables were used to 

develop recommended crash-modification functions.
3 

CHAPTER 1. INTRODUCTION

IMPORTANCE OF PAVEMENT SAFETY

Approximately one-quarter of highway fatalities in the United States occur at or near horizontal 

curves. Roadway-departure crashes consistently account for more than half of fatalities and fatal 

crashes in the United States. Contributing factors to these roadway-departure crashes include 

excessive vehicle speed, distracted driving, and driver error. At some locations, however, the 

deterioration of pavement-surface friction may also be a factor, particularly during wet weather. 

The Federal Highway Administration (FHWA) estimates that 70 percent of wet-pavement 

crashes are preventable through improved pavement friction; recent studies on the effects of 

pavement-surface friction validate this estimate (FHWA Office of Safety n.d.a.; FHWA Office 

of Safety n.d.b.). 

The research documented in this report was conducted as part of the Federal Highway 

Administration’s (FHWA’s) Evaluation of Low-Cost Safety Improvements Pooled Fund Study 

(ELCSI-PFS). FHWA established this PFS in 2005 to research the effectiveness of the safety 

improvements identified by the National Cooperative Highway Research Program’s Report 500 

Series as part of the implementation of the American Association of State Highway and 

Transportation Officials’ Strategic Highway Safety Plan. The ELCSI-PFS research studies 

provide a crash modification factor and benefit–cost economic analysis for each targeted safety 

strategy identified as a priority by the PFS member States. Using both naïve and comparison 

group (C-G) analyses of crash data on various pavement-surface treatments, HFSTs showed a 

significant reduction in wet-pavement crashes after application. The lack of reference sites 

prevented a state-of-the-art EB analysis. In the years since the ELCSI-PFS was initiated, several 

State highway agencies deployed HFSTs on a large scale, providing a greater number of 

treatment sites for a more detailed analysis of the crash-reduction benefits of this treatment. With 

significant interest in HFSTs as a safety countermeasure by most State highway agencies, the 

need for reliable CMFs and B/C ratios is great.

HFSTs 

HFSTs are specialty pavement-surface treatments that restore or enhance friction. HFSTs are 

commonly used for spot treatments of curves, ramps, intersections, and steep grades where 

friction demand is higher than what conventional paving materials can provide. HFSTs are

installed by spreading a thin layer of polymeric resin binder (typically epoxy or polyester) over 

the pavement surface, then broadcasting or dropping a 1- to 3-millimeter abrasion- and 

polish-resistant aggregate onto the resin layer (figure 1 through figure 4). Calcined bauxite is the 

most commonly used aggregate for HFSTs worldwide, as it provides exceptional skid resistance 

not typically achieved with conventional pavement aggregates. Calcined bauxite also maintains 

high skid resistance over time. Recognizing the safety benefits of HFSTs, FHWA deployed them 

as a focus technology under the Every Day Counts 2 Program and continues to support their use 

(FHWA EDC2 n.d.).
4 

Source: FHWA.

Figure 1. Photograph. Manual HFST installation.

Source: FHWA.

Figure 2. Photograph. Fully automated HFST installation. 
5 

Source: FHWA.

Figure 3. Photograph. HFST post installation.

Source: FHWA.

Figure 4. Photograph. Curve with an HFST installed. 

The use of HFSTs as safety countermeasures in the United States has grown exponentially over 

the past 10–15 yr (Cheung and Julian 2016). Although used as a bridge deck preservation 

treatment since the 1950s, as of 2005, there were only one or two known HFST demonstration 

installations on curves or ramps in the United States (Cheung and Julian 2016). By 2018, at least 

43 States had at least one HFST installation, with multiple States reporting over 100 installations, 
6 

accounting for millions of square yards of HFSTs (figure 5). Adequately assessing the cost and 

safety benefits is important as highway agencies continue to install HFSTs. 

Source: FHWA.

S.Y. = square yard.

Figure 5. Map. Implementation of HFSTs as of December 2018. 

SCOPE AND OBJECTIVES

This report provides reliable CMFs and B/C ratios for different types (in terms of applications to 

date) of HFSTs and recommends materials and specifications (as appropriate) for applications. 

This report also notes where and under what conditions to use HFSTs to effectively reduce 

roadway-departure crashes. This report will help State departments of transportation (DOTs) 

select pavement treatments for projects. 
7 

The objectives of this study included the following:

• Conduct a literature review of current HFST studies and their installation practices with 

relevant details.

• Identify HFST installations that have been used for safety improvements, documenting 

materials used, relevant specifications, and application practices, and recommend a set of 

HFST installations for safety and performance evaluation. 

• Determine appropriate statistical methodologies and analysis procedures and tools. 

• Design a data-collection plan, including selecting reliable and applicable data sources, 

data needs, and data-collection techniques and equipment. 

• Work with volunteer States to collect crash, road geometric design, traffic, and 

performance data; pavement-surface characteristics; weather and environmental 

information; and other data for HFSTs installed in these States. 

• Analyze the data using the most reliable and applicable statistical methodologies for 

scientifically determining the safety effectiveness of different HFSTs and develop 

reliable CMFs and/or crash-modification functions (CMFunctions) and B/C ratios. 

• Develop statistical models for different types of HFSTs over time.

• Recommend a set of HFST installations with material requirements and specifications on 

the manner of installation; provide information on what, where, when, why, and how to 

use HFSTs; and identify road designs, facilities, and weather conditions that can benefit 

the most from HFSTs. 

• Identify and recommend potential HFST materials and specifications that can be used for 

systemic safety improvements on a longer length of roadway. 

• Develop this report and DTFH61-13-D-00001, Task B9, High Friction Surface 

Treatment (HFST) Quick Reference, for HFST types, materials, specifications, 

applications, performance, testing, proactive maintenance, and data-collection needs

(FHWA 2020b). 

• Develop FHWA-HRT-20-062, Developing Crash-Modification Factors for 

High-Friction Surface Treatments: Friction Change Report, documenting friction 

changes for surface treatments using calcined bauxite aggregate over time and curves and 

intersections for before, through, and after-the-road geometric design (Merritt et al.

2020). 

• Prepare solicitation and evaluation criteria to identify volunteer highway agencies to 

implement systemic HFST installations for future evaluations.

9 

CHAPTER 2. LITERATURE REVIEW

This literature review summarizes a number of State highway, FHWA, and international studies 

on HFSTs in recent years, as well as some specification requirements from various agencies. 

This literature review is divided into three sections. The first section provides a brief overview of 

HFSTs and how they relate to safety. The second section summarizes the findings of recent 

studies by various agencies that evaluated the application and field performance of HFSTs and 

their effects on safety. The bulk of the material in the second section comes from case study 

evaluations rather than large-scale safety evaluations. The third section reviews specifications 

from industry and State highway agencies for installing HFSTs. 

OVERVIEW OF HFSTs 

The following overview of HFSTs is adapted from the Evaluation of Pavement Safety 

Performance (Merrett et al. 2015): 

HFSTs are specialty pavement treatments that restore or enhance friction. HFSTs are

commonly used for spot treatments of curves, intersections, and steep grades where 

friction demand is higher than conventional paving materials can provide. HFSTs are

installed by spreading a resin binder (e.g., epoxy, methacrylate, or polyester) over the 

pavement surface followed by broadcasting or dropping a 1- to 3-millimeter abrasionand polish-resistant aggregate onto the resin. Calcined bauxite, which exhibits 

exceptional polish resistance, is the most commonly used aggregate for HFSTs

worldwide. However, similar aggregates that maintain excellent microtexture properties 

over time have also been used as the aggregate. Although a form of HFSTs has been used 

extensively for bridge decks and seals the bridge deck surface, it does not provide any 

documented preservation for pavements (pp. 36 and 37). 

The American Traffic Safety Services Association explains that the key to cost effectively 

employing HFSTs is to identify sites where they will achieve the greatest impact (ATSSA 2013). 

One of the most successful applications of an HFST is at existing horizontal curves with a 

history of wet-pavement crashes where geometric improvements to change the radius of 

curvature and/or superelevation are not feasible and warnings to reduce vehicle speed are

ineffective. HFST applications on other sites with high occurrences of wet-pavement crashes, 

such as ramps, intersections, and tangents, are not well documented. 

In addition to a high occurrence of wet-pavement crashes, low pavement friction is another 

indicator to identify sites that would benefit from HFSTs. The Pavement Friction Management 

Technical Advisory (T 5040.38) recommends using pavement friction measurements along with 

crash and traffic data to complete the following (FHWA 2010): 

• Evaluate pavement design, construction, and maintenance practices to ensure pavement 

surfaces with good friction characteristics are provided. 
10 

• Identify and investigate locations with elevated wet-weather crash rates relative to 

comparable locations for the purposes of minimizing locations with elevated 

friction-related crash rates.

• Provide data to prioritize projects to improve highway safety. 

Pavement-friction data are not only used to identify potential sites for HFSTs but also to evaluate 

the performance of an HFST immediately after installation and over time. The Pavement Friction 

Management Technical Advisory (T 5040.38) lists four types of full-scale friction test 

equipment: locked wheel, fixed slip, side force, and variable slip (FHWA 2010). De León et al. 

(2016) explain that, even though the locked-wheel method (i.e., ASTM E274, Standard Test 

Method for Skid Resistance of Paved Surfaces Using a Full-Scale Tire) is used widely in the 

United States, the other methods operate continuously, especially on curves, and provide a better 

indication of friction number values (FN40R) corresponding to braking with antilock brakes 

(ASTM 2011). 

HFST EVALUATION STUDIES

Various agencies conducted studies to evaluate the performance of HFSTs on highway surfaces 

other than bridge decks. Most of these studies evaluated pavement friction, and many reported

crash counts before and after HFST installation; HFST construction and durability were also 

evaluated. This section summarizes the most recent HFST studies and is by no means an 

exhaustive compilation of HFST evaluations, but it provides some of the more relevant 

information as it relates to this current study. 

The following observations by Merritt et al. (2015) are applicable to the studies summarized in 

this section:

• Recent studies indicate, as would be expected, that higher skid resistance measurements 

are associated with lower crash rates, particularly wet-road-related collisions.

• Studies comparing the safety improvements after specific skid resistance improvement 

treatments are rare and the data and evaluation methods poor. 

• These limited studies indicate reductions in crashes following treatment.

A study of HFST sites in Florida indicated that HFSTs may not provide the desired crash 

reduction at wide curves/tangents, intersection approaches, and intersections (Wilson et al. 

2016). Wilson et al. (2016) concluded that the only statistically significant change in crash rates 

was the decrease in wet-weather accidents on tight curves. These results may not be reliable 

since they were based on a naïve before–after comparison that did not control for regression to 

the mean (RTM) and other changes in crashes that were unrelated to the treatment (e.g., those 

due to differences in weather between the before and after periods). In addition, researchers 

controlled traffic volume changes using crash rates rather than properly through safety 

performance functions (SPFs). 
11 

2015 Washington State: Kwik Bond Polymers® High Friction Surface Treatment

(Anderson et al. 2015) 

This report presented the interim findings of a study that evaluated the performance of the Kwik 

Bond Polymers® HFST in terms of pavement friction and crash reduction. Based on the results 

of the study, Anderson (2015) recommended Washington State Department of Transportation 

(WSDOT) use this product. This HFST consists of a polyester resin binder and calcined bauxite 

aggregate; it was applied to two interstate on-ramps in the Seattle area. The treatment was 

installed with automated installation equipment without notable problems. Table 1 summarizes 

the installation information and initial results, and the measurements right after the treatment 

show that the friction resistance was improved significantly. WSDOT plans to conduct friction 

testing and visual surveys on a yearly basis for a minimum of 5 yr and collect and analyze crash 

data for the same period to assess the long-term performance and B/C of this HFST. 

Table 1. Summary of 2015 WSDOT HFST evaluation (interim results)

(Anderson et al. 2015). 

Site Description Date HFST Materials

Cost per Square 

Yard

(U.S. Dollars)

Average Friction 

Before⁑

Average Friction 

After

(07/2015)

Westbound I–90 

on-ramp

(from southbound

148th Avenue) 

06/2015 Polyester resin 

binder with 

calcined bauxite 

aggregate

35.50 39 83

Westbound I–90 

HOV on-ramp

(from southbound

148th Avenue) 

06/2015 Polyester resin 

binder with 

calcined bauxite 

aggregate

35.50 40 86

Southbound I–5 

on-ramp

(from eastbound

SR 526)

06/2015 Polyester resin 

binder with 

calcined bauxite 

aggregate

35.50 38 75

⁑Date unknown.

HOV = high-occupancy vehicle. 

2015 Kentucky: Kentucky Transportation Cabinet’s High Friction Surface Treatment and 

Field Installation Program (Von Quintus and Mergenmeier 2015) 

This report provided an overview of effort by Kentucky Transportation Cabinet (KYTC) to 

establish an HFST program to reduce crashes and fatalities as part of their Strategic Highway 

Safety Plan. Von Quintus and Mergenmeier (2015) discussed standards, guidance, and 

specifications for using HFSTs as a safety countermeasure, including the various elements to 

consider when selecting candidate roadway segments for HFSTs, and insights on lessons learned 

from HFST deployment in Kentucky since 2009. 

Von Quintus and Mergenmeier (2015) explained that part of KYTC’s success with HFSTs was 

due to their site-selection process and the supporting data analysis that identified locations likely 

to benefit the most from using FSTs. Von Quintus and Mergenmeier (2015) described the 

evolution of this effort through the following processes: 
12 

• Process I—the 30 Worst was Kentucky’s first HFST screening system in 2009 that 

analyzed 3 yr of crash data on the two-lane, two-way rural State road system; curves; and 

ramps to identify the 30 highest crash sections with high wet/dry ratios. 

• Process II—2010 Roadway Departure Safety Implementation Plan was developed in 

2010 in collaboration with FHWA and analyzed 4 yr of crash data on the two-lane, 

two-way rural State road system, including roadway-departure crashes, segregating 

3,000-foot sections and identifying them by county, route, and milepoint (FHWA Office 

of Safety n.d.c.). Sections with eight or more wet/total crashes and a minimum wet/total 

crash ratio of 0.35 were deemed HFST candidate sites.

• Process III—the EB methodology was implemented as a more advanced process 

incorporating EB methodology to estimate the number of crashes at sites if an HFST was 

not installed by using SPFs. 

Once the screening process was completed, field assessments were conducted to corroborate the 

viability of HFSTs. Factors considered during field assessments include, but are not limited to: 

drainage, superelevation, driveways, future planned work, sight distances, crash analysis, 

ponding of water, constructability, existing pavement condition, HFST limits, and HFST 

treatment of one or both lanes. The post HFST installation timeframe was deemed inadequate to 

perform a more rigorous analysis. Thus, a simple 3-year before to a 1 to 4 yr after installation 

(depending on the age of the treatment) crash comparison was performed for curves and ramps. 

The analysis revealed substantial yearly crash reductions for both wet and dry crashes on both 

curves and ramps. However, the benefits were likely overestimated since they did not account for 

bias in selecting sites for treatment and the resulting RTM. 

2014 Kentucky: Evaluation of High Friction Surface Treatments (Scully 2014)

This report presented the University of Kentucky Transportation Center’s program to test HFST 

sites in Kentucky with a portable dynamic friction tester (DFT), a stationary spot measurement 

device. A DFT was used because the majority of HFST sites are curves and are difficult to test 

with the KYTC locked-wheel skid tester. The report contains 1-page summaries for the 

90 curves and 15 ramps tested, including descriptions of the sites, observed distresses, and 

photos. Scully (2014) noted that distresses varied from site to site, with some exhibiting no 

distresses and others exhibiting cracking and HFST peel off. For seven new HFST installations 

from 2014, the friction before and after HFST application was measured. Scully (2014) noted an 

FN40R before treatment ranging from 27 to 40 and an FN40R after treatment ranging from 81 to 

96. For older sites, testing revealed an FN40R ranging from 65 to 80 after nearly 4 yr of traffic.

2012 Washington State: Evaluation of Tyregrip® High-Friction Surfacing (Anderson et al. 

2012) 

This report presented the interim findings of a WSDOT study that evaluated the construction and 

performance of the Tyregrip® HFST in terms of pavement friction and crash reduction. Based on 

the results of the study, Anderson (2012) recommended WSDOT use this product. The 

installation consisted of a two-part epoxy binder with calcined bauxite aggregate applied to an 

interchange on-ramp with a high accident occurrence. Automated installation equipment was 
13 

used, but there were problems with the aggregate spreader that resulted in poor aggregate 

distribution. Hand placement and application of a second layer the following night were 

required. 

Table 2 summarizes the installation information and initial results, and measurements right after 

HFST installation show a modest increase in friction resistance. At the request of the supplier, 

WSDOT allowed the application of another Tyregrip® layer at no cost in May 2011. The 

measurements in May 2011 show a significant increase in friction resistance after the second 

layer was applied. A section of the ramp was left untreated comparison purposes (see the 

“Untreated section” row in table 2). WSDOT conducted friction testing and visual surveys on a 

yearly basis for a minimum of 5 yr. WSDOT also collected and analyzed crash data for the same 

period to assess the long-term performance and the B/C of the Tyregrip® HFST (Anderson et al. 

2017).

Table 2. Summary of 2012 WSDOT HFST evaluation (interim results)

(Anderson et al. 2012). 

Site Description

Installation 

Dates 

HFST 

Materials

Cost per 

Square Yard

(U.S. Dollars)

Friction 

Before 

(05/2011)

Friction 

After

(11/2010)

Friction 

After

(05/2011)

Crash 

Count 

Before

Ramp from 

southbound 

164th Avenue to 

westbound 

SR 14

08/2010

05/2011

Untreated 

section

— 47.4 44.1 44.5

27 crashes 

in 3 yr

(20 wet)

Two-part 

epoxy resin 

with 

calcined 

bauxite 

aggregate

36.50 47.4 54.1 76.7

—No information available.

2009 Kansas: Evaluation of High Friction Surface Locations in Kansas (Meggers 2015) 

This report presented the findings of a study that evaluated the long-term effectiveness and 

durability of four HFST installations as part of the FHWA Surface Enhancements at Horizontal 

Curves (SEaHC) Program. These HFSTs consisted of a Poly Carb Type III epoxy-based overlay 

with flint aggregate applied to two asphalt pavement highway sections and two concrete 

interchange ramps. The treatment was installed with automated installation equipment without 

notable problems. Evaluations in 2009, 2010, and 2013 consisted of visual inspection and 

friction, surface pull-off, and permeability testing.

Table 3 summarizes the installation information and friction testing results, showing a significant 

improvement in skid resistance after HFST installation. However, the skid resistance on the 

concrete ramps quickly deteriorated; in 2013, skid resistance was near the pretreatment values.

The K–99 installation was milled off and replaced with asphalt in 2012 due to a significant 

number of isolated failures (e.g., potholes and HFST peel off). The other three sections were 

evaluated in 2013 and 2014; K–5 and the US 54 ramp exhibited partial HFST wear, while the 

I–635 ramp exhibited significant HFST wear. Kansas Department of Transportation (KDOT) 

concluded that the premature HFST failure on K–99 was due to insufficient surface preparation 

(i.e., sand blasting) before installation. KDOT now uses the AASHTO MP 41-19, Standard 
14 

Specification for High Friction Surface Treatment for Asphalt and Concrete Pavements Using 

Calcined Bauxite, which suggests shot blasting asphalt pavements before applying HFSTs 

(AASHTO 2019). KDOT installed HFSTs at four sites in 2014 and two sites in 2015. The new 

sites are divided by aggregate type, with four using bauxite and two using flint. KDOT is 

conducting a long-term evaluation of these sites with the goal to achieve an HFST lifespan of 

7–10 yr on both asphalt and concrete pavements. 

Table 3. Summary of 2009 KDOT HFST evaluation (Meggers 2015). 

Site Description Date HFST Materials

Cost per 

Square Yard

(U.S. Dollars)

Friction 

Before⁑

Friction 

After 

(08/2009)

Friction 

After 

(10/2010)

Friction 

After 

(11/2013)

K–99 (asphalt) 08/2009 Two-part epoxy

with flint 

aggregate

✝ NB 47.8

SB 50.1

NB 87.3

SB 88.3

NB 80.0

SB 77.8

— 

K–5 (asphalt) 08/2009 Two-part epoxy

with flint 

aggregate

✝ NB 35.1

SB 35.1

NB 87.8

SB 82.1

NB 68.6

SB 59.9

NB 58.0

SB 55.7

Ramp from 

northbound I–35 

to northbound 

I–635 (concrete)

08/2009 Two-part epoxy

with flint 

aggregate

✝ 40.5 69.7 56.3 48.6

Ramp from 

eastbound K–96 

to eastbound 

US 54 (concrete)

08/2009 Two-part epoxy

with flint 

aggregate

✝ 46.4 76.1 66.1 47.9

⁑Date unknown.

—No information available.

✝Not reported.

NB = northbound; SB = southbound.

2009 Vermont: Ennis Paint, Inc. Tyregrip® High Friction Surface System (Kipp and 

Sanborn 2014) 

The Vermont Agency of Transportation (VTrans) conducted a study that evaluated the 

performance of the Tyregrip® HFST. The installation consisted of a highly modified exothermic 

epoxy resin two-part binder with calcined bauxite aggregate applied to a 266-foot section of the 

westbound lane of Route 9 (i.e., VT 9) in Woodford, VT. The section is on a curve with a steep 

grade of 8 percent and is considered a high crash location. 

VTrans intended to install the treatment according to the manufacturer’s recommendations using 

automated equipment; however, due to mechanical issues and trouble operating the truck on the 

steep grade, the treatment was placed by hand. Partial repairs were conducted in 2009; in 2010, 

significant cracking and delamination were observed, requiring milling and replacing the section 

in 2011. Failure was attributed to the underlying cracking and distresses of the existing 

pavement. Table 4 summarizes the installation information and results. The HFST showed 

promise in reducing crashes in the short time it was in use. Based on the results of this trial 

project, VTrans planned to evaluate a section of new or nondistressed pavement with automated 

installation.
15 

Table 4. Summary of 2009 VTrans HFST evaluation (Kipp and Sanborn 2014). 

Site Description Date HFST Materials

Cost per 

Square Yard

(U.S. Dollars)

Friction 

Before⁑

Friction 

After⁑

Crash 

Count 

Before

Crash 

Count

After

VT 9 westbound

lane

10/2009 Two-part epoxy 

resin with 

calcined bauxite 

aggregate

26 ✝ ✝ 13 crashes 

in 3 yr 

1 crash in 

3 yr 

⁑Date unknown.

✝Not reported.

2008–2010 South Carolina: High Friction Surface Course (HFSC) Before and After Study

(Riddle 2012) 

South Carolina Department of Transportation (SCDOT) conducted a simple before and after 

study on the performance of six HFST installations across the State. One of the installations was 

on a curvilinear four-lane divided highway, while the other five installations were on freeway 

ramps with a high percentage of wet crashes. The installations on ramps included signing 

improvements that also may have increased safety. Table 5 summarizes the installation 

information and some of the results. The installations occurred in 2008 and 2010, and crash data 

were available from 3 to 6 yr before and 1 to 3 yr after. The data showed an average crash 

reduction of 70 percent and a wet-crash reduction of 77 percent. B/C ratios of 23.8 and 3.93 were 

calculated for total and wet crashes, respectively.

SCDOT reported that recent HFST installations on pavement sections with open-graded courses 

presented challenges and they are updating their specification to require a double-layer system 

for such applications for better epoxy coverage.

Table 5. Summary of 2008–2010 SCDOT HFST evaluation (Riddle 2012). 

Site Description Date HFST Materials

Total Cost

(U.S. Dollars 

Rounded to 

Nearest 

Thousand) 

Total

Crashes per 

Year 

Before

Total

Crashes per 

Year 

After

Wet 

Crashes per 

Year 

Before

Wet 

Crashes per 

Year 

After

I–526 at US 17 11/2010 * 74,000 4 0 3.8 0 

I–85 at US 29 06/2009 * 25,000 8.3 0 6.3 0 

US 25 

(near S–41)

10/2008 * 1,200,000 9.6 4.2 8.1 2.6

SC 31 at SC 9 04/2010 * 160,000 8.2 2.5 3.4 1.2

SC 31 at SC 544 04/2010 * 60,000 2.1 1.9 0.4 0 

SC 31 at US 501 04/2010 * 160,000 12.6 5 5.2 2.5

*The specification requirement for all sites was a two-part cold-applied modified exothermic nonvolatile organic 

compound epoxy resin binder treatment containing an epoxy/amine binder covered with natural or pigmented 

aggregates with bauxite aggregate.

2006 Florida: Evaluation of a High Friction Pavement Surface Treatment (Savolainen et al. 

2009) 

Florida Department of Transportation (FDOT) conducted a study to determine the effectiveness 

of HFSTs for improving pavement friction and reducing crashes. The installation consisted of a 
16 

highly modified exothermic epoxy resin two-part binder with calcined bauxite aggregate applied 

to an I–75 on-ramp. Table 6 summarizes the installation information and shows a significant 

increase in friction resistance right after HFST installation. Due to limited availability of crash 

data for the site after HFST installation, surrogate measures of safety, including mean and 

variance of vehicle speeds, vehicles traveling over the advisory speed limit, and vehicles 

encroaching across the lane edge lines, were evaluated to determine the effectiveness of the 

HFST. After HFST installation, all surrogate measures decreased. 

Table 6. Summary of 2006 FDOT HFST evaluation (Savolainen et al. 2009). 

Site Description Date HFST Materials

Cost per 

Square Yard

(U.S. Dollars)

Friction 

Before

(04/2006)

Friction 

After

(05/2006)

Crash 

Count 

Before

Crash 

Count 

After

Ramp from 

Royal Palm

Boulevard to 

northbound I–75

05/2006 Highly modified 

exothermic 

epoxy resin 

two-part binder

with calcined 

bauxite 

aggregate

✝ 35 104 11 crashes 

in 4 yr 

2 crashes 

in 1 yr

✝Not reported.

1999 Wisconsin: Investigative Study of the ItalgripTM System (Bischoff 2008) 

In 1999, Wisconsin Department of Transportation (WisDOT) installed the ItalgripTM HFST at 

five different locations with high crash rates due to wet/icy surfaces. Four installations were on 

bridge decks, while one was a section of portland cement concrete (PCC) pavement on State 

Trunk Highway (STH) 16 with a history of accidents due to black ice. Table 7 summarizes the 

information for the installation on STH 16 (not the bridge decks) and shows a significant 

improvement in skid resistance. Although the friction decreased after 5 yr, the FN40Rs were still 

higher than the pre installation values. Results also show there were no crashes during the 3-year 

period after HFST installation, compared to five accidents during the 3-year period before HFST 

installation. Bischoff (2008) reported annual progressive aggregate surface loss due to traffic and 

snowplow blades. By 2005, a surface loss of 60 percent was estimated at the STH 16 site. 

Table 7. Summary of 1999 WisDOT HFST evaluation (Bischoff 2008). 

Site Description Date HFST Materials

Cost per 

Square Yard

(U.S. Dollars)

Friction 

Before⁑

Friction 

After⁑

Friction 

5 yr 

After⁑

Crash 

Count 

Before

Crash 

Count 

After

Eastbound 

STH 16 in 

Waukesha County

10/1999 Two-part 

polymer resin 

with manmade 

aggregate of 

reworked steel 

slag

20 43.3 75.4 57.4

5 

crashes 

in 3 yr 

0 

crashes 

in 3 yr Westbound 

STH 16 in 

Waukesha County

42.5 75.7 60.3

⁑Date unknown.
17 

2012 California: Northern California US 199 – Del Norte County, Case Study (Cheung and 

Julian 2015) 

This California Department of Transportation (Caltrans) case study prepared by FHWA 

describes the factors leading to an HFST installation along a section of US 199, a two-lane rural 

highway in northern California with a high frequency of wet crashes. Previous measures to 

improve safety, including center lane rumble strips, speed and chevron warning signs, and an 

open-graded asphalt overlay, were not effective at reducing accidents. The site is located within 

the Redwood National and State Parks, an environmentally sensitive location, and 

realignment/geometric improvements at the site require a lengthy and involved process. Table 8

summarizes the information for this case study, though a detailed evaluation with 

before-and-after friction and crash data was not provided. 

Table 8. Summary of 2012 Caltrans HFST application (Cheung and Julian 2015). 

Site Description Date HFST Materials

Total Cost

(U.S. Dollars)

Friction 

Before⁑

Friction After

(11/2010)

Crash Count 

Before

US 199 Summer 

2012

Double layer of 

epoxy-resin 

binder with 

calcined bauxite 

aggregate

250,000 ✝ ✝ 26 wet crashes 

from 2006 to 

2009

⁑Date unknown.

✝Not reported.

In 2012, an HFST was installed, which minimized impacts on wildlife and did not require a 

detour estimated at longer than 8 hr due to the remote location of the site. Figure 6 shows the 

design timeframe, construction duration, and B/C analysis of installing an HFST over realigning 

the curve.

Source: FHWA.

Figure 6. Graphic. Comparison of an HFST versus curve realignment (Cheung and 

Julian 2015). 

The literature reviewed previously in this section consists of small-scale case studies and safety 

analyses limited to simple before-and-after crash count comparisons, typically with small 

samples, where data were available. Hereafter, the literature reviewed is for studies that are 

considered more rigorous evaluations of HFSTs in terms of scope or that looked at the 

relationship between crashes and skid resistance, not necessarily HFST installations. 
18 

2015 FHWA: Evaluation of Pavement Safety Performance (Merritt et al. 2015) 

Merritt et al. (2015) evaluated the safety effects of various pavement treatments, including 

HFSTs. Due to limitations in readily available data, including suitable reference sites and traffic 

volume counts, a preferred EB before–after study could not be performed for HFSTs; simpler 

naïve and C-G before–after analyses were performed depending on the data available 

(Hauer 1997). All treatment sites were either curves or interchange ramps, and 

intersection-related, animal, and snow/slush/ice crashes were not included. Merritt et al. (2015) 

noted that the results are likely biased toward exaggerating the expected crash reductions due to 

the methodological limitations and reported the results using a method-correction factor applied 

in the development of the HSM. The results using the naïve and C-G methods are shown in 

table 9 and table 10, respectively, for total and wet-road crashes. Despite the methodological 

concerns noted by Merritt et al. (2015), the results indicated that HFSTs can significantly reduce

crashes on curves and ramps, particularly wet-road crashes.

Table 9. Results for the naïve before–after study based on all sites (Merritt et al. 2015). 

Group

Number of 

Sites

Crashes 

After

Wet-Road 

Crashes 

After

CMFunction

(and Standard Error) for 

Total Crashes

CMFunction

(and Standard Error) for 

Wet-Road Crashes

Biased

With HSM 

RTM 

Correction Biased

With HSM 

RTM 

Correction

All ramps 27 111 19 0.387

(0.041)

0.484 0.169

(0.041)

0.211

All curves 43 104 45 0.502

(0.052)

0.628 0.298

(0.048)

0.373

Table 10. Results for the before–after with C-G study for treatment sites for which 

comparison sites were available (Merritt et al. 2015). 

Group

Number of 

Sites

Crashes 

After

Wet-Road 

Crashes 

After

Total Crashes—C-G Wet-Road Crashes—C-G 

Biased

With HSM 

RTM 

Correction Biased

With HSM 

RTM 

Correction

Ramps 12 77 8 0.522

(0.092)

0.653 0.111

(0.042)

0.139

Curves 35 104 45 0.607

(0.067)

0.759 0.385

(0.064)

0.481

2014 Texas Transportation Institute: Evaluating the Need for Surface Treatments to Reduce 

Crash Frequency at Horizontal Curves (Pratt et al. 2014) 

Pratt et al. (2014) developed an analysis framework, which includes three aspects, to assess the 

need for surface treatments at curves. The first aspect is developing models to predict vehicle 

speeds prior to and traversing a curve. The second aspect is using these predicted vehicle speeds 

to determine the side friction demand at several points within the curve, which is a function of 

vehicle speed, superelevation, grade, and curve radius. The third aspect is determining the side 

friction provided for the estimated travelling speed, which uses a formula to estimate skid 

numbers (SNs) for a desired vehicle speed with a measured SN as the baseline.
19 

Pratt et al. (2014) also developed several SPFs relating crashes on curves of various crash types 

to traffic volumes, roadway geometry, and measured SNs. Separate SPFs were developed for 

two-lane, four-lane undivided, and four-lane divided roadways. The crash types for which SPFs 

were developed did not include property-damage-only (PDO) crashes but otherwise included all, 

wet-weather, ROR, and wet-weather ROR crashes. The parameter estimates for SNs in the 

various models can be used to calculate CMFs for SNs. The results indicated that, as skid 

resistance increases, crashes decrease, particularly wet-weather crashes. The implied CMFs for 

wet-weather crashes, for an increase in SN of 1.0, were 0.981, 0.967, and 0.969 for two-lane, 

four-lane undivided, and four-lane divided roadways, respectively. The data summary did not 

note the pavement types for the curves used by Pratt et al. (2014), but since data collection was 

not focused on HFSTs, it is likely that few, if any, sites had HFSTs installed. The mean SNs 

recorded were 44.2, 38.1, and 35.0 for two-lane, four-lane undivided, and four-lane divided 

roadways, respectively, well below what was expected with the installation of an HFST. 

Although some higher SNs were recorded, including at least one curve with an SN of 99, the 

distribution of SNs was not recorded; thus, the relevancy of CMFs based on SNs as they relate to 

HFSTs could not be assessed.

The site characteristic guidelines from Pratt et al. (2014) were input into a spreadsheet called the 

Texas Curve Margin of Safety that predicts margin of safety, crash frequency (including CMFs) 

and travel path distribution. The Texas DOT uses this tool when considering HFSTs for curves to 

conduct a margin of safety analysis and estimate its potential benefit, including the expected 

crash frequency reduction.

2016 Virginia Tech Transportation Institute: A New Approach for Managing Pavement 

Friction and Reducing Road Crashes (McCarthy et al. 2016) 

McCarthy et al. (2016) conducted research to determine if adding routine skid resistance data to 

SPFs could improve crash predictions and help identify locations that would benefit from 

HFSTs. Skid resistance measurements with a continuous fixed-slip measurement device that 

reports friction data approximately every 3 ft were taken for approximately 2,000 lane-miles in 

the Salem District of the Virginia Department of Transportation (VDOT). In addition to the 

measurements, crash and traffic data from 2010 to 2012 were analyzed. Only accidents with wet 

or dry pavement-surface conditions were taken into account, excluding snow, mud, and others. 

To relate the data collected to the VDOT locked-wheel skid tester measurements, McCarthy 

et al. (2016) applied a correction factor to the GripNumber of 0.007 for each mph greater or less 

than 40 mph. McCarthy et al. (2016) also used a moving average of 60 ft to identify the lowest 

friction within each 0.1-mile segment since locked-wheel skid tests are conducted at 40 mph 

over a 1-second interval and the SN value approximates the average over 58.7 ft. 

The analysis found the GripNumber statistically significant for the SPFs, showing that crashes 

are expected to decrease with increased pavement friction. The SPFs were subsequently applied 

to the EB approach for preliminary screening of network segments with a high crash risk and 

prioritized them for HFST installations. A companion paper, discussed next, provides more 

details on the approach. 
20 

2016 Virginia Tech Transportation Institute: Pioneering the Use of Continuous Pavement 

Friction Measurements to Develop New Safety Performance Functions, Improve the Accuracy 

of Crash Count Predictions, and Evaluate Possible Treatments for the Roads in Virginia

(de León et al. 2016) 

de León et al. (2016) reported on McCarthy et al. (2016) but also discussed the B/C analysis of 

pavement treatments to improve friction, asphalt overlays, and HFSTs. To calculate the benefits 

of a pavement treatment at a specific roadway section, the SPFs developed by McCarthy (2016), 

including available friction, were used to derive CMFs by dividing the SPF prediction with the 

assumed reading after treatment divided by the CMF estimate with the current reading. The scale 

of the GripNumber ranged from 0 to 1.0. An increased GripNumber of 0.1 provided CMFs of 

0.894, 0.905, and 0.946 for interstate, primary, and secondary routes, respectively. The CMFs 

were then applied to the EB estimate of expected crashes at the site. Because the data used to 

develop the SPFs included all roadways, with friction levels typically lower than that expected 

for HFSTs, it is unknown if the SPFs applied were accurate for the high range of frictions 

associated with HFSTs. The models did not account for any interactive effects between friction 

and horizontal curvature, although both variables were included in one set of models. HFSTs can 

be beneficial to safety on curves where a higher level of friction is demanded. de León et al. 

(2016) found the cost savings from crash avoidance through friction treatments was considerable 

and justified implementing a comprehensive pavement friction management program.

2012 Texas Transportation Institute: Using High Friction Surface Treatments to Improve 

Safety at Horizontal Curves (Brimley and Carlson 2012) 

Brinley and Carlson (2012) cited and used the findings from several studies on HFSTs by 

different organizations to complete the following: 

• Quantify the benefits of applying HFSTs on curves using research findings showing their 

effectiveness at reducing crashes.

• Provide a background on the historical and modern use of HFSTs. 

• Show how Federal programs support the use of HFSTs. 

• Present the physical components of a common HFST application. 

• Derive the value of HFSTs based on a review of studies on their effectiveness.

• Project the benefits of applying HFSTs under various scenarios with consideration of 

product cost and expected life. 

• Recommend HFST placement.

The studies Brinley and Carlson (2012) cited for estimates of safety effectiveness were based on 

only one or a few sites and are not reliable estimates of the safety effectiveness of HFSTs. 
21 

2015 National Center for Asphalt Technology: High Friction Surface Treatment Alternative 

Aggregates Study (Heitzman 2015) 

Heitzman (2015) examined the performance of seven alternative aggregate sources in the United 

States to determine if they provided similar friction performance to calcined bauxite, which is an 

imported product. The alternative aggregates were granite, flint, basalt, silica sand, steel slag, 

emery, and taconite. The research was divided into three studies: LAB‐1, FIELD, and LAB‐2. 

LAB‐1 evaluated HFST test slabs with calcined bauxite and seven alternative aggregates under 

accelerated laboratory polishing and testing procedures. None of the seven alternative HFST 

aggregates provided friction comparable to calcined bauxite based on DFT measurements.

FIELD evaluated the friction performance of HFST pavement test sections with the same eight 

aggregates under heavy truck loading in the west end super-elevated curve at the National Center 

for Asphalt Technology Pavement Test Track. The calcined bauxite section maintained higher 

friction levels over the 24 mo of accelerated truck traffic conditioning. LAB-2 evaluated the 

influence of particle size on HFST friction performance and examined other laboratory aggregate 

tests as a simpler approach to qualifying friction aggregates in HFST specifications. Particle 

shape and angularity did not correlate to friction. Heitzman (2015) concluded the friction for the 

evaluated alternative HFST aggregates was not equal to calcined bauxite.

2011 Virginia Tech Transportation Institute: Review of High-Friction Surface Technologies 

– Constructability and Field Performance (Flintsch et al. 2011) 

Flintsch et al. (2011) reviewed the constructability and performance of six HFSTs available in 

the United States. Flintsch et al. (2011) discussed the application procedures and key items for 

successful installation of the HFSTs, including the epoxy preparation composition, epoxy 

aggregate placement, existing pavement compatibility with HSFTs, the humidity and moisture of 

the pavement surface and aggregate, ambient temperature, and curing times. Flintsch et al. 

(2011) took friction and texture measurements with a DFT, circular track meter, and grip tester at 

bridge deck overlay locations in Tennessee, Virginia, and Wisconsin where the six HFSTs were 

installed. Flintsch et al. (2011) concluded HFSTs provide very high initial levels of friction and 

macrotexture. Based on limited data, Flintsch et al. (2011) estimated that some HFSTs can 

maintain high friction properties for 10 yr. 

2015 Oklahoma State University: 3D Laser Imaging based Real Time Pavement Surface 

Evaluation for High Friction Surfacing Treatments (Li et al. 2015) 

Researchers at Oklahoma State University developed a 1-millimeter resolution three-dimensional 

laser imaging sensor technology. This technology was used to evaluate 23 HFST installations 

across the United States that were part of the FHWA SEaHC Program. Cracking, rutting, 

macrotexture, roadway geometry, and HFST surface loss were assessed with the technology at 

highway speeds and with full lane width coverage. Friction was also measured with FHWA’s

highway friction fester (HFT) to show FN40R comparisons before and after HFST installation. 

Li et al. (2015) found the HFST sections had statistically higher surface texture (i.e., mean 

profile depth (MPD)) and FN40Rs than the adjacent sections without HFSTs. Li et al. (2015) 

reported several sites with HFSTs exhibiting isolated failures, but available software was not 

capable of estimating HFST surface loss.
22 

HFST SPECIFICATIONS

U.S. Specifications 

A review of current State highway agency specifications/special provisions for furnishing, 

applying, and accepting HFSTs was conducted, including those for California, Florida, Georgia, 

Illinois, Kansas, Kentucky, Maryland, Nebraska, Pennsylvania, South Carolina, Tennessee, 

Texas, and Virginia. AASHTO MP 41-19 was reviewed as well, providing a baseline 

specification off which many State specifications are based. Although specific requirements 

(e.g., test methods and thresholds) are not provided for brevity, the following items are covered 

in the majority of State highway agency specifications. 

Materials

Most specifications either indicate the type of binder resin to use or require it to be a material 

from the agency’s qualified/approved products list (QPL/APL). Different QPLs/APLs can be 

provided for asphalt and concrete pavements. Epoxy resin is the most common binder resin, but 

polyester resin is becoming more common; methyl methacrylate (MMA) is another option 

generally permitted. Physical requirements and corresponding test methods for properties, such 

as viscosity, gel time, durometer hardness, tensile and compressive strengths, cure rate, water 

absorption, and adhesive strength, are provided in most specifications. AASHTO MP 41-19 

provides separate requirements for polymeric and MMA resins. 

Virtually all the specifications reviewed specify calcined bauxite for the aggregate and 

physical/mineralogical requirements and corresponding test methods for properties, such as 

polish stone value, resistance to degradation, gradation, moisture content, and minimum 

aluminum oxide content. Most agencies use AASHTO T 96/ASTM C131, Standard Method of 

Test for Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in 

the Los Angeles Machine, for resistance to degradation, but Tennessee Department of 

Transportation (TDOT) recently switched to ASTM D7428, Standard Test Method for 

Resistance of Fine Aggregate to Degradation by Abrasion in the Micro-Deval Apparatus, which 

may be more appropriate for smaller aggregates like those used for HFSTs (AASHTO 2002; 

ASTM 2015). 

Most specifications require certification from the binder and aggregate manufacturers or test 

results from an independent laboratory demonstrating the materials meet agency specifications. 

Installer Qualifications

Most specifications require installers have prior experience installing HFSTs, including a 

description of previous experience (e.g., minimum of three projects and 5,000–10,000 square 

yards), test results (e.g., friction measurements), and references from owner agencies. Some 

agencies allow work from installers not meeting minimum requirements if they are approved by 

the manufacturer and a manufacturer representative is on site during the installation.
23 

Construction Practices

Items covered under the construction section of the majority of specifications include materials

storage, weather and pavement-surface temperature requirements, pavement surface preparation, 

hand and mechanical mixing, application of the HFST, and curing. 

Surface preparation items include removing pavement markings in areas to be treated, washing 

contaminated surfaces, pretreating cracks, cleaning asphalt surfaces with mechanical sweepers 

and air washing/vacuum sweeping, shot blasting, and vacuum sweeping concrete pavements.

If mechanical application is required, hand mixing and application is typically allowed for areas 

where mechanical application is not practical or economical, such as segments less than 250 ft in 

length or areas less than 200–800 square yards, depending on the agency. Mechanical application 

is not permitted by some agencies on sites with steep slopes (e.g., greater than 2.5 percent).

Quality control (QC) plans are required by most agencies. QC plans require an installer to 

describe in detail all personnel; equipment; materials; surface preparation, resin mixing and 

application, and aggregate application methods; and curing, sweeping, and opening to traffic 

requirements. An installer then submits the QC plan to the agency for approval prior to 

installation.

Some agencies require a test strip be constructed prior to full-scale installation. Test strips allow 

an installer to demonstrate their installation practices so the agency can verify the full-scale 

installation meets project requirements.

Acceptance Testing

The majority of specifications require the HFST be tested within 60 to 90 d after installation and 

meet a minimum FN40R per AASHTO T 242, Standard Method of Test for Frictional 

Properties of Paved Surfaces Using a Full-Scale Tire, of at least 65. TDOT requires FN40R of 

70, and KYTC requires FN40R of 75 (AASHTO 2018). 

Some agencies require additional testing of the in-place surface characteristics. For example, 

Georgia Department of Transportation (GDOT) specifies testing with a DFT within 5 d after 

installation and again 90 d after installation, requiring a value greater than 0.9 at 5 d and a value 

greater than 0.8 required at 90 d. TDOT and the Maryland State Highway Administration both 

require a minimum mean texture depth (per ASTM E965, Standard Test Method for Measuring 

Pavement Macrotexture Depth Using a Volumetric Technique) or MPD (per ASTM E2157, 

Standard Test Method for Measuring Pavement Macrotexture Properties Using the Circular 

Track Meter) of 1.0 mm (ASTM 2019a; ASTM 2019b). 

Some agencies require a performance guarantee on HFSTs. For example, SCDOT requires the 

HFST be tested within 360 to 390 d after installation (after approximately 1 yr) and the average 

FN40R be at least 64 with no significant aggregate surface loss (e.g., at least a 90-percent intact 

surface).
24 

International Specifications

The United Kingdom has the most extensive HFST usage history outside the United States with 

HFST installations dating back to the 1960s and widespread usage since the late 1980s (RSTA 

2013). In 2011, the Road Surface Treatments Association (RSTA) produced Code of Practice for 

High Friction Surface Treatments, which provides best practices for selecting and installing 

high-friction surfacing systems to maximize their performance and durability (RSTA 2011). 

Code of Practice for High Friction Surface Treatments is based on HFST installations that 

improve skid resistance at high-risk locations in the United Kingdom. High-risk locations are 

highlighted as approaches to major junctions, approaches to pedestrian crossings, sites with 

grades steeper than 10 percent, and curves with radii smaller than 500 m (1,640 ft). 

Table 11 summarizes the current Highways England requirements for minimum aggregate 

polished stone value (PSV) based on site characteristics (e.g., roadway type and traffic), showing 

that only HFSTs can provide the necessary friction at certain locations with a relatively high 

investigatory level friction threshold (Highways England 2006). 

Code of Practice for High Friction Surface Treatments cites the Monitoring Local Authority 

Safety SchemES study of road safety schemes in the United Kingdom that found an overall 

weighted crash reduction of 35.1 percent (10/90 percent rural/urban site balance) for HFSTs 

between 1991 and 2001 (Gorell and Tootill 2001). Code of Practice for High Friction Surface 

Treatments also highlights the British Board of Agrément Highways Agency Product Approval 

Scheme (BBA HAPAS) certification process for ensuring that both the HFST product and HFST 

installer are certified for use on a project. BBA HAPAS provides some best practices for surface 

preparation prior to HFST installation, recommended specifications for aggregate properties, and 

guidance on performance criteria for projects with a performance guarantee.
25 

Table 11. Road surfacing aggregate PSV requirements based on site characteristics from Highways England Design Manual 

for Roads and Concrete Bridges (Highways England 2006). 

Site Category Site Description IL

Minimum PSV Required for Given IL, Traffic Level, and Type of Site

Traffic (CV/Lane/Day) at Design Life

0–250 251–

500

501–

750

751–

1,000

1,001–

2,000

2,001–

3,000

3,001–

4,000

4,001–

5,000

5,001–

6,000

Over 

6,000

A1 Motorways where traffic is generally 

free-flowing in a relatively straight line

0.30 50 50 50 50 50 55 55 60 65 65

0.35 50 50 50 50 50 60 60 60 65 65

A2 Motorways where some braking regularly 

occurs (e.g., on 300-meter approach to an 

off-slip)

0.35 50 50 50 55 55 60 60 65 65 65

B1 Dual carriageways where traffic is generally 

free-flowing in a relatively straight line

0.30 50 50 50 50 50 55 55 60 65 65

0.35 50 50 50 50 50 60 60 60 65 65

0.40 50 50 50 55 60 65 65 65 65 68+

B2 Dual carriageways where some braking 

regually occurs (e.g., on 300-meter

approach to an off-slip)

0.35 50 50 50 55 55 60 60 65 65 65

0.40 55 60 60 65 65 68+ 68+ 68+ 68+ 68+

C Single carraigeways where traffic is 

generally free-flowing in a relatively 

straight line

0.35 50 50 50 55 55 60 60 65 65 65

0.40 55 60 60 65 65 68+ 68+ 68+ 68+ 68+

0.45 60 60 65 65 68+ 68+ 68+ 68+ 68+ 68+

G1/G2 Gradients >5 percent longer than 50 m per 

HD 28, Skid Resistance

0.45 55 60 60 65 65 68+ 68+ 68+ 68+ HFS

0.50 60 68+ 68+ HFS HFS HFS HFS HFS HFS HFS

0.55 68+ HFS HFS HFS HFS HFS HFS HFS HFS HFS

K Approaches to pedestrian crossings and 

other high-risk situations

0.50 65 65 65 68+ 68+ 68+ HFS HFS HFS HFS

0.55 68+ 68+ HFS HFS HFS HFS HFS HFS HFS HFS

Q Approaches to major and minor junctions 

on single and dual carriageways where 

frequent or sudden braking occurs in a 

generally straight line

0.45 60 65 65 68+ 68+ 68+ 68+ 68+ 68+ HFS

0.50 65 65 65 68+ 68+ 68+ HFS HFS HFS HFS

0.55 68+ 68+ HFS HFS HFS HFS HFS HFS HFS HFS

R Roundabout circulation areas 0.45 50 55 60 60 65 65 68+ 68+ HFS HFS

0.50 68+ 68+ 68+ HFS HFS HFS HFS HFS HFS HFS

S1/S2 Bends (radii <500 m) on all types of 

roadways, including motorway link roads, 

and other hazards that require combined 

braking and cornering

0.45 50 55 60 60 65 65 68+ 68+ HFS HFS

0.50 68+ 68+ 68+ HFS HFS HFS HFS HFS HFS HFS

0.55 HFS HFS HFS HFS HFS HFS HFS HFS HFS HFS

© 2016 Crown. 

1 m = 3.28 ft.

CV = commercial vehicles; HFS = high-friction surface; IL = investigatory level.
26 

LITERATURE REVIEW CONCLUSIONS

The literature review of HFSTs at the commencement of this current study in 2015 found few 

materials of direct relevance, likely because this is a relatively new treatment with a limited 

number of installations in the United States. The bulk of literature on HFSTs is related to case 

study evaluations of one or only a few HFST installations. Some literature was found 

investigating the available friction from various HFST materials. Some literature was also found 

relating crash expectancy to the level of road friction available. However, there was a lack of 

information on the safety effects of HFSTs as measured through a large-scale and 

methodologically reliable study. A review of HFST specifications from various States was also 

undertaken. 

The literature review provided relevant information considered during this current study, 

including the following: 

• HFST are installed to improve safety. Therefore, RTM is a significant issue to contend 

with for a safety evaluation.

• Most HFST are installed on curves and interchange ramps. Limited use of HFSTs for 

intersection approaches or short sections on steep grades has been reported, but not 

enough to be considered for a safety evaluation. 

• Pavement friction is significantly improved immediately after HFST installation, but 

there is no established trend for how friction decreases over time. 

• Long-term performance of HFSTs is not consistent. Early failures are observed at some 

installations, while others see excellent long-term performance. This is likely due to a 

general lack of institutional knowledge of best practices for HFST materials and 

installation specifications in the early stages of deployment. Some studies attribute early 

failures to issues during the installation process, including the following: 

o Inappropriate pavement surface preparation. 

o Installation on highly distressed pavements. 

o Inappropriate resin binder thickness on coarse-textured and open-graded surfaces. 

o Deterioration due to heavy traffic when using aggregates other than calcined bauxite. 

• Available pavement friction has a measurable and significant effect on expected crash 

rates.

• Studies on the long-term safety benefits of HFSTs are limited because few sites have 

been evaluated and/or state-of-the-art statistical methodologies cannot be applied. The 

results indicate large crash reductions, particularly wet-weather crashes.

• The safety effects of HFSTs are likely related to the friction demand and the level of 

friction provided prior to treatment. The development of CMFunctions that are sensitive 

to these variables is desirable.
27 

CHAPTER 3. DATA COLLECTION

OVERVIEW OF THE DATA-COLLECTION PROCESS

Researchers developed and submitted to FHWA a thorough data-collection plan before 

beginning the data-collection process at the commencement of Phase II of this current study. The 

plan summarized the types of data to be collected and listed the sites from various States 

included in the current study. Key components of the data-collection process included the 

following: 

• Identifying HFST sites. 

• Collecting crash data. 

• Collecting friction data. 

• Colleting roadway and pavement data. 

• Identifying reference sites. 

Climatic data collection was initially part of the data-collection effort. However, the research 

team found the effort required to collect usable climatic data over a large sample of HFST 

locations unjustifiable, especially considering the limited benefits based on the results of a 

previous pavement safety performance study (Merritt et al. 2015). 

IDENTIFYING HFST SITES 

The process to identify sites for inclusion in this current study was summarized in a memo to 

FHWA at the end of Phase I. The research team contacted all State highway agencies with 

known HFST programs to identify sites available for inclusion in this current study. Preference 

was given to States with large and/or mature HFST programs able to provide a large population 

of sites for analysis.

Specific criteria for selecting sites included the following: 

• Site type—curves, ramps, and intersections were the focus of the HFST evaluation. 

Although there is a large population of HFST installations on bridges throughout the 

United States, the purpose of bridge treatments is typically preservation (i.e., sealing the 

bridge deck surface) with friction enhancement as a secondary benefit. A number of the 

curves or ramps included in the final analysis had bridge decks within the treated section. 

• In-place and planned sites—site selection sought to capture a mixture of existing HFST 

sites as well as planned sites. Existing sites provide more after-period crash data, 

particularly for older sites. Planned sites provide an opportunity to test pavement friction 

before and after HFST installation. Planned sites were limited to those installed during 

the 2016 and early 2017 construction season. 

• HFST materials—HFST sites selected for evaluation were limited to those consisting of 

calcined bauxite aggregate over a polymeric resin binder. Although alternative aggregates 
28 

have been used in a number of States, calcined bauxite is the only aggregate permitted in 

AASHTO MP 41-19 and most State specifications (AASHTO 2019). 

Table 12 summarizes the initial number of sites by State and site type (e.g., curve, ramp, 

intersection) initially proposed before beginning the data-collection process. 

Table 12. Initial sites planned for testing.

State

Number of Curves Number of Ramps Number of Intersections

In Place Planned In Place Planned In Place Planned

Georgia 26 98 — — — — 

Kentucky 54 0 23 0 1 0 

Louisiana 18 77 — — — — 

Pennsylvania 85 22 — — — — 

Texas 5 20 — — — — 

West Virginia 25 14 — — — — 

Arkansas — — 0 24 6 0 

Florida — — 15 3 4 0 

South Carolina — — 24 0 2 0 

Total 213 231 62 27 13 0 

—No information available.

Friction data were considered essential for the analysis, so the ability to efficiently collect 

friction data was a key consideration in site selection. Additional considerations in site selection 

included the following: 

• Availability of before-and-after friction data—priority was given to locations where 

researchers could collect friction data before and after HFST installation while ensuring 

the ability to collect after-period crash data. An estimate of before-treatment friction was 

important to link any change in safety to friction improvement. Four States (Arkansas, 

Louisiana, Georgia, and Pennsylvania) were identified as having a number of HFST sites 

that could be tested before and after HFST installation during the timeframe of this 

current study. 

• Proximity to other sites—focusing on sites clustered in jurisdictions in relative proximity 

to one another allowed for more efficient friction data collection by minimizing driving 

distance between sites. Preference was given to sites on a single roadway over sites 

scattered on various roadways.

• Number of sites in the same jurisdiction—States with a large number of sites provide the 

opportunity to evaluate a large population of HFST sites more efficiently in terms of 

crash data and friction data collection than States with only one or two sites. 

Table 13 summarizes the final list of sites planned for inclusion in this current study based on the 

initial data-collection plan. The final number of sites for each State included in the crash-data 

analysis is summarized in the following section. 
29 

Table 13. Final States and number of sites for data collection.

State

Number of Curves Number of Ramps Number of Intersections

In Place Planned In Place Planned In Place Planned

Arkansas 5 0 17 0 6 0 

Georgia 43 93 — — — — 

Kentucky 58 0 27 0 1 0 

Louisiana 27 90 — — — — 

Pennsylvania 85 22 — — — — 

West Virginia 25 14 — — — — 

Total 243 219 44 0 7 0 

—No information available.

Final Site Selection

Rationale for final site selection included the following for each State:

• Arkansas: 

o Large number of planned ramp locations for before–after evaluation.

o Close proximity to other States with a large number of sites.

• Georgia: 

o Large number of sites in place and planned for 2016. 

o Close proximity to other States with a large number of sites. 

o Sites are generally clustered together in a few counties, making friction data 

collection efficient.

o Curve selection represents a systemic approach to HFST deployment.

• Kentucky: 

o Large number of curves and ramps in place.

o Close proximity to other States with a large number of sites.

o Some of the oldest HFST installations that provide significant after-period evaluation 

and HFST performance over time.

• Louisiana: 

o Several curves in place and a large number planned for 2016. 

o Close proximity to other States with a large number of sites.

o Curve site selection represents a systemic approach (different from Georgia) to HFST 

deployment. 

• Pennsylvania: 

o Large number of sites in place and planned for 2016. 

o Primarily curves (similar to Georgia and Louisiana) but not representing a systemic 

approach as in Georgia and Louisiana. 

• West Virginia: 

o Large number of sites in place and planned for 2016. 

o Many of the in-place sites are 4 to 5 yr old. 

o Close proximity to other States with a large number of sites.
30 

Reasons for Exclusion 

The following factors led to the exclusion of some sites from the original HFST site list:

• Reliability of friction data—some of the operational limitations of the HFT combined 

with site characteristics either precluded friction data collection on site or resulted in 

unusable friction data after processing. Ramps in Florida and South Carolina were 

excluded for this reason. 

• Small number of sites—intersections were ultimately excluded from the final analysis 

due to the relatively small number of sites. There was significant variability of the 

characteristics of intersections, and most significantly, it was difficult to collect friction 

data at intersections. 

• HFSTs applied to only one lane—for a number of two-lane rural-roadway curves, only 

one of the two lanes (or only one direction) had an HFST installation. Since crash data on 

two-lane roadways are not always coded with the direction of travel, it was difficult to 

assign crashes to one direction of travel and determine the true effects of HFST 

installation. 

• Little or no after-period crash data—a number of planned sites that were tested for 

friction before HFST installation were delayed due to construction issues. Therefore, 

researchers lacked sufficient after-period crash data or post installation friction data.

• Removal of sites—a number of sites—particularly older ones—intended for inclusion no 

longer existed when the friction data-collection process began. Several of these sites were 

still tested for friction in case they could be useful reference sites.

CRASH DATA

Crash data were collected for each HFST and reference site for the periods before and after 

HFST installation. Ideally, a minimum of 3 yr of before-period and 3 yr of after-period crash 

data were compiled. While 3 yr of before-period data were generally not a problem, 3 yr of 

after-period data were only possible for older HFST installations. Many HFST sites included in 

this current study were installed in 2016 and 2017, limiting after-period data. The amount of 

after-period data was highly dependent on the currency of an agency’s crash database. 

Researchers decided to collect and use before–after analysis crash data for all sites with a 

minimum of a 1-year after period.

Method

The collection of crash data varied from State to State in terms of how the data were collected 

and the level of information contained in the crash data. Crash data were either provided directly 

by volunteer States or gathered from agency crash databases. A few States provided summary 

crash data for specific HFST sites using their internal processes for assigning crashes to the 

curve/ramp of interest.
31 

For crash data not provided directly by volunteer States, researchers used either publicly 

available or agency-provided crash databases. Although raw crash data with all possible 

variables were desired, researchers were limited to the various levels of data available for each 

State. The following were of most importance in the crash data: 

• Crash date. 

• Location type (e.g., intersection, nonintersection). 

• Crash type (e.g., ROR, sideswipe). 

• Crash severity (e.g., PDO, injury, fatal). 

• Vehicle type. 

• Pavement condition. 

• Reported driver action. 

• Direction of travel. 

• Reference location (e.g., milepost). 

The Summary of Data Collection by State section summarizes the data available for each State. 

Challenges

Researchers encountered a number of challenges when collecting crash data for HFST 

installations, including the following: 

• Accurately capturing curve-related crashes—because HFSTs on curves are typically short 

(as are the curves on which most HFSTs are installed) accurate milepost information 

and/or global positioning system (GPS) coordinates in crash records is critical. 

Researchers surmised that many crashes without accurate milepost information and/or 

GPS coordinates went uncaptured for this reason. When available, researchers relied on 

curve-related crash coding when milepost information and/or GPS coordinates were not 

available or when there was ambiguity about where on the curve a crash occurred.

• Accurately capturing ramp-related crashes—similar to the challenge for curves, it was 

difficult to ensure the accuracy of ramp-related crashes if GPS coordinates were missing 

or if one or both of the intersecting roadways was not recorded. When available, 

researchers relied on ramp-related crash coding when ramp identifiers and/or GPS 

coordinates were ambiguous or not available. 

FRICTION DATA COLLECTION

Underlying pavement and HFST friction were essential components of the data for analysis. 

Agency-provided friction data for HFST sites were limited—depending on the State—and were

collected using a variety of methods, making useful comparisons difficult. To help ensure 

consistency of the pavement and HFST friction data used in the analysis, researchers collected 

friction data on all HFST sites, including underlying and/or surrounding pavement. 
32 

Method

Friction data have been traditionally collected in the United States using an ASTM E274 

locked-wheel skid tester (LWST). An LWST is a trailer-based testing system that measures 

friction by completely locking up the test wheel and recording the average sliding force for 3 s 

and reporting a 1-second average after reaching the fully locked state (i.e., 100-percent slip). 

Thus, with a 40-mph test speed, a 1-second test time is equivalent to testing the pavement surface 

for approximately 59 ft. The fully locked requirement means that measurements can only be 

recorded periodically over short intervals. Reporting one test per mile results in approximately 

1.1 percent of the pavement surface being tested. Because testing with a trailer-based unit with a 

locking wheel in tight curves and intersections can be challenging or prohibitive, an LWST may 

not adequately characterize pavement friction in these locations.

Continuous friction-measurement systems report measurements as frequently as every 1 ft over 

the section of pavement. This allows for any variations in friction to be measured and quantified. 

Because of this advantage of continuous friction measurement and the nature of the location of 

most HFST installations, friction data for this current study were collected using FHWA’s

6875H HFT (figure 7). The HFT is a continuous fixed-slip measurement device that complies 

with ASTM E2340, Standard Test Method for Measuring the Skid Resistance of Pavements and 

Other Trafficked Surfaces Using a Continuous Reading, Fixed-Slip Technique, and provides a 

continuous measurement of friction at prevailing highway speed (ASTM 2006). Friction data are 

reported by the HFT every 1 ft over the length of the pavement surface. The HFT uses a 

smooth-tread test tire that complies with ASTM E1551, Standard Specification for a Size 4.00-8 

Smooth Tread Friction Test Tire, located in the left wheelpath of the lane with a slip ratio of 

14 percent (ASTM 2016). The HFT applies a 0.5-millimeter water film to the pavement surface 

in front of the test tire during testing. The HFT is also equipped with a texture laser to estimate 

pavement texture as MPD. 
33 

Source: FHWA. 

Figure 7. Photograph. FHWA’s HFT. 

Operating as a low slip ratio, the HFT measures friction numbers closer to the peak friction 

number for a pavement surface, as opposed to the fully locked (i.e., 100 percent slip) friction

LWST. HFT measurements are more representative of the friction available from a pavement 

surface for vehicles with antilock brakes. As such, the friction numbers reported by the HFT are 

generally higher than those measured by highway agencies using an LWST. Because friction 

measurement is highly dependent on the method used by the device (e.g., fixed slip versus fully 

locked), the tire used by the device, and the characteristics of the pavement surface itself 

(e.g., micro- or macrotexture), friction numbers from the HFT are not directly comparable to the 

LWST. Therefore, differences in friction between HFSTs and the underlying pavement as 

reported in this current study were evaluated based on HFT friction test results only. 

Testing Protocol

In general, one run was used to collect friction data in each lane at each site. However, if any 

anomalies were observed during friction data collection, repeat measurements were taken as 

needed and unreliable data were disregarded. Event markers were used to identify reference 

points in the data that could be retraced to field locations and anomalies within the test site, 

including bridge decks, pavement changes, pavement markings within the lane, and 

inconsistencies during testing (e.g., braking, deviation from wheelpaths). 

A test speed of 40 mph was selected as the target for all sites, regardless of higher posted speeds. 

However, a number of sites limited testing speed to anywhere from 25 to 35 mph due to curve 

radius or prevailing traffic conditions. During trial runs, researchers noted that speeds less than 

25 mph resulted in unreliable friction data, so 25 mph was established as the minimum test 

speed.
34 

Of particular importance for this current study was collecting friction data before and after HFST 

installation. This was possible for most planned sites, but for in-place sites, friction data were 

collected on the underlying pavement leading up to (i.e., lead-in) and away from (i.e., lead-out) 

the HFST as a surrogate measure of pavement friction without an HFST. However, 

lead-in/lead-out pavement friction is not the same as the actual pavement prior to HFST 

installation due to the effects of aging on pavement friction and potential variation in friction 

through a curve. The ability to collect lead-in and lead-out friction data on surrounding pavement 

varied widely from site to site depending on the location of the site in relation to other curves, 

intersections, and so on. Researchers attempted to collect approximately 300–500 ft of lead-in 

and lead-out friction data whenever possible. For a number of sites, only 100–200 ft of data were

collected.

Data Reporting 

The HFT was selected for friction data collection due to its ability to measure friction 

continuously and report results at high resolution (i.e., every 1 ft), thereby capturing any changes 

in friction within a given curve or ramp. However, the quality of the pre-HFST friction data 

collected was inadequate. The issues related to friction data collection are discussed in the 

following section and in the companion report FHWA-HRT-20-062. 

Friction data were manually processed for each site, and the average friction number over the 

HFST and the underlying or abutting (i.e., lead-in/lead-out) pavement was reported. For sites 

with two or more lanes, the average friction number for all lanes was reported. Any significant 

differences in friction between lanes was noted in case further evaluation was needed to 

determine the cause. Any significant difference in lead-in and lead-out friction was also noted. 

For the purposes of reporting herein, HFT Mu is used to distinguish HFT friction measurements 

from other friction measurements, and where appropriate, HFT(S) is used to designate HFT 

friction at speed in mph. 

Challenges

Researchers encountered many challenges with friction data collection that effectively limited 

the number of sites tested and included in the current study. Some of these challenges included 

the following: 

• Inability to collect friction data on stop-controlled and most signal-controlled 

intersections—the HFT must be operating at a relatively constant speed, as braking and 

acceleration affect the friction data. As such, researchers were unable to test HFST sites 

at stop-controlled intersections. Likewise, signal-controlled intersections were difficult to 

test, as testing needed to coincide with a green signal. Although researchers able to test 

several signal-controlled intersections, the total number was small.

• Inability to collect friction data on tight-radius ramps and curves—because the HFT must 

operate at a minimum of 25 to 30 mph, many tight-radius ramps and curves with advisory 

speeds less than 25 mph were excluded from testing due to safety concerns.
35 

• Inability to collect friction data on ramps terminating at intersections—many of the ramps 

in the current study terminated at intersections, requiring the HFT to stop collecting data 

well in advance of the end of the ramp. While it was still possible to test many ramps 

with this configuration, several had to be eliminated. 

• Lag in friction data at the beginning of HFST sections—due to the nature of collecting 

friction data via the HFT, friction numbers build over the first 100–200 ft. This required 

some post processing of the friction data to ensure the true friction numbers were 

reported and not skewed by this lag area. In some cases (i.e., short HFST sections), the 

data were unusable. 

• Unforeseen mechanical failures—several mechanical breakdowns of the HFT led to 

delays in collecting data and ultimately reduced the scope of friction data collection. 

ROADWAY AND PAVEMENT DATA

Similar to crash data, roadway and pavement data collection varied greatly from State to State in 

terms of the level of information available and how the data were collected. Roadway and 

pavement data were either provided directly by volunteer States, either as summary data or from 

a roadway information database, or gathered from publicly available databases. The following 

sections summarize the data sought by the researchers. Data provided by individual States are 

summarized in the Summary of Data Collection by State section. 

Roadway Data 

Roadway data collected for this current study included the following: 

• Roadway classification (e.g., urban, rural, interstate, noninterstate). 

• Traffic (i.e., annual average daily traffic (AADT)). 

• Number of lanes. 

• Lane width. 

• Median type (e.g., divided, undivided). 

• Geometrics (e.g., curvature, cross-slope/superelevation, grade). 

• Shoulder type and width. 

• Roadside features (e.g., driveways, guardrails). 

• Safety features (e.g., striping, special signage, rumble strips/stripes). 

• Intersection control type (e.g., stop-controlled, signalized). 

Pavement Data

Pavement data collected for this current study included the following: 

• Original/underlying pavement:

o Asphalt pavement type (e.g., dense graded, open graded, surface treatment). 

o Concrete pavement type and texture characteristics (e.g., tined, turf drag, grooved, 

diamond ground). 

o Pavement age. 
36 

o Pavement condition. 

o Pavement materials (e.g., aggregate materials, asphalt/concrete mixture). 

• HFST information: 

o Installation date. 

o HFST vendor/product. 

o Installation process used (e.g., manual or automated application). 

o Single/double layer application. 

o Limits/length of installation. 

o Material properties (e.g., binder and aggregate type/properties). 

o Any documented issues during installation. 

To help supplement agency-provided roadway data, researchers collected other field data during 

friction data collection, including the following: 

• Pavement cross-slope using an inclinometer mounted to the HFT—while these data were 

collected from the majority of sites, analysis of the data did not provide reliable results 

for accurately assessing the cross-slope that could be used in the final crash-data analysis.

• GPS data to assist in estimating curve radius and locating sites—an algorithm for 

estimating curve radius from GPS data was developed and used when agency-provided 

curve data were not available.

• Dashcam video to assist in assessing site features and pavement type and 

pavement/HFST condition—this included documentation of shoulder and median 

characteristics, advisory speed, and other safety features (e.g., chevrons, arrow boards, 

guardrails). These data were not valuable for the final crash-data analysis but served as a 

backup to verify certain site characteristics if needed.

Challenges

Researchers encountered numerous challenges with roadway data collection that limited its 

usefulness in the final crash-data analysis. Some of these challenges included the following: 

• Limited traffic data history—although researchers gathered current traffic data 

(i.e., AADT) for most sites, historical data were largely unavailable. 

• Limited traffic data for ramps—for a number of ramps, traffic data were unavailable, as 

agencies do not commonly collect ramp traffic data.

• Underlying pavement type and history—while researchers assessed the basic pavement 

type (e.g., bituminous concrete or PCC), they were unable to discern specific types of 

each (e.g., open-graded, dense graded, or SMA for bituminous surfaces). Agency 

pavement history databases were generally unhelpful in confirming information other 

than basic pavement type. 
37 

• Curve radius discrepancies—although several States provided curve data (e.g., degree of 

curvature or radius of curvature) from geographic information system (GIS)-based 

roadway inventories, a given curve may consist of multiple arc segments with different 

radii, making it difficult to assign a single radius to each curve. In general, the tightest 

curve radius was assigned to the site after checking that the segment length for the arc 

was reasonable. Curve radii for ramps were estimated from GPS data collected during 

friction data collection, as this information is not typically found in agency curve 

inventories.

• Outdated roadway inventory databases—particularly for rural roadways and ramps, 

roadway inventory databases are usually not updated annually, and information found in 

such databases may be out-of-date.

• Unverifiable HFST installation date—few States maintain construction-related details, 

such as actual installation dates, in centralized databases. In many cases, researchers

contacted agency district or county personnel to determine actual installation timeframes. 

In some cases, only the construction acceptance data, which were from many months or 

more than 1 yr after actual installation, were available. 

REFERENCE SITES

Identifying reference sites for this current study proved to be one of the most challenging aspects 

of the data-collection effort. Most States do not have a readymade database of curves and ramps. 

Additionally, the large variety of curve and ramp characteristics made identifying reference sites 

difficult. The methods used to identify candidate reference sites are summarized in the following 

section. 

Upstream/Downstream

One of the methods used to identify reference sites was to look for similar sites upstream and 

downstream or in the same vicinity (e.g., city, county, district) as HFST sites, ideally on the same 

roadway. This method worked best with ramps, as agencies generally design interchanges to 

consistent standards. However, a number of rural roadway curve reference sites directly 

upstream or downstream from one or more HFST sites were identified with this method.

Challenges associated with this method included the following: 

• Finding the RampID or roadway mileposts for the reference ramp or curve of interest so

crash data could be collected.

• Gathering additional roadway information (e.g., traffic, curvature) already compiled by 

the agency for the HFST site but not for reference sites (i.e., additional roadway data 

gathering).

Curve Inventory

Another method for identifying reference sites was using agency curve inventories or GIS-based 

curve data to identify curves of similar geometry to the treated curves. However, this method 
38 

only helps identify curves with similar geometry and still requires comparison of other 

characteristics (e.g., traffic) to determine the suitability of a reference site. Challenges associated 

with this method include the following: 

• Ramp curvature data are not typically found in curve databases. 

• GIS-based curve data are generally segmented by smaller arcs of varying length within a 

given curve, and identifying which arc best represents the curve of interest within the 

treated sites, then matching to arcs of potential reference sites, is a tedious process. 

Similar Systemic Criteria

For States with systemic HFST programs (Georgia and Louisiana for the current study), 

reference sites can be identified from untreated roadways with characteristics similar to those 

identified for treatment by the systemic process. Under systemic treatment programs, curves 

without any crash history may be treated. Challenges associated with this method include the 

following: 

• Systemic programs inherently treat locations/curves without any crash history, and 

selecting sites from nontreated roadways further increases the number of null-crash sites.

• Roadways with a systemic treatment likely have different crash rates than treated 

roadways despite having the same characteristics.

• After identifying a particular roadway, individual curves and mileposts must be identified 

so crash data can be obtained, which can be a tedious process; curve 

databases/inventories can help with this. 

SUMMARY OF DATA COLLECTION BY STATE

Arkansas 

Arkansas has a large but relatively new HFST program. The Arkansas Department of 

Transportation (ARDOT) provided an initial list1 of approximately 50 in-place and planned 

HFST sites for curves, ramps, and intersections, with all sites installed between July 2015 and 

August 2016. From the initial list, approximately 30 ramps and intersections were selected for 

data collection. While the initial data-collection plan involved testing all the planned ramps 

before HFST installation, delays in friction data collection led to testing after HFST installation. 

In addition to ramps and intersections, friction data were also collected from several curves. 

 

1

These are not published data. ARDOT provided this informal list to researchers for analysis in 2016. ARDOT 

explicitly stated that this list with specific locations cannot be published.
39 

Crash Data 

Statewide crash databases2 were provided by ARDOT for 2011–2017 for researchers to gather

data for treated and reference sites.

Reference Sites

Reference sites were identified in the vicinity of HFST sites. For curves, similar curves 

upstream, downstream, and along adjacent roadways were identified. For ramps, similar ramps in 

the vicinity (i.e., the same or adjacent county) were identified. After sites were chosen, GIS data 

were used to identify the RoadID and mileposts for collecting crash and roadway data.

Roadway Data 

ARDOT provided a summary of roadway data based on the list of treated and reference sites 

from researchers. Roadway data included the following: 

• Classification (e.g., urban, rural). 

• Pavement type. 

• Safety features. 

• Roadside features. 

• Traffic (i.e., AADT for 2011–2016). 

• Speed limit (e.g., posted and advisory). 

• Median and shoulder info. 

• Number and width of lanes. 

Friction Data

Researchers planned to test most ramps for friction before and after HFST installation, but 

construction delays prohibited collecting friction data before HFST installation; therefore, all 

friction data were collected after HFST installation. Lead-in and lead-out pavement were used to 

estimate friction of the underlying pavement. Table 14 summarizes the friction data for Arkansas 

HFST sites.

Table 14. Summary of friction data for Arkansas sites.

Type Number

Speed (mph)

HFST Friction 

(HFT Mu) 

Underlying 

Pavement Friction 

(HFT Mu) 

Change in Friction

(Percent)

Range Average Range Average Range Average Range Average

Ramp 17 27.30–

43.10 

39.30 0.80–1.00 0.95 0.33–

0.71

0.49 +35–203 +104

Curve 5 39.30–

40.70 

40.00 0.89–0.95 0.91 0.41–

0.61

0.52 +48–122 +81

Intersection 6 38.10–

40.80 

39.70 0.79–0.91 0.85 0.35–

0.69

0.43 +25–160 +108

 2

These are not published data. ARDOT shared the data from this internal database with researchers for analysis 

in 2016.
40 

Georgia

Georgia has one of the largest HFST programs in the United States in terms of the total number 

of sites and treated area. GDOT’s program is relatively new, with installations beginning in 

2014. Sites provided by GDOT for this study resulted from a systemic HFST program called 

Sharp Curve Treatment Process focusing on rural roadway curves. Ball-bank indicator readings 

of 12 or more at curves when driven at the posted speed limit were selected for HFST

installations. 

GDOT provided an initial list3

 of approximately 40 in-place and 285 planned sites across 

55 counties primarily on rural roadway curves. Based on the site-selection criteria discussed 

previously, sites were selected to maximize the efficiency of friction data collection while also 

maximizing the number of planned sites for friction testing before and after HFST installation. 

Researchers established an initial list of 26 in-place and 98 planned sites. The final site list for 

this current study included 43 in-place sites and 93 planned sites. Of the 93 planned sites, 

85 were tested before and after HFST installation; the remainder were only tested before HFST 

installation.

Crash Data 

GDOT provided summary crash data4

 for before and after periods for individual sites. Crash data 

ranged from 2009 to 2018 depending on the HFST installation date.

Reference Sites

GDOT did not provide any reference sites. 

Roadway Data 

GDOT provided a summary of roadway data based on the list of treated and reference sites from 

researchers. Roadway data included the following: 

• Classification (e.g., urban, rural). 

• Traffic (i.e., current and previous AADT, truck percentage). 

• Speed limit (e.g., posted and advisory). 

• Median and shoulder type and width. 

• Maintenance type and year. 

Friction Data

The Georgia sites provide a large population of planned sites where before and after HFST 

installation friction data were collected. A number of in-place sites also provided additional 

friction data using lead-in and lead-out pavement friction to estimate underlying pavement 

 

3

These are not published data. GDOT provided this internal list to researchers for analysis in 2016. 4

These are not published data. GDOT shared the data from this internal database with researchers for analysis in 

2019.
41 

friction. Table 15 summarizes the friction data for Georgia HFST sites, including a summary by 

in-place and planned (i.e., before/after HFST installation friction testing) sites.

Table 15. Summary of friction data for Georgia sites.

Type Number

Speed (mph)

HFST Friction 

(HFT Mu) 

Underlying 

Pavement Friction 

(HFT Mu) 

Change in Friction

(Percent)

Range Average Range Average Range Average Range Average

In place 43 35.50–42.00 39.90 0.84–1.00 0.98 0.48–0.76 0.61 +32–105 +64

Planned 93 37.30–42.50 40.00 0.74–1.00 0.96 0.58–0.87 0.73 +1–65 +33

Kentucky

Kentucky has one of the oldest and most extensive HFST programs with sites dating back to 

2009. KYTC provided an initial list5 of approximately 136 curves and ramps, all in-place sites. 

From this list, approximately 54 curves, 23 ramps, and 1 intersection were included in the initial 

data-collection plan. Actual data collection captured 58 curves, 14 of which the HFST was

removed and 2 not on the original list; and 24 ramps, 3 of which the HFST was removed. 

Crash Data 

Researchers gathered crash data for before and after periods from a publicly available crash 

database maintained by the Kentucky State Police (2018). Crash data as far back as 2006 were 

queried to capture before periods for sites installed in 2009. Data through 2018 were available 

for most queries. Due to limitations on the size of the crash dataset, crash data for curves were 

queried by county, route, and milepost, capturing both treated and reference sites on a particular 

route. Crash data for ramps were queried by county and intersecting interchange routes to 

capture both treated and reference sites. Only crashes with a “Ramp Indicator” field were 

included. 

Reference Sites

Reference sites were identified in the vicinity of HFST sites. For curves, similar curves 

upstream, downstream, and along adjacent roadways were identified. For ramps, similar ramps in 

the vicinity (i.e., the same or adjacent county) were identified. Rather than assigning each 

reference site to a specific treated site, three broad categories of reference sites were identified: 

rural roadway curves, clover-leaf-type ramps, and large-radius connector ramps. 

After reference sites were identified, KYTC’s online GIS database was used to identify the 

RouteID and mileposts for querying crash and roadway data (KYTC n.d.a.; KYTC n.d.b). 

Roadway Data 

KYTC’s online GIS database was used to query various roadway data for treated and reference 

sites, including curve radius and traffic (i.e., current and previous AADT). Curve radii for ramps 

were unavailable and captured using GPS data during friction testing. Other roadway data were 

 5

These are not published data. KYTC provided this informal list to researchers for analysis in 2018.
42 

manually documented for treated sites during friction data collection and subsequent review of 

video logs, including the following: 

• Speed limit (e.g., posted and advisory). 

• Median and shoulder type and approximate width. 

• Other safety features (e.g., chevrons, arrow boards, guardrails). 

Friction Data

The Kentucky sites provide a large population of in-place HFST sites for friction testing, ranging 

in age from 18 mo to 6.5 yr. Lead-in and lead-out pavement were used to estimate friction of the 

underlying pavement. Table 16 summarizes the friction data for Kentucky HFST sites where 

friction data were captured.

Table 16. Summary of friction data for Kentucky sites.

Type Number

Speed (mph)

HFST Friction 

(HFT Mu) 

Underlying 

Pavement Friction 

(HFT Mu) 

Change in Friction

(Percent)

Range Average Range Average Range Average Range Average

Curves 42 30.20–43.70 38.40 0.80–1.00 0.95 0.38–0.74 0.55 +27–155 +76

Ramps 20 33.40–42.10 38.90 0.91–1.00 0.98 0.35–0.69 0.50 +45–186 +102

Louisiana

Louisiana has a large and relatively new HFST program with most HFST installations completed 

after 2016. Sites provided by the Louisiana Department of Transportation and Development 

(LADOTD) were from a systemic treatment approach for rural roadway curves. The 

characteristics of rural roadway curves deemed suitable candidates for HFSTs and other 

curve-related countermeasures included the following: 

• Two-lane State-maintained rural roadways. 

• AADT of 2,500–7,500. 

• Lane width 12 ft or greater. 

• Shoulder width of 2–6 ft.

• Degree of curve greater than 3.5 degrees (i.e., radius <1,640 ft). 

LADOTD provided an initial list6

 of 226 sites across 34 parishes with HFST installations 

planned for 2016. Based on the site-selection criteria discussed previously, sites were selected to 

maximize the efficiency of friction data collection while also maximizing the number of planned 

sites for friction testing before and after HFST installation. Researchers established an initial list 

of 18 in-place and 77 planned sites for testing. While nine of the original planned sites were 

ultimately not installed, researchers added several additional sites to their list. The final site list 

included 27 in-place and 90 planned sites. Due to construction delays, researchers were unable to 

test any of the planned sites after HFST installation.

 

6

These are not published data. LADOTD provided this informal list to researchers for analysis in 2016.
43 

Crash Data 

LADOTD provided crash data for 2010–20177, capturing the before period for all sites. 

However, because only approximately eight sites were installed in 2016, only those eight sites 

had after-period crash data. Crash data for potential reference sites were also provided. 

Reference Sites

LADOTD provided a curve inventory for State-maintained roadways with characteristics similar 

to HFST sites. Researchers identified reference site candidates from the inventory based on curve 

radii and lengths similar to HFST sites.

Roadway Data 

In 2017, LADOTD provided roadway data8 for the list of treated and reference sites identified by 

researchers. Additionally, LADOT provided a list of any other curve safety countermeasures 

implemented at various HFST sites. Roadway data included the following: 

• Classification (e.g., urban, rural). 

• Traffic (i.e., current AADT). 

• Pavement type and width. 

• Number of lanes. 

• Shoulder type and width. 

• Median type. 

Friction Data

Louisiana had a large population of planned sites where friction data could be collected before 

and after HFST installation, but construction delays prohibited collecting friction data after 

HFST installation. However, a number of in-place sites provided HFST friction data as well as 

an estimate of underlying pavement friction data using lead-in and lead-out pavement friction. 

Table 17 summarizes the friction data for Louisiana HFST sites, including a summary by 

in-place and planned (i.e., before/after HFST installation friction testing) sites.

Table 17. Summary of friction data for Louisiana sites.

Type Number

Speed (mph)

HFST Friction 

(HFT Mu) 

Underlying 

Pavement Friction 

(HFT Mu) 

Change in Friction

(Percent)

Range Average Range Average Range Average Range Average

In place 27 38.50–41.50 40.00 0.89–1.00 0.97 0.41–0.57 0.49 +74–138 +100

Planned 89 39.30–42.20 40.40 N/A N/A 0.37-0.9 0.60 N/A N/A

N/A = not applicable.

 7

These are not published data. LADOTD provided these data to researchers for analysis in 2017. 8

These are not published data. LADOTD provided these data to researchers for analysis in 2017.
44 

Pennsylvania 

Pennsylvania has one of the largest and oldest HFST programs in the United States, dating back 

to a pilot project in 2007, with implementation beginning in earnest in 2012. As of August 2018, 

the Pennsylvania Department of Transportation (PennDOT) reported approximately 42 mi of 

HFSTs over 248 locations. 

PennDOT provided an initial list9

 of 150 in-place and 91 planned sites. From this list, 85 in-place 

and 22 planned curves were included in the initial data-collection plan. Additional sites were 

added as data collection began because several sites with multiple curves were split into separate 

sites. This resulted in data collection on approximately 200 sites, including 131 in-place and 

37 planned sites (only 25 of which were tested after HFST installation). While these were all 

believed to be curves, 14 of the sites were classified as ramps and 21 were classified as 

intersections (i.e., intersection within a curve) during data collection. The remaining 32 sites 

consisted of sites not on the original list, sites where the HFST was removed, or sites where only 

one lane/direction was treated. The final list of curves for analysis consisted of 103 in-place and 

17 planned sites. 

Crash Data 

Crash data were queried from statewide crash databases from 1998–201710, covering before and 

after periods for all of the HFST sites except those installed in 2017. 

Reference Sites

Reference sites were identified from PennDOT’s statewide curve inventory11. Sites located on 

the same roadway in the same county as a treated site were selected. Reference sites were 

selected to ensure their lengths, radii and traffic volumes were within the same range as treated 

sites.

Roadway Data 

Curve radii for treated and reference sites were obtained from PennDOT’s curve inventory. 

Traffic data were obtained from PennDOT online databases. Other roadway data were 

documented for treated sites during friction data collection and subsequent review of video logs, 

including the following: 

• Speed limit (e.g., posted and advisory). 

• Median and shoulder type and approximate width. 

• Other safety features (e.g., chevrons, arrow boards, guardrails). 

 

9

These are not published data. PennDOT provided this informal list to researchers for analysis in 2016.

10These are not published data. PennDOT provided these data to researchers for analysis in 2018.

11These are not published data. PennDOT provided these data to researchers for analysis in 2019.
45 

Friction Data

Pennsylvania provided a large population of in-place HFST sites for friction testing, ranging in 

age from 3 mo to 4.5 yr, with one site nearly 10 yr old. For these sites, lead-in and lead-out 

pavement was used to estimate friction of the underlying pavement. A number of planned sites 

provided friction data before and after HFST installation. Table 18 summarizes the friction data 

for Pennsylvania HFST sites where friction data were captured.

Table 18. Summary of friction data for Pennsylvania sites.

Type Number

Speed (mph)

HFST Friction 

(HFT Mu) 

Underlying 

Pavement Friction 

(HFT Mu) 

Change in Friction

(Percent)

Range Average Range Average Range Average Range Average

In place 98 28.70–42.20 37.40 0.74–1.00 0.93 0.37–0.86 0.54 +16–168 +76

Planned 14 28.40–41.60 34.60 0.75–1.00 0.90 0.42–0.73 0.56 +17–131 +70

West Virginia

West Virginia has an active HFST program with an extensive number of sites throughout the 

State, the oldest dating back to 2011. The West Virginia Department of Transportation 

(WVDOT) provided an initial list12 of 49 locations, including 35 in-place and 14 planned sites 

set for installation in 2016. From this list, 25 in-place and 14 planned sites were included in the 

initial data-collection plan. Actual data collection captured 41 sites (2 sites were added), with 

6 classified as ramps, intersections, or bridge decks and not included in the final analysis to 

maintain a focus on curves. Delays prohibited friction data collection at the 14 planned sites. 

Since they were installed prior to friction data collection, all 35 sites were considered in place 

when tested.

Crash Data 

WVDOT provided summary crash data13 for before and after periods for each HFST site. Crash 

data ranged from 2009 to 2017 depending on the HFST installation date. Summary crash data 

provided by WVDOT included the following: 

• Date and time of crash. 

• Collision type (e.g., rear end, sideswipe, single vehicle, angle). 

• Injury and fatal (KABC). 

• Light condition. 

• Road surface condition. 

• Animal involvement. 

• Intersection indicator. 

• Vehicle type. 

 

12This is not published data. WVDOT provided this informal list to researchers for analysis in 2016.

13This is not published data. WVDOT provided these data to researchers for analysis in 2017.
46 

Reference Sites

WVDOT did not provide any reference site candidates. 

Roadway Data 

WVDOT provided summary roadway data for each site, including the following: 

• Traffic (i.e., current and previous AADT). 

• Number and width of lanes. 

• Median type. 

• Shoulder width. 

• Curvature degree. 

• Posted speed limit. 

Other roadway data were documented for treated sites during friction data collection and 

subsequent review of video logs, including the following: 

• Advisory speed limit. 

• Other safety features (e.g., chevrons, arrow boards, guardrails). 

Friction Data

West Virginia provided a large population of in-place HFST sites for friction testing, ranging in 

age from 5 mo to 6 yr. Lead-in and lead-out pavement were used to estimate friction of the 

underlying pavement. Table 19 summarizes the friction data for West Virginia HFST sites where 

friction data were captured.

Table 19. Summary of friction data for West Virginia sites.

Type Number

Speed (mph)

HFST Friction 

(HFT Mu) 

Underlying 

Pavement Friction 

(HFT Mu) 

Change in Friction

(Percent)

Range Average Range Average Range Average Range Average

Curves 29 28.00–41.60 35.20 0.67–1.00 0.87 0.45–0.81 0.60 +7–87 +47

SUMMARY OF DATA USED FOR THE BEFORE–AFTER SAFETY EVALUATION

Researchers used data from Arkansas (ramps), Kentucky (curves and ramps), Pennsylvania 

(curves), and West Virginia (curves). Researchers were unable to use data collected for other site 

types and States for a rigorous evaluation for a variety of reasons, including the following: 

• The recorded installation dates (i.e., construction acceptance) in Louisiana did not 

provide an after period for crash analysis. 

• Reference sites were unavailable in Georgia, and sites with confirmed installation dates 

with available after-period crash data were few.
47 

Summaries of the treatment and reference data for the site types and States used for the safety 

evaluation are in table 20 through table 23. Note that differences in the number of sites and 

friction change reported in table 20 through table 23 may be different from those reported in 

table 14 through table 19, as friction data were collected on more sites than were used in the 

crash-data analysis.
48 

Table 20. Summary of curve treatment data used for safety evaluation. 

State Sites 

Curve radius 

(ft)

Friction 

Increase 

(Percent)

Average 

AADT Crashes/Mile-Year

Range Average Range Average Before After

Total Injury Wet ROR HOSSD

Before After Before After Before After Before After Before After

West 

Virginia

26 98–

1,910

717 +7–87 +43 3,954 3,952 4.78 2.02 2.47 1.40 2.00 0.47 2.93 1.71 0.00 0.00

Pennsylvania 95 119–

2,536

586 +14–

168

+74 8,089 7,434 7.20 3.76 2.76 1.32 3.70 0.95 1.76 0.58 0.58 0.37

Kentucky 36 148–

955

500 +27–

155

+74 4,087 3,657 12.39 2.97 3.28 0.82 8.28 0.73 8.25 1.02 1.32 0.77

Table 21. Summary of ramp treatment data used for safety evaluation. 

State Sites

Average AADT Crashes/Ramp/Year

Before After

Total Injury Wet ROR

Before After Before After Before After Before After

Arkansas 15 8,491 9,293 2.64 6.47 0.57 1.27 1.33 0.40 0.11 0.20

Kentucky 21 11,587 11,561 7.43 1.59 1.47 0.40 5.47 0.43 0.91 0.05

Table 22. Summary of curve reference site data.

State Sites

Curve radius (ft) AADT Crashes/Mile-Year

Range Average Range Average Total Injury Wet ROR HOSSD

Pennsylvania 1,011 121–

2,534

1,151 342–

43,950

6,797 2.16 0.80 0.48 0.35 0.21

Kentucky 128 109–

1,508

605 474–

17,903

3,181 4.38 1.24 2.09 2.12 0.51

Note: Kentucky reference sites were used for West Virginia evaluation. 

Table 23. Summary of ramp reference site data. 

State Sites Average AADT

Crashes/Ramp/Year

Total Injury Wet ROR

Arkansas 67 6,199 0.80 0.22 0.19 0.08

Kentucky 71 8,183 0.87 0.16 0.39 0.03
49 

CHAPTER 4. ANALYSIS

ANALYSIS OBJECTIVES

The objective of the crash-data analysis was to estimate the change in target crashes after HFST

installation. Only nonintersection, nonanimal-related crashes and crashes not involving snow or 

ice were considered. Crash types examined include the following: 

• Total. 

• Injury. 

• ROR. 

• Wet-road. 

• HOSSOD. 

Further questions of interest that were examined include the following: 

• Do effects vary by levels of traffic volumes?

• Do effects vary by the site-specific expected crash frequency prior to treatment?

• Do effects vary by State and the improvement in friction?

• Do effects vary with the value of geometric variables, such as curvature?

Meeting these objectives placed some special requirements on the data collection and analysis, 

including the need to accomplish the following: 

• Select a large enough sample size to detect, with statistical significance, small changes in 

safety for some crash types.

• Carefully select untreated reference sites to properly account for changes in safety not 

due to HFSTs, including RTM, traffic volume changes, and time trends. 

• Properly account for traffic volume changes. 

• Pool data from multiple jurisdictions where possible to improve the reliability of the 

results and facilitate broader applicability of the research products.

As discussed in the Data Collection chapter, roadway, traffic volume, and crash data were 

acquired for curves in Pennsylvania, West Virginia, and Kentucky and for ramps in Arkansas 

and Kentucky to facilitate the crash-data analysis. The States also provided information related to 

the installation of HFSTs (i.e., location and date). 

ANALYSIS METHODOLOGY

The general analysis methodology applied is the EB before–after approach. The methodology is 

well documented by Hauer (1997). The advantages of the EB method include the following:

• Accounting for RTM. 

• Overcoming the difficulties of using crash rates in normalizing for volume differences 

between before and after periods.

• Reducing the level of uncertainty in the estimates of safety effects. 
50 

• Providing a foundation for developing guidelines for estimating the likely safety 

consequences of contemplated installations. 

• Accounting for differences in crash experience and reporting practice in amalgamating 

data and results from diverse jurisdictions. 

The EB before-after approach consists of the following three steps: 

1. Predict what safety would have been in the after period had the status quo been 

maintained.

2. Estimate what the actual safety was in the after period.

3. Compare the two. 

The EB method requires the calibration of SPFs, as outlined in the next section, relating crashes 

of different types and severities to traffic flow and other relevant factors for each jurisdiction for 

sites without HFSTs with appropriate adjustments for temporal effects. This calibration enables 

simultaneous accounting for temporal and possible RTM effects, as well as those related to 

changes in traffic volume. 

DEVELOPMENT OF SPFs 

Fundamental to the EB method is the use of the SPFs to represent the roadway conditions before 

installation. Where sufficient data are available for a reference population of sites similar to 

HFST sites, SPFs should be calibrated directly for the jurisdiction and analysis period of interest. 

Crash, traffic, and geometric data are required for SPF development for a sample of reference 

sites similar to those for which the SPF would be applied. The data are required for each year of 

the analysis period (i.e., the period of the before and after data at HFST sites). Friction data 

should also be included for SPF development since pavement friction influences the expected 

number of crashes. However, a friction variable could not be included in the SPFs for this current 

study because friction data were not collected at reference sites due to budget limitations. The 

EB methodology requires annual SPF estimates of expected crash frequencies at HFST sites. 

Therefore, annual friction data are required at reference and HFST sites for proper inclusion of a 

friction variable in developing and applying SPFs. Additionally, because FN40Rs are 

device-specific, any SPFs incorporating friction are specific to the device used for data

collection.

SPFs were directly calibrated with generalized linear modeling (GLM) using the R software 

package, which allows the specification of a negative binomial error structure that is recognized 

as more appropriate for crash counts than the normal distribution assumed in conventional 

regression modeling (The R Foundation n.d.). The GLM procedure also estimated the 

overdispersion parameter of the negative binomial distribution used in the EB estimation (k). 

Crash counts at locations in the reference group were used as estimates of the dependent 

variable, which was the expected number of crashes per year by type and severity, while 

corresponding road characteristics and traffic data were used as estimates of the independent 

variables.
51 

SPECIFICS OF THE EB BEFORE–AFTER EVALUATION

Overall Safety Effects

In the EB before–after evaluation of the effect of an HFST, the calculation of change in safety 

for a given crash type at a treated site is given in figure 8: 

Figure 8. Equation. Change in EB before–after evaluation. 

Where: 

B = expected number of crashes in the after period without an HFST installed. 

A = number of reported crashes in the after period with an HFST installed. 

Because of changes in safety resulting from changes in traffic volume, from RTM, and from 

trends in crash reporting and other factors, the count of crashes before a treatment by itself is not 

a good estimate of B—a reality now commonly accepted (Hauer 1997). Instead, B is estimated 

from an EB method in which an SPF is used to first estimate the number of crashes expected 

each year of the before period at locations with traffic volumes and other characteristics similar 

to a treatment site being analyzed. The sum of these annual SPF estimates (P) is then combined 

with the count of crashes in the before period at the treatment site (x) to obtain an estimate of the 

expected number of crashes before treatment (m). This estimate of m is shown in figure 9:

Figure 9. Equation. Calculation of m.

Where w is weight. The value of w is estimated as shown in figure 10:

Figure 10. Equation. Calculation of w.

The value of k is estimated from the SPF calibration process with the use of a maximum 

likelihood procedure. 

A factor is then applied to m from figure 9 to account for the length of the after period, 

differences in traffic volumes between the before and after periods, and other unknown 

differences between these two periods accounted for by using the yearly factor of the SPF. This 

factor is the sum of annual SPF predictions for the after period divided by P. The result, after 

applying this factor, is an estimate of B. The procedure also produces an estimate of the variance 

of B. 

The estimate of B is then summed over all sites in a treatment group of interest (Bsum) and 

compared with the count of crashes during the after period in that group (Asum). The variance of 

B is also summed over all sites in the group of interest. 
52 

The index of safety effectiveness (θ) is estimated as shown in figure 11:

Figure 11. Equation. Calculation of θ. 

The standard deviation of θ is given in figure 12:

Figure 12. Equation. Standard deviation of θ. 

The percent change in crashes is 100(1−θ); thus, a value of θ = 0.70 with a standard deviation of 

0.12 indicates a 30-percent reduction in crashes with a standard deviation of 12 percent. 

Effects on Different Severity and Impact Types

The EB methodology for crashes of different severity and impact types is essentially the same as 

previously outlined. The difference is that crashes of interest are used along with SPFs specific to 

these crash types.

Effects of Design, Traffic, Operational, and Safety Characteristics

Where samples were large enough, researchers isolated sites with certain levels or ranges of a 

given variable and estimated the separate effects for each category. This current study explored 

the effects of several variables on the value of the CMF, including the following: 

• Level of safety before installation measured as the expected crash frequency. 

• Traffic volume levels. 

• Friction levels before and after HFST installation. 

• Curvature (for curve sites). 

This univariate categorical analysis informed the exploration of continuous CMFunctions to 

mathematically relate CMFs to one or more of these variables.

SPFs 

This section presents the SPFs developed and used in the EB methodology to estimate the 

expected number of crashes in the after period without an HFST installed. 

GLM was used to estimate model coefficients assuming a negative binomial error distribution, 

which is consistent with the State of research in developing these models. Alternative models 

were evaluated by comparing the magnitude and statistical significance of the variables included 

as well as the value of k, which is in itself a reliable goodness-of-fit measure. A smaller k value 

indicates a model that better captures the overdispersion in the data. 

Where possible, separate SPFs were developed for each State and for different site and crash 

types.
53 

Pennsylvania Curves 

The model form for Pennsylvania curve SPFs shown in table 24 is shown in figure 13. 

Figure 13. Equation. Model form for Pennsylvania curve SPFs. 

Where: 

α = constant parameter (estimated in the modeling process). 

β1 = parameter for AADT (estimated in the modeling process). 

β2 = parameter for radius (estimated in the modeling process). 

rad = curve radius in feet.

ln = natural log. 

Table 24. SPF parameter estimates for Pennsylvania curves. 

Crash Type 

ln(α) 

(Standard 

Error)

β1 

(Standard 

Error) 

β2 

(Standard 

Error) 

k

(Standard 

Error) 

Total −3.6534

(0.4157)

0.5979

(0.0480)

−0.6951

(0.0626)

0.4447

(0.0501)

Injury −4.1633

(0.5917)

0.5362

(0.0680)

−0.6403

(0.0891)

0.4947

(0.1005)

ROR −3.6113

(1.0163)

0.4241

(0.1201)

−0.9934

(0.1618)

2.4365

(0.4157)

Wet-road −3.9216

(0.8582)

0.5181

(0.1009)

−1.2098

(0.1358)

1.5632

(0.2470)

HOSSOD −3.9891

(1.1465)

0.4409

(0.1324)

−1.3525

(0.1884)

1.2549

(0.3796)

Kentucky Curves

The model form for Kentucky curve SPFs shown in table 25 is shown in figure 14.

Figure 14. Equation. Model form for Kentucky curve SPFs. 

Where Length is curve length (miles). 
54 

Table 25. SPF parameter estimates for Kentucky curves. 

Crash Type

ln(α) 

(Standard 

Error) 

β1 

(Standard 

Error) 

β2 

(Standard 

Error) 

β3 

(Standard 

Error) 

k

(Standard 

Error) 

Total −3.9789

(0.9141)

0.7126

(0.1071)

−0.0011

(0.0003)

0.7387

(0.1737)

0.5541

(0.0832)

Injury −4.2862

(1.0280)

0.6210

(0.1208)

−0.0009

(0.0003)

0.9288

(0.1706)

0.3319

(0.0863)

ROR −2.5408

(1.1253)

0.4254

(0.1357)

−0.0011

(0.0003)

0.5950

(0.1998)

0.6168

(0.1016)

Wet-road −4.3685

(1.2709)

0.7722

(0.1698)

−0.0015

(0.0004)

1.0000 

(N/A) 

1.0637

(0.1606)

HOSSOD −4.7706

(1.3216)

0.6506

(0.1760)

−0.0017

(0.0004)

1.0000 

(N/A) 

0.5676

(0.1711)

N/A = not available. 

West Virginia Curves

Reference sites were not available for West Virginia. Thus, SPFs from the neighboring State of 

Kentucky were used. Annual factors for applying these SPFs in the EB methodology were 

obtained from statewide West Virginia crash counts available online. 

Arkansas Ramps 

The model form for Arkansas ramp SPFs shown in table 26 is shown in figure 15.

Figure 15. Equation. Model form for Arkansas ramp SPFs. 

Table 26. SPF parameter estimates for Arkansas ramps. 

Crash Type

ln(α) 

(Standard 

Error) 

β1 

(Standard 

Error) 

k

(Standard 

Error) 

Total −5.8976

(1.1122)

0.6577

(0.1300)

1.0849

(0.2578)

Injury −6.5328

(1.3908)

0.5897

(0.1631)

1.2231

(0.4118)

ROR * * * 

Wet-road −9.5594

(1.9032)

0.9157

(0.2196)

1.6125

(0.5477)

*Apply a proportion of 0.10 to total crash SPF.
55 

Kentucky Ramps

The model form for Kentucky ramp SPFs shown in table 27 is shown in figure 16.

Figure 16. Equation. Model form for Kentucky ramp SPFs. 

Table 27. SPF parameter estimates for Kentucky ramps. 

Crash Type

ln(α) 

(Standard 

Error) 

β1 

(Standard 

Error) 

k

(Standard 

Error) 

Total −3.4367

(0.9194)

0.3960

(0.1122)

1.2741

(0.2416)

Injury −6.0170

(1.2126)

0.4971

(0.1447)

1.5146

(0.3830)

ROR −8.5549

(1.6135)

0.5801

(0.1797)

0.9891

(0.6748)

Wet-road −4.9251

(1.2596)

0.5801

(0.1797)

0.9891

(0.6748)

57 

CHAPTER 5. BEFORE–AFTER ANALYSIS RESULTS

A safety impact was calculated for each State, and where feasible, for combined data for multiple 

States. The latter was possible for curves, for which results from three States were combined.

The combined data allowed a disaggregate analysis to explore the effects of design, traffic, 

operational, and safety characteristics.

AGGREGATE RESULTS

Table 28 through table 31 provide the estimated aggregate CMFs and standard errors broken 

down by crash type, site type, and State. 

Aggregate Results for Curves

The results in table 28 through table 31 indicate significant benefits in terms of low CMFs for 

each State and for Kentucky, Pennsylvania, and West Virginia combined, especially for the 

primary crash types targeted by HFST programs: wet-road, ROR, and HOSSOD. While the 

average after period for the three States combined was 3.3 yr, the data for the longer after periods 

were too limited to facilitate a meaningful analysis of the effects over time.

Because of the relative consistency in the results for Kentucky, Pennsylvania, and West Virginia, 

the CMFs based on the three States combined were recommended for estimating the benefits of 

contemplated treatments.

Table 28. Aggregate results for Pennsylvania curves. 

Crash Type

Observed 

Crashes after 

Treatment

EB Expected 

Crashes after 

Treatment

CMF

Estimate

(95-Percent

Confidence 

Interval)

Standard Error

(p-Value)

Total 143 269.990 0.529

(0.431–0.627)

0.050

(<0.010) 

Injury 50 101.550 0.490

(0.341–0.639)

0.076

(<0.010) 

ROR 22 51.160 0.428

(0.240–0.616)

0.096

(<0.010) 

Wet-road 36 139.610 0.257

(0.167–0.347)

0.046

(<0.010) 

HOSSOD 14 33.260 0.416

(0.183–0.649)

0.119

(<0.010) 
58 

Table 29. Aggregate results for West Virginia curves. 

Crash Type

Observed 

Crashes after 

Treatment

EB Expected 

Crashes after 

Treatment

CMF

Estimate

(95-Percent

Confidence 

Interval)

Standard Error

(p-Value)

Total 12 29.140 0.403

(0.156–0.650)

0.126

(<0.010) 

Injury 8 8.830 0.885

(0.230–1.540)

0.334

(0.740) 

ROR 10 17.180 0.569

(0.187–0.951)

0.195

(0.030) 

Wet-road 3 12.480 0.230

(0.000–0.497)

0.136

(<0.010) 

HOSSOD * * * * 

*No crashes identified.

Table 30. Aggregate results for Kentucky curves. 

Crash Type

Observed 

Crashes after 

Treatment

EB Expected 

Crashes after 

Treatment

CMF

Estimate

(95-Percent

Confidence 

Interval)

Standard Error

(p-Value)

Total 174 468.710 0.370

(0.305–0.435)

0.033

(<0.010) 

Injury 48 94.640 0.505

(0.346–0.664)

0.081

(<0.010) 

ROR 60 264.670 0.226

(0.163–0.289)

0.032

(<0.010) 

Wet-road 43 343.550 0.125

(0.086–0.164)

0.020

(<0.010) 

HOSSOD 45 48.030 0.926

(0.597–1.255)

0.168

(0.670) 

Table 31. Aggregate results for curves for Kentucky, Pennsylvania, and West Virginia

combined. 

Crash Type

Observed 

Crashes after 

Treatment

EB Expected 

Crashes after 

Treatment

CMF

Estimate

(95-Percent

Confidence 

Interval)

Standard Error

(p-Value)

Total 329 767.840 0.430

(0.375–0.485) 

0.028

(<0.010) 

Injury 106 205.020 0.515

(0.442–0.588) 

0.037

(<0.010) 

ROR 92 333.490 0.279

(0.216–0.342) 

0.032

(<0.010) 

Wet-road 82 495.540 0.168

(0.129–0.207) 

0.020

(<0.010) 

HOSSOD 59 81.290 0.691

(0.485–0.897) 

0.105

(<0.010) 
59 

Aggregate Results for Ramps

The results for Arkansas and Kentucky, shown in table 32 and table 33, respectively, were highly 

inconsistent. The exception was wet-weather crashes, for which the benefits were large—

although the results for Arkansas were based on few crashes. The benefits for total crashes and 

injury crashes were large for Kentucky, while there were negligible effects for these crashes in 

Arkansas. This anomaly can be explained by the relatively small proportion of wet-weather 

crashes, the main HFST target, in Arkansas compared to Kentucky as seen in the data summary 

in table 34. 

To achieve the substantial benefits of HFSTs, ramps should be identified for treatment based on 

a high proportion of wet-weather crashes. The CMF results for Kentucky in table 33 are 

recommended for evaluating the potential benefits of HFSTs. The Kentucky sample of treatment 

sites was too small to facilitate a meaningful analysis of the effects over time.

Table 32. Aggregate results for Arkansas ramps. 

Crash Type

Observed 

Crashes after 

Treatment

EB Expected 

Crashes after 

Treatment

CMF

Estimate

(95-Percent

Confidence 

Interval)

Standard Error

(p-Value)

Total 97 92.430 1.045

(0.800–1.290)

0.125

(0.730) 

Injury 19 17.720 1.086

(0.535–1.637)

0.281

(0.770) 

ROR 3 3.640 0.788

(0.000–1.697)

0.464

(0.660) 

Wet-road 6 44.630 0.133

(0.025–0.241)

0.055

(<0.010) 

Table 33. Aggregate results for Kentucky ramps. 

Crash Type

Observed 

Crashes after 

Treatment

EB Expected 

Crashes after 

Treatment

CMF

Estimate

(95-Percent

Confidence 

Interval)

Standard Error

(p-Value)

Total 183 860.440 0.212

(0.177–0.247)

0.018

(<0.010) 

Injury 46 125.130 0.365

(0.245–0.485)

0.061

(<0.010) 

ROR 6 29.510 0.202

(0.037–0.367)

0.084

(<0.010) 

Wet-road 49 621.070 0.079

(0.055–0.103)

0.012

(<0.010) 
60 

DISAGGREGATE RESULTS (FOR CURVES) 

Where samples were large enough, as was the case for curves, researchers isolated sites with 

certain levels or ranges of a given variable and estimated the separate effects for each category. 

This univariate categorical analysis was first conducted to explore continuous CMFunctions. 

Univariate Categorical Analysis Results 

For this analysis, sites were separated into two categories of four key influential variables. The 

boundary for each category was determined by trial and error to provide the greatest difference 

in the CMF between the two categories while maintaining a significant number of sites and 

crashes in each.

Based on the HFT Mu measurement value, the results in table 34 suggest the beneficial effects 

are larger (i.e., CMFs are smaller) for larger friction absolute and percentage increases, AADTs

are lower, and expected crashes per mile-year are greater before HFST installation. CMF results 

for friction increase are only applicable to friction measurements with the HFT device and within 

the context of HFSTs—not from other pavement treatments. The simplified friction increase 

category was used since the friction data collected for this current study was inadequate for a

higher-resolution friction representation of the pavement due to factors previously presented in 

chapter 3. 

Table 34. Curve CMF results from the univariate categorical analysis. 

Category

Number 

of Sites

After-Period 

Crashes

CMF

(Standard Error) 

Total Wet Total Wet 

HFT Mu friction percentage increase >0.4 46 78 22 0.319

(0.400) 

0.124

(0.028)

HFT Mu friction percentage increase <0.4 73 197 47 0.469

(0.045)

0.176

(0.028)

HFT Mu friction percentage increase >55 72 165 44 0.378

(0.037)

0.138

(0.022)

HFT Mu friction percentage increase <55 47 110 25 0.482

(0.066)

0.200

(0.044)

AADT <3,000 48 60 15 0.272

(0.042)

0.097

(0.026)

AADT >3,000 96 267 67 0.493

(0.041)

0.200

(0.027)

Crashes/mile-year >9.48 44 166 44 0.341

(0.032)

0.123

(0.020)

Crashes/mile-year <9.48 100 161 38 0.584

(0.069)

0.290

(0.053)
61 

Investigation of CMFunctions

Researchers used multiple variable regression modeling investigate the effects of a number of 

factors, including AADT, curvature, expected crash frequency before treatment, and friction 

improvement, on the CMF. The objective was to investigate whether CMFunctions can be 

developed to capture the effects of these factors and more precisely estimate CMFs for 

prospective HFST installations. 

In the GLM regression approach taken to estimate CMFunctions, each site was considered an 

observation. The observed crash frequency in the after period was modeled using a negative 

binomial model with the EB estimate of expected crashes in the after period had no HFST been 

installed used as an offset. Variables affecting the CMF value were then added to the model. This 

approach is illustrated in figure 17 where EBafter is the EB estimate of expected crashes in the 

after period. The value of observed crashes after being divided by the EB estimate is an 

approximate CMF for a site, as seen in figure 18. This approximate CMF is modeled as a 

function of site characteristics. This function is a CMFunction that predicts a CMF value for an 

individual site based on the characteristics in the model seen in figure 19.

Figure 17. Equation. Calculation for observed crashes.

Where:

Observed Crashes After = number of reported crashes at a site after HFST installation.

f = percentage HFST friction increase.

site characteristics = characteristic of interest (e.g., friction, traffic, rainfall).

Figure 18. Equation. Approximate CMF for a site. 

Figure 19. Equation. CMF for an individual site based on the site characteristics.

The CMFunctions developed were not as robust as might have been obtained with larger 

samples. The direction of effect for some variables, such as AADT, was not always consistent, 

and the statistical significance of estimated parameters tended to be poor. Nevertheless, there 

were useful insights, suggesting value in developing robust CMFunctions in future research. 

These insights confirmed the indications from the univariate categorical analysis that there was a 

logical and consistent relationship between CMFs and three variables: friction improvement as 

measured by the percentage HFST friction increase, AADT, and expected crash frequency before 

HFST installation. Other friction-related variables, such as estimated FN40Rs before HFST 

installation and the actual change in friction, were not as significant as the one eventually 

selected.
62 

Table 35 and table 36 indicate the estimated parameters and standard errors of the recommended 

CMFunctions for total and wet-weather crashes. Asterisks indicate the data ranges where the 

CMFunction is applicable in table 36. 

The functional form of the CMFunctions is shown in figure 20.

Figure 20. Equation. Functional form for CMFunctions. 

Table 35. CMFunction parameter estimates for curves. 

Parameter

Parameter Estimate (Standard Error)

Total Crashes Wet-Weather Crashes

In(α) −2.9798 (1.0250) 0.8158 (0.3894)

β1 −0.0131 (0.0110) −0.0417 (0.0253)

β2 −0.6437 (0.2512) −0.6200 (0.4594)

β3 0.3299 (0.1228) N/A

N/A = not available.

Table 36. Applicable ranges of variables for curve CMFunctions. 

Variable

Values at Site with Minimum Values at Site with Maximum

m (total) f AADT m (total) F AADT

m 0.47* 37 2,270 38.05* 43 3,561

f 6.66 12* 2,400 1.57 168* 3,330

AADT 0.97 23 650* 5.40 48 24,853* 

*Variables within the applicable range.

In applying the CMFunction estimates in conjunction with the point CMF estimates for the 

States combined in table 31 for a prospective HFST installation, whichever estimate provides the 

most conservative value can be used (i.e., the larger CMF be used). If a site has a variable value 

outside the applicable range of the CMFunction, the value of the variable at the extremity should 

be used to estimate the CMF. 

The combination of a higher expected crash frequency and the associated smaller CMF yielded

the greatest crash reductions and the highest B/C ratios. These impacts were somewhat mitigated 

by the larger CMF values implied by the CMFunction for the higher AADTs at sites with high 

crash frequencies.
63 

CHAPTER 6. B/C ANALYSIS

METHODOLOGY

B/C analyses allow agencies to assess, compare, and prioritize potential HFST installations by 

considering the quantified benefits in terms of crash reduction and lifespan—not just the 

installation cost. 

In this current study, separate analyses are provided for ramps (based on Kentucky sites) and 

curves (based on sites for Kentucky, Pennsylvania, and West Virginia combined) considering the 

benefits for total crashes. The annualized cost of an HFST installation is computed as follows in 

figure 21.

Figure 21. Equation. Annualized cost of an HFST.

Where:

Annual Cost = cost of treatment per year if annualized over its life.

C = treatment cost. 

R = discount rate (as a decimal). 

N = expected service life (years).

Based on information provided by States, conservative values of 5 yr for service life and $35 per 

square yard for an HFST installation cost were used.

Based on information from the Office of Management and Budget and FHWA 

recommendations, a real discount rate of 7 percent was used to determine the annual cost of the 

HFST installation (U.S.C. 2015). 

For the benefit calculations, the most recent FHWA mean comprehensive crash costs 

disaggregated by crash severity, location type, and speed limit were used as a base (Council et al. 

2005). Council et al. (2011) developed these costs based on 2001 crash costs and found the unit 

costs (in 2001 U.S. dollars) for PDO and KABC crashes for posted speed limits less than 50 mph 

(assumed for ramps) were $7,068 and $91,917, respectively; for all speed limits combined 

(assumed for curves), the unit crash costs were $7,428 and $158,177, respectively. These were 

updated to 2019 U.S. dollars by applying the ratio of the projected value of a statistical life of 

$9.8 million (projected from latest published values of $9.4 and $9.6 million for 2014 and 2016, 

respectively) to the 2001 value of $3.8 million (USDOT 2014; USDOT 2016; Zaloshnja et al. 

2006). Applying this ratio of 2.58 to the unit costs for PDO and KABC crashes, and then 

weighting by the frequencies of PDO and KABC crashes in the after period (223 and 106 for 

curves and 137 and 46 for ramps, respectively) unit costs for total crashes of $144,744 and 

$68,719 were obtained curves and ramps, respectively.

The total crash reduction was calculated by subtracting the actual crashes in the after period from 

the expected crashes in the after period had an HFST not been installed. The number of crashes 

prevented per year was obtained by dividing the total crash reduction (438.84 for curves and 
64 

677.44 for ramps) by the average number of after-period years per site (3.285 for curves and 

5.476 for ramps). The annual benefit (i.e., crash savings) is the product of the total crash 

reduction per year and the aggregate cost of a crash (all severities combined). The B/C ratio is 

calculated as the ratio of the annual benefit to annual cost.

The annual benefit (i.e., crash savings) of $19,300,113 and $9,063,309 for curves and ramps, 

respectively, is the product of crash reductions per year (133.59 and 123.71, respectively) and the 

aggregate costs of a crash, all severities combined ($144,744 and $68,719, respectively). The 

B/C ratios are estimated to be 6.00 for curves and 18.74 for ramps. The DOT recommends a 

sensitivity analysis by assuming values of a statistical life of 0.56 and 1.41 times the 

recommended value (USDOT 2016). These factors were applied directly to the estimated B/C 

ratios to get a range of 3.36 to 8.45 for curves and 10.50 to 26.44 for ramps. These results 

suggest that HFSTs, even with conservative assumptions on cost, service life, and the value of a 

statistical life, are cost effective—especially for ramps.
65 

CHAPTER 7. DISCUSSION AND CONCLUSIONS

SAFETY EVALUATION AND RECOMMENDATIONS

This current study was limited in scope because the amount of data anticipated at the beginning 

did not materialize in a timely manner for a variety of reasons. Nevertheless, defendable and 

application-ready CMF results were obtained and the cost effectiveness of HFSTs was 

confirmed. 

The objective of this current study was to estimate the effect of HFSTs on crashes by evaluating 

different site types from several States. The state-of-the-art EB before–after methodology 

evaluated the effects on various crash types (e.g., total, injury, wet-road, ROR, and HOSSOD) 

using data collected in Arkansas (ramps), Kentucky (curves and ramps), Pennsylvania (curves), 

and West Virginia (curves). Data were also collected in Louisiana and Georgia but did not 

include enough after-period data or reference sites for a rigorous EB evaluation. A 

complementary evaluation was conducted under this current study for friction change of HFSTs 

over time, the results of which are documented in the companion report FHWA-HRT-20-062. 

Friction was tested on several older HFST installations where previous friction data were

collected. All friction was tested with FHWA’s HFT, a continuous fixed-slip measurement 

device. The friction data collected for this current study and documented in this report were 

evaluated for friction change before and after HFST installation, for HFST and existing 

pavement over time, and through a curve. 

The results for curves indicated significant benefits in terms of low CMFs for each State and 

Kentucky, Pennsylvania, and West Virginia combined, especially for the primary crash types 

targeted by HFST programs: wet-road, ROR, and HOSSOD. CMFs of 0.279, 0.168, and 0.691, 

respectively, were estimated for the three States combined. Because of the relative consistency in 

the results for the three States, the CMFs based on the three States combined were recommended 

for estimating the benefits of a contemplated HFST installation and for use in the CMF 

Clearinghouse. 

The results for ramps for Arkansas and Kentucky were highly inconsistent. The exception was 

wet-weather crashes, for which the benefits were large. The benefits for all crashes and injury 

crashes were large for Kentucky, while there were negligible effects for these crashes in 

Arkansas. This anomaly can be explained by the relatively small proportion of wet-weather 

crashes, the main HFST target, in Arkansas compared to Kentucky. To achieve the substantial 

benefits of HFSTs, ramps should be identified for treatment based on a high proportion of 

wet-weather crashes.

A thorough disaggregate analysis of the before–after evaluation data was only feasible for 

curves. For this, univariate categorical and multivariable regression analyses were used to 

investigate the effects on the CMFs of a number of variables. There was a logical and consistent 

relationship between CMFs and three variables: friction improvement, AADT, and expected 

crash frequency before HFST installation. These variables were used to develop the 

recommended CMFunctions. 
66 

An economic analysis suggested HFSTs, even with conservative assumptions on cost, service 

life, and the value of a statistical life, are cost effective—especially for ramps. Table 37

summarizes the B/C ratios along and the recommended CMFs. 

Table 37. B/C ratios and recommended CMFs. 

Crash Type Curves Ramps

CMF for total crashes

(95-percent confidence interval)

0.430

(0.375–0.485)

0.212

(0.177–0.247)

CMF for injury crashes

(95-percent confidence interval)

0.515

(0.442–0.588)

0.365

(0.245–0.485)

CMF for ROR crashes

(95-percent confidence interval)

0.279

(0.216–0.342)

0.202

(0.037–0.367)

CMF for wet-road crashes

(95-percent confidence interval)

0.168

(0.129–0.207)

0.079

(0.055–0.103)

CMF for HOSSOD crashes

(95-percent confidence interval)

0.691

(0.485–0.897)

N/A

B/C ratio based on total crashes

(sensitivity range)

6.000 

(3.360–8.450) 

18.740 

(10.500–26.440) 

N/A = not available.

PERFORMANCE EVALUATION AND RECOMMENDATIONS

Some of the objectives for this current study centered around recommending materials, 

specifications, and HFST installation evaluations. Although researchers collected data from a 

large number of HFST installations encompassing a wide variety of ramp and curve 

characteristics from six States, the data gathered were inadequate for conclusive 

recommendations regarding materials, specifications, or installation evaluations. 

It was impossible to identify specific materials (i.e., HFST products) beyond the basic 

components used for all the projects: polymeric binder resin (e.g., epoxy or polyester) and 

calcined bauxite aggregate. Friction measurements confirmed the materials used for virtually all 

projects evaluated were indicative of what was expected from an HFST, with calcined bauxite 

aggregate demonstrating high levels of friction, regardless of HFST age. 

There are currently no established standards for rating HFST condition, making performance 

evaluation difficult. Due to safety concerns for researchers, only a subjective windshield survey 

of HFST condition was captured for the sites tested. While this information was helpful, it was 

inadequate for conclusive recommendations on HFST performance over time, especially without 

knowing the condition of the underlying pavement prior to installation. However, based on 

discussions with representatives from the agencies involved with these HFST programs, the 

following are some reasons for premature failure of HFSTs based on their experience: 

• Underlying pavement type and condition—while HFSTs can be applied to virtually any 

pavement surface, they should not be applied to heavily distressed pavements, including 

those with extensive fatigue cracking (e.g., alligator and block cracking), deep rutting, or 

durability-related cracking (e.g., deep spalls, map cracking).

• Inadequate surface preparation—a mechanical bond between the HFST resin binder and 

underlying pavement surface is critical for preventing debonding or delamination of the 
67 

HFST. In general, HFSTs adhere well to clean, dry asphalt surfaces. However, concrete 

surfaces require shot blasting to ensure a mechanical bond and prevent debonding. 

AASHTO MP 41-19 provides guidelines for surface preparation.

• Moisture beneath the pavement surface—for asphalt pavements where subsurface 

moisture migrates to the surface, the moisture can become trapped in the asphalt layers 

after HFSTs are applied. This can lead to stripping and degradation of the underlying 

asphalt pavement and eventual failure, also causing failure of the HFST.

• Inadequate resin binder thickness—the thickness of the resin binder should be 

approximately 50 percent of the nominal maximum aggregate size. The resin binder 

application rate is dependent on the texture depth of the existing pavement, with coarseand open-textured pavements requiring more resin binder than lower-texture pavements. 

If the resin binder application rate is not adjusted for texture, inadequate thickness that 

does not securely hold the HFST aggregate in place—leading to premature aggregate and 

overall friction loss—can occur. Additionally, installing HFSTs on pavement surfaces 

with significant cross-slope steep grade requires additional care to prevent the resin from 

flowing downhill and leaving inadequate thickness at the high side. Resin binder 

thickness should be regularly checked during installation and further verified by 

application rate/material usage.

• Poor installation practices—while this is a broad category, issues with improper 

installation include improper formulation and/or inadequate proportioning and mixing of 

the resin binder components, which results in the binder resin not curing; dirty aggregate, 

which inhibits a bond between the resin binder and aggregate; proper timing of aggregate 

placement onto the resin binder so the resin binder securely holds the aggregate in place; 

and proper timing of sweeping of excess aggregate so aggregate is not pulled out of the 

resin binder before it is fully cured. 

The application practices and methods for the projects evaluated under this current study varied 

greatly from State to State and even within States. While all projects were single-layer HFSTs, 

application processes varied widely from a variety of semiautomated to fully automated 

practices, and in some specific situations, manual installation. However, the application practices 

for each project were not specifically documented, making any recommendations on best 

practices based on observed performance impractical.

While each volunteer State provided specifications, most were in the process of revising their 

specifications from lessons learned through widespread HFST installations and evaluations. 

There was no consistent specification used across all projects researchers could point to as best 

practice. AASHTO MP 41-19 provides a good baseline specification to which State-specific 

materials, QC, testing requirements, and installation practices can be added based on local 

experience. A properly developed HFST specification should include the following key items: 

• QC plan requirements—a good QC plan ensures the installer has thoroughly documented 

all aspects of installation, from material supply and installation equipment/methods to 

contingency plans and opening to traffic. The QC plan also gives an agency a clear plan 
68 

to review before installation and documentation to hold the installer accountable during 

construction. 

• Material properties for resin binder and aggregate—key resin binder and aggregate 

properties are provided in AASHTO MP 41-19. Additional State-specific requirements 

for these materials based on previous experience can be added as needed. Many agencies 

have a prequalified product approval process to ensure proposed materials were

evaluated.

• Sampling and testing methods—HFST resin binder and aggregate materials should be 

regularly sampled and tested during installation to identify any changes in the materials 

from what was accepted for installation.

• Packaging and materials certification—HFST resin binder and aggregate materials are 

specialty materials that require additional care during storage. If an agency does not test 

the materials themselves, certification of the material properties should be provided by 

the installer and/or material suppliers.

• Surface preparation requirements—critical for HFST performance, surface preparation 

includes removing striping and other pavement markings, cleaning and roughening the 

pavement surface, and protecting the prepared surface prior to HFST installation. 

AASHTO MP 41-19 provides guidance on proper surface preparation requirements. 

• Resin binder and aggregate installation requirements—resin binder installation 

requirements include the required thickness and/or application rate and any restrictions 

on installation conditions (e.g., pavement/air temperature, precipitation). Aggregate 

installation requirements include application rate and timing of aggregate placement to 

ensure it will properly embed in the resin binder. 

• Acceptance requirements—requirements generally center around uniformity and 

appearance of the finished surface, complete curing of the resin binder, texture depth, 

and/or friction of the finished surface. Friction testing is typically performed after some 

amount of traffic exposure to provide a wear-in period for the HFST, anywhere from 3 to 

90 d after installation.

• Test strip requirements—test strips allow an installer to demonstrate their installation 

processes and methods to the agency on a smaller scale before full installation. Any 

issues with installation can be identified and remedied before full installation.

Because the focus of this report was curves and ramps, researchers cannot confidently 

recommend any different specifications or practices for HFST installations on longer length 

sections of roadway. While installation practices for HFSTs for longer sections of roadway 

should not differ from targeted curves and ramps, the B/C of such installations have not been 

evaluated.
69 

ACKNOWLEDGMENTS

The authors acknowledge the contributions from the various ELCSI-PFS States that provided 

valuable input for this report. The authors specifically acknowledge the following ELCSI-PFS 

and non-ELCSI-PFS States for providing data used or considered for use in this current study: 

• Arkansas

• Florida 

• Georgia

• Iowa

• Kentucky

• Louisiana 

• Oklahoma 

• Pennsylvania 

• South Carolina 

• Tennessee

• Texas

• Wisconsin 

• West Virginia.

71 

REFERENCES

AASHTO. (2002). Standard Method of Test for Resistance to Degradation of Small-Size Coarse 

Aggregate by Abrasion and Impact in the Los Angeles Machine, AASHTO T 96/

ASTM C131, American Association of State Highway and Transportation Officials, 

Washington, DC. 

AASHTO. (2008). Guide for Pavement Friction, American Association of State Highway and

Transportation Officials, Washington, DC. 

AASHTO. (2018). Standard Method of Test for Frictional Properties of Paved Surfaces Using a 

Full-Scale Tire, AASHTO T 242, American Association of State Highway and 

Transportation Officials, Washington, DC. 

AASHTO. (2019). Standard Specification for High Friction Surface Treatment for Asphalt and 

Concrete Pavements Using Calcined Bauxite, AASHTO MP 41-19, American

Association of State Highway and Transportation Officials, Washington, DC. 

Anderson, K.W., Russell, M., Sheets, J., Burkey, D., Southwick, M., Uhlmeyer, J.S., and 

Weston, J. (2012). Evaluation of Tyregrip® High-Friction Surfacing, Washington State 

Department of Transportation, Olympia, WA. 

Anderson, K.W., Russell, M., Sheets, J., Burkey, D., Southwick, M., Uhlmeyer, J.S., and 

Weston, J. (2017). Evaluation of Tyregrip® High-Friction Surfacing – Final Report, 

Washing State Department of Transportation, Olympia, WA. 

Anderson, K.W., Russell, M., Uhlmeyer, J.S., and Weston, J. (2015). Kwik Bond Polymers® 

High Friction Surface Treatment, Washington State Department of Transportation, 

Olympia, WA.

ASTM. (2001). Standard Test Method for Measuring Pavement Macrotexture Depth Using a 

Volumetric Technique, ASTM E 965-96, ASTM International, West Conshohocken, PA. 

ASTM. (2005). Standard Practice for Calculating Pavement Macrotexture Mean Profile Depth, 

ASTM E 1845-01, ASTM International, West Conshohocken, PA. 

ASTM. (2006). Standard Test Method for Measuring the Skid Resistance of Pavements and 

Other Trafficked Surfaces Using a Continuous Reading, Fixed-Slip Technique, 

ASTM E2340, ASTM International, West Conshohocken, PA. 

ASTM. (2011). Standard Test Method for Skid Resistance of Paved Surfaces Using a Full-Scale 

Tire, ASTM E274, ASTM International, West Conshohocken, PA. 

ASTM. (2015). Standard Test Method for Resistance of Fine Aggregate to Degradation by 

Abrasion in the Micro-Deval Apparatus, ASTM D7428, ASTM International, 

West Conshohocken, PA. 
72 

ASTM. (2016). Standard Specification for a Size 4.00-8 Smooth Tread Friction Test Tire, 

ASTM E1551, ASTM International, West Conshohocken, PA.

ASTM. (2019a). Standard Test Method for Measuring Pavement Macrotexture Depth Using a 

Volumetric Technique, ASTM E965, ASTM International, West Conshohocken, PA. 

ASTM. (2019b). Standard Test Method for Measuring Pavement Macrotexture Properties Using 

the Circular Track Meter, ASTM E2157, ASTM International, West Conshohocken, PA. 

ATSSA. (2013). Safety Opportunities in High Friction Surfacing, American Traffic Safety 

Services Association, Fredericksburg, VA. 

Bischoff, D. (2008). Investigative Study of the Italgrip System, Wisconsin Department of 

Transportation, Madison, WI. 

Brimley, B. and Carlson, P. (2012). Using High Friction Surface Treatments to Improve Safety 

at Horizontal Curves, Texas Transportation Institute, College Station, TX. 

Cheung, J. and Julian, F. (2015). Northern California US 199 - Del Norte County, Case Study, 

Federal Highway Administration, Washington, DC. 

Cheung, J. and Julian, F. (2016). “‘Curve’ Appeal: High Friction Surface Treatments Key to 

Safer Pavements,” Pavement Preservation Journal, 9(2), FP2

, Inc., Austin, TX 

Council, F., Zaloshnja, E., Miller, T., and Persaud, B. (2005). Crash Cost Estimates by Maximum 

Police-Reported Injury Severity Within Selected Crash Geometries, Report No. 

FHWA-HRT-05-051, Federal Highway Administration, Washington, DC. 

de León, E., Katicha, S., Flintsch, G., and McGhee, K. (2016). “Pioneering the Use of 

Continuous Pavement Friction Measurements to Develop New Safety Performance 

Functions, Improve the Accuracy of Crash Count Predictions, and Evaluate Possible 

Treatments for the Roads in Virginia.” Presented at the 95th Annual Meeting of the 

Transportation Research Board, Washington DC. 

FHWA EDC2. (n.d.). “High Friction Surface Treatment Initiative.” (website) Washington, DC. 

Available online: http://www.fhwa.dot.gov/everydaycounts/edctwo/2012/friction.cfm, 

last accessed June 27, 2019. 

FHWA Office of Safety. (n.d.a.). “High Friction Surface Treatment Initiative.” (website) 

Washington, DC. Available online: 

https://safety.fhwa.dot.gov/roadway_dept/pavement_friction/high_friction, last accessed 

June 27, 2019. 

FHWA Office of Safety. (n.d.b.). “Pavement Friction.” (website) Washington, DC. Available 

online: https://safety.fhwa.dot.gov/roadway_dept/pavement_friction/index.cfm, last 

accessed June 27, 2019. 
73 

FHWA Office of Safety. (n.d.c.). “Roadway Departure Safety.” (website) Washington, DC. 

Available online: http://safety.fhwa.dot.gov/roadway_dept/#facts, last accessed June 27, 

2019. 

FHWA. (2010). 2010 FHWA Technical Advisory 5040.38 on Pavement Friction Management, 

Federal Highway Administration, Washington, DC. Available online: 

https://www.fhwa.dot.gov/pavement/t504038.cfm, last accessed June 27, 2019. 

FHWA. (2020a). “Crash Modification Factors Clearinghouse.” (website), Washington, DC. 

Available online: http://www.cmfclearinghouse.org, last accessed June 11, 2020. 

FHWA. (2020b). High Friction Surface Treatment (HFST) Quick Reference, Report No. 

DTFH61-13-D-00001, Task B9, Federal Highway Administration, Washington, DC. 

Flintsch, G.W., de León, E., McGhee, K.K., and Roa, J.A. (2011). “Review of High-Friction 

Surface Technologies – Constructability and Field Performance.” Presented at the 90th 

Annual Meeting of the Transportation Research Board, Washington, DC.

Gorell, R.S.J and Tootill, W. (2001). Monitoring Local Authority Road Safety Schemes Using 

MOLASSES, TRL Report TRL512. TRL Limited, Crowthorne, United Kingdom. 

Hauer, E. (1997). Observational Before-After Studies in Road Safety – Estimating the Effect of 

Highway and Traffic Engineering Measures on Road Safety, Emerald Group Publishing 

Limited, Bingley, United Kingdom.

Heitzman, M., Turner, P., and Greer, M. (2015). High Friction Surface Treatment Alternative 

Aggregate Study, National Center for Asphalt Technology, Auburn, AL. 

Highways England. (2006). Design Manual for Roads and Concrete Bridges, Surfacing 

Materials for New and Maintenance Construction, Report No. HD 36/06, 

Highways England, London, United Kingdom. 

Kentucky State Police. (2018). “Kentucky Collision Analysis for the Public.” (website) 

Frankfurt, KY. Available online: http://crashinformationky.org, last accessed April 21, 

2020. 

Kipp, W. and Sanborn, D. (2014). Ennis Paint, Inc. Tyregrip High Friction Surface System, 

Vermont Agency of Transportation, Montpelier, VT.

KYTC. ( n.d.b.). “Photolog.” (website) Frankfurt, KY. Available online: 

http://maps.kytc.ky.gov/photolog, last accessed April 21, 2020. 

KYTC. (n.d.a.). “Highway Information View and Extract Interface (HIVEi) KYTC – Query.” 

(website) Frankfurt, KY. Available online: 

http://datamart.business.transportation.ky.gov/EDSB_SOLUTIONS/HISEXTRACTS/def

ault.aspx, last accessed April 21, 2020. 
74 

Li, J.Q., Wang, K.C.P., Yang, G., and Zhan, J.Y. (2015). 3D Laser Imaging based Real Time 

Pavement Surface Evaluation for High Friction Surfacing Treatments, Oklahoma State 

University, Stillwater, OK.

Li, S., Noureldin, S., Jiang, Y., and Sun, Y. (2012). Evaluation of Pavement Surface Friction 

Treatments, Report No. FHWA/IN/JTRP-1012/04. Joint Transportation Research 

Program, Indiana Department of Transportation, Indianapolis, IN.

McCarthy, R., Flintsch, G.W, Katicha, S.W.,McGhee, K.K., and Flintsch, A. (2016). “A New 

Approach for Managing Pavement Friction and Reducing Road Crashes.” Presented at 

the 95th Annual Meeting of the Transportation Research Board, Washington DC. 

Merritt, D.K. Tallon, R.A., and Watson, R.P. (2020). Developing Crash-Modification Factors for 

High- Friction Surface Treatment: Friction Change Report, Report No. 

FHWA-HRT-20-062, Federal Highway Administration, Washington, DC. 

Merritt, D.K., Lyon, C.A., and Persaud, B. (2015). Evaluation of Pavement Safety Performance, 

Report No. FHWA-HRT-14-065. Federal Highway Administration, Washington, DC. 

Pratt, M.P., Greedipally, S.R., Pike, A.M., Carlson, P.J., Celoza, A.M., Lord, D. (2014). 

Evaluating the Need for Surface Treatments to Reduce Crash Frequency on Horizontal 

Curves, Texas Transportation Institute, College Station, TX. 

RSTA. (2011). RSTA Code of Practice for High Friction Surfacing, Road Surface Treatments 

Association, Wolverhampton, United Kingdom. 

RSTA. (2013). “High Friction Surfacing,” Highways Magazine, 2013(October), Hemming 

Group, London, United Kingdom. 

Savolainen, P.T., Reddy, V., Santos, J.B., and Datta, T . (2009). “Evaluation of a High Friction 

Pavement Surface Treatment.” Presented at the 88th Annual Meeting of the 

Transportation Research Board, Washington, DC. 

Scully, T.C. (2014). Evaluation of High Friction Surface Treatments 2014, Kentucky 

Transportation Center, Lexington, KY. 

The R Foundation. (n.d.). “The R Project for Statistical Computing.” (website) Vienna, Austria. 

Available online: https://www.r-project.org, last accessed June 11, 2020. 

U.S.C. (2015). Circular A-94: Guidelines and Discount Rates for Benefit–Cost Analysis of 

Federal Programs. Available online: https://www.wbdg.org/FFC/FED/OMB/OMBCircular-A94.pdf, last accessed April 21, 2020. 

USDOT. (2014). “Guidance on Treatment of the Economic Value of a Statistical Life (VSL) in 

U.S. Department of Transportation Analyses – 2014 Adjustment.” (memo) June 13, 2014. 

http://www.dot.gov/sites/dot.gov/files/docs/VSL_Guidance_2014.pdf, last accessed 

June 27, 2019. 
75 

USDOT. (2016). “Guidance on Treatment of the Economic Value of a Statistical Life (VSL) in 

U.S. Department of Transportation Analyses – 2016 Adjustment.” (memo) August 8, 

2016. 

https://www.transportation.gov/sites/dot.gov/files/docs/2016%20Revised%20Value%20o

f%20a%20Statistical%20Life%20Guidance.pdf, last accessed June 27, 2019. 

Von Quintus, H. and Mergenmeier, A. (2014). Case Study: Kentucky Transportation Cabinet’s 

High Friction Surface Treatment and Field Installation Program, Report No. 

FHWA-SA-15-038. Federal Highway Administration, Washington, DC. 

Wilson, B.T., Brimley, B., Zhang, J. Mukhopadhyay, A., Mills, J., and Holzschuher, C. (2016). 

“Benefit–Cost Analysis of Florida High-Friction Surface Treatments.” Presented at the 

95th Annual Meeting of the Transportation Research Board, Washington, DC. 

Zaloshnja, E., Miller, T., Council, F., and Persaud, B. (2006). “Crash Costs in the United States 

by Crash Geometry,” Accident Analysis and Prevention, 38(4), pp. 644–651, Elsevier, 

Amsterdam, Netherlands. 


HRDS-20/09-20(300)E Recycled

Recyclable