Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

PUBLICATION NO. FHWA-HRT-20-062 NOVEMBER 2020 

Developing Crash-Modification Factors 

for High-Friction Surface Treatments: 

Friction Change Report 
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 

improvement identified as a priority by the PFS member States.

This study complements the FHWA study Developing Crash-Modification Factors for 

High-Friction Surface Treatments (FHWA-HRT-20-061) and documents results from friction 

testing of high-friction surface treatments (HFSTs).(1) This study used a highway friction tester, a 

continuous fixed-slip measurement device that provides a continuous friction plot, to measure 

and collect pavement friction values of the HFST and the existing pavement surface before

HFST installation. In some cases, the study used the friction of the pavement leading up to and 

away from existing HFST sites to estimate the friction of the underlying pavement for 

comparison to HFST friction. This report presents the evaluation results of friction change before 

and after HFST installation, friction change of the HFST and existing pavement over time, and 

friction change within a curve. This report may benefit safety engineers, pavement engineers, 

and safety planners by providing greater insight into applications of HFST 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-20-062

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

4. Title and Subtitle

Developing Crash-Modification Factors for High-Friction Surface

Treatments: Friction Change Report

5. Report Date

November 2020

6. Performing Organization Code

7. Author(s)

D.K. Merritt, R.A. Tallon, and R.P. Watson

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-00001

12. Sponsoring Agency Name and Address

Federal Highway Administration

1200 New Jersey Avenue SE

Washington, DC 20590

13. Type of Report and Period

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 and Operations

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

This report documents results and observations from friction testing of high-friction surface treatments (HFSTs) as

part of an FHWA research study, Developing Crash-Modification Factors for High-Friction Surface Treatments.

(1)

This larger study provides high-quality crash-modification factors and benefit–cost ratios for HFST with calcined

bauxite aggregate and recommends materials and specifications for applying HFST to effectively reduce

roadway-departure crashes. Friction testing was performed to document the friction of the HFST and underlying

pavement. For a number of sites, friction testing was performed on the existing pavement surface before HFST

installation and on the HFST after installation. In some cases, testing of the pavement leading up to and away from

existing HFST sites was used to estimate the friction of the underlying pavement for comparison to HFST friction.

To evaluate friction change of HFST over time, friction testing was also performed on several older HFST

installations where previous friction data had been collected. All friction testing was performed with a highway

friction tester, a continuous fixed-slip measurement device that provides a continuous plot of friction reported in

0.30-meter (1-foot) increments through the section of pavement tested. The advantage of continuous friction

measurement is that variations in friction (e.g., through a curve) can be documented. 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 within a curve.

17. Key Words

High-friction surface treatment, HFST, highway friction

tester, HFT, continuous friction measurement

18. Distribution Statement

No restrictions. This document is available to the

public through the National Technical Information

Service, Springfield, VA 22161.

http://ntis.gov

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

43

22. Price

N/A

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

TABLE OF CONTENTS

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

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

High-Friction Surface Treatment ........................................................................................... 4

Collection of Friction Data ...................................................................................................... 5

Method ................................................................................................................................. 5

Testing Procedure ................................................................................................................. 7

Data Reporting ..................................................................................................................... 7

Challenges ............................................................................................................................ 8

CHAPTER 2. FRICTION CHANGE BEFORE AND AFTER HFST INSTALLATION ...... 9

Evaluation Overview ................................................................................................................ 9

Data Summary .......................................................................................................................... 9

Observations ........................................................................................................................... 11

CHAPTER 3. FRICTION CHANGE OVER TIME................................................................. 13

Evaluation Overview .............................................................................................................. 13

Data Summary ........................................................................................................................ 14

HFST Sites With Historical Friction Data ......................................................................... 14

HFST Sites From Safety Performance Evaluation ............................................................. 18

Observations ........................................................................................................................... 22

CHAPTER 4. FRICTION CHANGE WITHIN CURVES ....................................................... 25

Evaluation Overview .............................................................................................................. 25

Data Summary ........................................................................................................................ 25

Observations ........................................................................................................................... 34

ACKNOWLEDGMENTS ........................................................................................................... 35

REFERENCES ............................................................................................................................. 37
iv

LIST OF FIGURES

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

Figure 2. Photograph. Machine laying aggregate for HFST. ........................................................... 4

Figure 3. Photograph. Close-up view of HFST after installation. .................................................... 5

Figure 4. Photograph. HFST finished surface. ................................................................................. 5

Figure 5. Photograph. The HFT. ...................................................................................................... 6

Figure 6. Graph. Summary of friction change for sites using lead-in and lead-out friction 

data. ....................................................................................................................................... 11

Figure 7. Graph. HFST friction versus age for sites with historical friction data. ......................... 17

Figure 8. Graph. HFST friction versus estimated traffic exposure for sites with historical 

friction data. .......................................................................................................................... 18

Figure 9. Graph. HFST friction versus age for safety performance evaluation sites. .................... 20

Figure 10. Graph. HFST friction versus estimated traffic exposure for all safety 

performance evaluation sites. ............................................................................................... 21

Figure 11. Graph. HFST friction versus estimated traffic exposure (safety performance 

evaluation sites with less than 3 million vehicles)................................................................ 22

Figure 12. Graph. Friction measurement within a curve before HFST installation. ...................... 26

Figure 13. Graph. Friction measurement within a curve after HFST installation. ......................... 27

Figure 14. Graph. Friction measurement within a curve (larger curve radius) before HFST 

installation............................................................................................................................. 28

Figure 15. Graph. Friction measurement within a curve (larger curve radius) after HFST 

installation............................................................................................................................. 29

Figure 16. Graph. Friction measurement within a curve (asphalt pavement) before HFST 

installation............................................................................................................................. 30

Figure 17. Graph. Friction measurement within an S-curve (lower AADT volume) before 

HFST installation. ................................................................................................................. 31

Figure 18. Graph. Friction measurement within an S-curve (lower AADT volume) after 

HFST installation. ................................................................................................................. 32

Figure 19. Graph. Friction measurement within an S-curve (higher AADT volume) before 

HFST installation. ................................................................................................................. 33
v 

LIST OF TABLES

Table 1. Number of HFST data collection sites by location and type. ............................................ 3

Table 2. Summary of friction change for sites with pre- and post-HFST friction data. .................. 9

Table 3. Summary of friction change for sites using lead-in and lead-out pavement friction 

data. ....................................................................................................................................... 10

Table 4. Summary of HFST sites with historical friction data. ...................................................... 14

Table 5. Friction change for HFST sites with historical friction data. ........................................... 15

Table 6. Estimated friction change for underlying pavement. ....................................................... 16

Table 7. Summary of safety performance evaluation HFST sites. ................................................. 19
vi 

LIST OF ACRONYMS

AADT average annual daily traffic

CMF crash-modification factor

EB eastbound 

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

FHWA Federal Highway Administration

HFST high-friction surface treatment

HFT highway friction tester

LWST locked-wheel skid trailer

TTI Texas Transportation Institute

WB westbound
1 

EXECUTIVE SUMMARY

This report summarizes several evaluations of the friction data collected as part of a safety 

performance evaluation of high-friction surface treatments (HFSTs). Developing 

Crash-Modification Factors for High-Friction Surface Treatments provides high-quality 

crash-modification factors and benefit–cost ratios for HFST with calcined bauxite aggregate and 

recommends materials and specifications for applying HFST to effectively reduce 

roadway-departure crashes.(1)

 For this safety performance evaluation, friction data were collected 

at several hundred HFST sites in six States and at several locations where previous (historical) 

friction data were available.

Three separate evaluations of friction change were performed:

1. Evaluation of friction before and after HFST application for sites in the six States 

included in the safety performance evaluation study. 

2. Evaluation of friction change over time for HFST sites with historical friction data, as 

well as current friction of HFST based on age and traffic exposure. 

3. Evaluation of friction change within curves to determine whether traffic-induced wear on 

pavement and HFST surfaces results in reduced friction within a curve.

The results of these evaluations verified the effectiveness of HFST. HFST was shown to 

dramatically increase the friction of virtually any pavement surface on both curves and ramps. 

HFST was also shown to maintain high levels of friction over time and under high traffic 

exposure. Although the evaluation of friction change within curves was not conclusive, the 

increased friction provided by HFST should minimize any potential reduction in friction within a 

curve over time.

3 

CHAPTER 1. INTRODUCTION

This report supplements the technical report of the Federal Highway Administration (FHWA)

study Developing Crash-Modification Factors for High-Friction Surface Treatments, which 

evaluated the safety performance of high-friction surface treatments (HFSTs).(1) For the safety 

performance study, crash, roadway, and friction data were collected from more than 500 existing 

and planned HFST sites for use in developing crash-modification factors (CMFs) and 

benefit–cost ratios for HFST. The friction data were used to develop CMFs based on friction 

change before and after HFST installation.

Table 1 summarizes the HFST sites that were tested for inclusion in the safety performance 

evaluation for this study, although not all sites were used for the final analysis. Two categories of 

sites were tested in the study: (1) existing HFST sites, which were tested in place; and (2) planned 

sites, which were tested before and after HFST installation. 

Table 1. Number of HFST data collection sites by location and type. 

State

Number of Curves Number of Ramps Number of Intersections

In Place Planned In Place Planned In Place Planned

AR 5 0 17 0 6 0 

GA 43 93 — — — — 

KY 58 0 27 0 1 0 

LA 27 90 — — — — 

PA 85 22 — — — — 

WV 25 14 — — — — 

Total 243 219 44 0 7 0

—No data. 

This report focuses on the evaluation of the friction data collected from these and additional 

HFST sites with friction history to assess friction change. Specifically, the following evaluations 

are summarized in this report:

1. Friction change before and after HFST installation. For sites tested as part of the safety 

performance evaluation, the friction of the pavement surfaces before and after HFST 

installation was evaluated. For existing HFST sites, the pavement friction leading up to 

(i.e., lead-in) and away from (i.e., lead-out) the HFST was used to estimate pavement 

friction before HFST installation.

2. Friction change over time. For HFST sites for which historical friction data were 

available, the friction was evaluated to assess change in HFST friction over time. In 

addition, HFST friction change based on age and estimated traffic exposure was 

evaluated.

3. Friction change within curves. Changes in pavement friction within curves before HFST 

installation and changes in HFST friction within curves after HFST installation were 

evaluated. 
4 

HIGH-FRICTION SURFACE TREATMENT

HFST is a specialty pavement treatment specifically used to restore or enhance friction. It is 

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

friction demand is higher than conventional paving materials can provide. HFST is installed by 

spreading a thin layer of polymeric resin binder (typically epoxy or polyester) over the pavement 

surface and then broadcasting or dropping an abrasion- and polish-resistant aggregate 1–3 mm 

(0.04–0.12 inches) in size onto the resin layer (figure 1 through figure 4). Calcined bauxite, which 

exhibits exceptional polish resistance and skid resistance not achieved with conventional 

pavement aggregates, is used to maintain high skid resistance over time. All HFST sites tested as 

part of this friction change evaluation used HFST consisting of calcined bauxite aggregate.

Source: FHWA.

Figure 1. Photograph. HFST installation. 

Source: FHWA.

Figure 2. Photograph. Machine laying aggregate for HFST. 
5 

Source: FHWA.

Figure 3. Photograph. Close-up view of HFST after installation.

Source: FHWA.

Figure 4. Photograph. HFST finished surface.

COLLECTION OF FRICTION DATA

The project team collected friction data used for the friction change evaluation to ensure 

consistency of the pavement and HFST friction data used in the analysis. 

Method

In the United States, friction data have traditionally been collected using an ASTM E274 

locked-wheel skid tester (LWST).(2) 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 (100-percent slip). Thus, at 

a 40-mph1 test speed, a 1-second test time is equivalent to testing the pavement surface for 

approximately 17.98 m (59 ft). The full-lock requirement means that measurements can be 

recorded only periodically over short intervals of time. Reporting one test per mile results in 

approximately 1.1 percent of the pavement surface being tested. In addition, testing with a trailer 

1

This document reports speed in mph. To calculate speed in km/h, multiply the speed in mph by 1.61. For 

example, 40 mph multiplied by 1.61 equals 64.4 km/h.
6 

unit locking wheel on tight horizontal curves or at intersections can be challenging or prohibitive. 

Thus, an LWST may not adequately characterize pavement friction at these locations.

Alternatively, continuous friction measurement systems measure friction continuously and report 

it at 0.30-meter (1-foot) increments over the section of pavement evaluated, allowing any 

variations in friction to be measured and quantified. Because of the advantage of continuous 

friction measurement and the location of most HFST installations, the project team collected 

friction data for this study using a highway friction tester (HFT) (figure 5). The HFT is an 

ASTM E2340 continuous fixed-slip measurement equipment device that provides a continuous 

measurement of friction at prevailing highway speed.(3) Friction data are reported by the HFT 

every 0.30 m (1 ft) over the length of the pavement surface being tested. The HFT uses an 

ASTM E1551 smooth-tread test tire located in the left wheel path of the lane and a slip ratio of 

14 percent.(4) The HFT applies a 0.5-millimeter (0.02-inch) 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 mean profile depth. 

Source: FHWA.

Figure 5. Photograph. The HFT. 

Because it operates at a low slip ratio, the HFT measures friction values closer to the peak friction 

value for a pavement surface, in contrast to the LWST, which uses the fully locked (100-percent) 

slip ratio. The HFT friction values are more representative of the friction available from a 

pavement surface for vehicles with antilock brakes, whereas the LWST friction values are more 

representative of the friction available from a pavement surface under a skidding (locked-wheel) 
7 

condition. At the low slip ratio of the HFT, the measurements are very sensitive to pavement 

microtexture and the properties of the test tire. As such, the friction values reported by the HFT 

are generally higher than those commonly measured by highway agencies using an ASTM E274 

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 (microtexture and macrotexture in particular), friction values from the HFT are not 

directly comparable to those from the LWST for all pavement surfaces. A 2013 Texas 

Transportation Institute (TTI) evaluation comparing the HFT to two different ASTM E274 

LWSTs found that the locked-wheel smooth-tread tire (ASTM E524) skid numbers were

approximately two-thirds of the corresponding HFT friction number for the pavement surfaces 

tested.

(5,6)

 However, because no HFST sections were included in the TTI evaluation, this 

correlation is not necessarily applicable to all surfaces, but it may provide a general 

understanding of the relative difference in readings. Pavement and HFST friction measurements 

presented herein are all based on data from the HFT.

Testing Procedure

In general, one run was used to collect friction data in each lane at each site. However, if any 

anomalies were observed during data collection, repeat measurements were made as needed and 

suspect data were disregarded. Event markers were used to identify reference points in the data 

that could be retraced to field locations, as well as any anomalies at the test site (e.g., bridge 

decks, pavement changes, pavement markings within the lane) and during testing (e.g., braking, 

deviation from wheel paths). 

A test speed of 40 mph was selected as the target test speed for most sites, regardless of higher 

posted speeds. For some sites, the test speed was limited to between 25 and 35 mph because of

curve radius or prevailing traffic conditions. During trial runs of friction testing, the project team 

noted that speeds below 25 mph resulted in unreliable friction data; therefore, 25 mph was 

established as the absolute minimum test speed.

For this study, it was important to collect friction data before and after HFST installation to better 

understand the change in friction as a result of HFST installation. This was possible for planned 

sites, but for existing HFST sites, friction data were collected on the lead-in and lead-out 

pavement as a surrogate measure of underlying pavement friction. The ability to collect lead-in 

and lead-out friction data on abutting pavement varied widely from site to site depending on the 

location of the HFST site in relation to other curves, intersections, and so forth. The project team

attempted to collect approximately 91.44–152.40 m (300–500 ft) of lead-in and lead-out friction 

data whenever possible. However, for some sites, only 60.96 m (200 ft) (or less) of data could be 

collected.

Data Reporting 

Friction data were manually processed for each HFST site, and the average friction values for the 

HFST and the underlying or abutting (lead-in and lead-out) pavement were reported. For sites 

with two or more lanes, the average friction value for all lanes was reported. The project team 

noted any significant differences in friction between lanes in case further evaluation was needed 
8 

to determine the cause. Any significant difference in lead-in and lead-out friction was also noted, 

and if the lead-in or lead-out pavement was different from the known surface under the HFST, 

only the data from the same type of surface were used. 

The HFT reports friction as a “Mu” value. For the purpose of reporting data herein, the Mu value 

measured by the HFT at speed S (mph) is represented as HFT(S). In general, data collected at 

40 mph were used for comparison purposes. Although a number of sites were tested at speeds 

other than 40 mph, a speed adjustment factor has not yet been determined for the HFT on 

surfaces with HFST, so only data collected at the same speed were compared. 

Additionally, although the HFT reports Mu values greater than 1.0 (for HFST in particular), for 

the purpose of this report, anything above 1.0 is simply reported as 1.0. The exception is for the 

evaluation of friction change within curves. 

Because of how friction data are collected by the HFT, friction values build up over the first 

60.96 m (200 ft) of an HFST site before stabilizing at the apparent friction value for the HFST. 

Challenges

The project team encountered many challenges with collecting friction data, which limited the 

number of sites that could be tested and included in the 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 because braking 

and acceleration can affect the friction data. As such, the project team was not able to test 

HFST sites at stop-controlled intersections. Likewise, signal-controlled intersections were 

difficult to test because testing needed to coincide with a green signal. Although the 

project team was able to successfully test several signal-controlled intersections, the total 

number of these intersections was very small.

• Inability to collect friction data on tight-radius ramps and curves. To obtain reliable 

friction data, the HFT must be operating at a minimum of 25 mph, so tight-radius ramps 

and curves with advisory speeds less than 25 mph had to be excluded from testing because 

of safety concerns for the friction testing operation. 

• Inability to collect friction data on most ramps terminating at intersections. Many of the 

ramp sites planned for inclusion in the study terminated at intersections, requiring the 

HFT to stop collecting data well before the end of the ramp. Although it was still possible 

to test many ramps with this configuration, several had to be eliminated for this reason. 
9 

CHAPTER 2. FRICTION CHANGE BEFORE AND AFTER HFST INSTALLATION

EVALUATION OVERVIEW

This chapter presents the results from an evaluation of friction data before and after HFST 

installation at HFST sites. Two sets of data were analyzed for this purpose: 

• Planned HFST sites where pre-HFST pavement friction data and postinstallation HFST 

friction data were measured over the same area. These sites were located in Georgia and 

Pennsylvania. In Louisiana, some planned sites were tested before HFST installation, but 

delays in HFST installation prevented postinstallation friction testing within the project 

timeframe. 

• Existing HFST sites where lead-in and lead-out pavement friction was used to estimate 

the friction of the underlying pavement. These sites were analyzed separately from the 

planned HFST sites because the lead-in and lead-out pavement friction was only an 

estimate of the pavement friction before HFST installation and likely changed with time

since the HFST was installed.

DATA SUMMARY

Table 2 summarizes friction change for sites in Georgia and Pennsylvania where pre- and 

post-HFST friction data were collected. All sites in Georgia were tested at 40 mph, and all sites in 

Pennsylvania were tested at either 30 mph or 35 mph because of site characteristics. All sites 

were asphalt pavement surfaces, and all were curve sites (no ramps). The project team measured 

the friction of the pavement surface before HFST installation over the approximate area where 

the HFST was to be installed, although the planned HFST installation area was not always well 

marked. The reported friction does not include lead-in and lead-out pavement for a true 

before-and-after comparison of friction data. 

Table 2. Summary of friction change for sites with pre- and post-HFST friction data.

State

Total 

Number of 

Sites

Nominal 

Test 

Speed

(mph)

Number of 

Sites at 

Nominal Speed

Pre-HFST 

Friction at 

Nominal Speed

(HFT Mu)

Post-HFST 

Friction at 

Nominal Speed

(HFT Mu)

Friction 

Change

(HFT Mu)

Friction 

Change 

(%) 

GA 85 40 85 0.58–0.87

Avg. = 0.73

0.74–1.0

Avg. = 0.96

+0.01–0.4

Avg. = 0.23

+1–65

Avg. = 33

PA 14 30 7 0.42–0.68

Avg. = 0.54

0.75–1.0

Avg. = 0.86

+0.11–0.55

Avg. = 0.32

+17–131

Avg. = 64

35 5 0.53–0.63

Avg. = 0.57

0.84–0.99

Avg. = 0.94

+0.27–0.46

Avg. = 0.37

+47–87

Avg. = 67

1 mph = 1.61 km/h.

Avg. = average.

Table 3 summarizes friction change for existing HFST sites using lead-in and lead-out friction 

data to estimate the underlying pavement friction. A total of 276 sites were tested, but for the 

purpose of comparing friction values, only the 191 sites tested at 40 mph are summarized in the 

table. These sites were predominantly curve locations, with a few ramps in Arkansas and 
10 

Kentucky that could be tested at 40 mph. All but 11 sites were asphalt pavement. Because of the 

wide range of pavement friction values, there was no distinguishable difference between concrete 

pavement and asphalt pavement sites. 

Table 3. Summary of friction change for sites using lead-in and lead-out pavement friction

data. 

State

Total 

Number 

of Sites

Number 

of 

HFT(40) 

Sites

Pavement 

Friction

(HFT(40)) 

HFST 

Friction 

(HFT(40)) 

Friction 

Change 

(HFT(40)) 

Friction 

Change 

(%) Pavement Type

AR 19 15 0.33–0.71

Avg. = 0.51

0.89–1.0

Avg. = 0.95

+0.25–0.67

Avg. = 0.44

+35–203

Avg. = 97

2 concrete

GA 42 40 0.48–0.76

Avg. = 0.60

0.84–1.0

Avg. = 0.98

+0.21–0.5

Avg. = 0.38

+32–105

Avg. = 65

All asphalt

KY 61 40 0.35–0.74

Avg. = 0.53

0.86–1.0

Avg. = 0.98

+0.26–0.65

Avg. = 0.45

+35–186

Avg. = 90

6 concrete

LA 27 27 0.41–0.49

Avg. = 0.51

0.89–1.0

Avg. = 0.97

+0.42–0.57

Avg. = 0.48

+74–138

Avg. = 100

All asphalt

PA 98 58 0.40–0.86

Avg. = 0.56

0.84–1.0

Avg. = 0.95

+0.13–0.55

Avg. = 0.39

+16–136

Avg. = 76

1 concrete, 

4 concrete/asphalt

WV 29 11 0.45–0.81

Avg. = 0.59

0.77–1.0

Avg. = 0.90

+0.06–0.44

Avg. = 0.32

+7–87

Avg. = 57

2 concrete

Avg. = average. 

Figure 6 shows pavement friction versus HFST friction for these 191 sites, providing an 

indication of the range of friction values for pavement versus HFST. 
11 

Source: FHWA.

Figure 6. Graph. Summary of friction change for sites using lead-in and lead-out friction 

data. 

OBSERVATIONS

The following are some of the key observations from the evaluation of friction data collected 

before and after HFST installation.

• HFST provided dramatic increases in pavement friction regardless of the friction of the 

existing pavement. Friction increased by between 1 and 131 percent, with an average 

increase of 33 percent for Georgia sites (which already had relatively high friction) and

67 percent for Pennsylvania sites tested at 35 mph (which had lower existing pavement 

friction). 

• Friction change for sites where lead-in and lead-out friction data were used to estimate the 

friction of the existing pavement was even more dramatic. Friction increased by between

7 and 203 percent, with an average increase of approximately 80 percent. However, the 

pavement friction values may not have accurately reflected actual pavement friction under 

the HFST before installation.
12 

• Although pre-HFST pavement friction for Pennsylvania sites was similar to the friction

estimated from lead-in and lead-out pavement, lead-in and lead-out pavement friction for 

Georgia sites was noticeably lower than pre-HFST pavement friction. 

• Friction values for HFST were consistently high across all sites in all States, with an 

average of more than 0.95 HFT Mu regardless of HFST age, pavement type, or site type 

(e.g., curve, ramp).
13 

CHAPTER 3. FRICTION CHANGE OVER TIME

EVALUATION OVERVIEW

This chapter discusses the evaluation of friction change over time for HFST and underlying 

pavement. Two sets of sites were evaluated for this purpose: 

• HFST sites where historical HFT friction data were available. Some HFST installations 

in the United States had previously been tested with the HFT and were tested again for 

this study. 

• HFST sites tested as part of the safety performance evaluation. The sites from the six 

States summarized in chapter 2 vary widely in age and provided valuable information on 

friction change over time even though initial friction values were not available. 

For each site, friction change was analyzed for both the HFST and the underlying pavement. For 

sites where pre-HFST friction data were not available, the friction of the underlying pavement 

was estimated from lead-in and lead-out pavement friction at each site. 

Friction change was analyzed in terms of age and traffic exposure. Friction change in terms of

age was evaluated by the raw age of the treatment and the change in friction between 

measurements. Friction change in terms of traffic exposure was based on total estimated traffic 

exposure, which was calculated by multiplying the average annual daily traffic (AADT) by 30 d 

by the number of months (either raw age or time between friction tests). Although this is not the 

exact traffic exposure because of changes in traffic volume over time, it should be within 

10 percent of actual and is primarily intended to demonstrate the relative order of magnitude of 

traffic exposure for each HFST site. Note that this calculation does not account for the mix of 

traffic (passenger vehicles versus trucks), which can have a significant impact on HFST wear, but 

still provides a better indication of the impact of traffic than age alone.

Table 4 lists the sites with historical HFT friction data that were evaluated. This table indicates 

HFST installation date and when pre-HFST and post-HFST friction data were collected. Traffic 

volumes (measured by AADT) for each site are also provided. Note that traffic data shown in 

table 4 are estimates for each lane at a particular site, which were calculated by dividing the total 

AADT for the roadway by the number of lanes. Although this averaging may result in slight 

underestimation or overestimation because of unequal directional or lane distribution factors, the 

intent is to show the relative order of magnitude of traffic exposure at the various sites. Ramp

traffic data for sites H03 and H04 were not available and were estimated based on intersecting 

mainline traffic data and ramp traffic data for similar interchanges in the vicinity of these sites.
14 

Table 4. Summary of HFST sites with historical friction data.

State

Site 

ID Site Type

AADT1

(yr) 

HFST 

Installation 

(mo/yr)

Pre-HFST Installation 

HFT Friction Data 

(mo/yr)

Post-HFST 

Installation HFT 

Friction Data 

(mo/yr)

OK H01 Rural road curve, 

EB and WB lanes

650 

(2012)

12/13 12/13 12/13, 12/14, 7/17

H02 Rural road curve, 

EB and WB lanes

280 

(2017)

3/14 12/13 12/14, 7/17

AL H03 Urban single-lane 

connector ramp

10,8002

(2017)

5/14 4/14 12/14, 7/17

H04 Urban single-lane 

connector ramp

14,0002

(2017)

5/14 4/14 12/14, 7/17

WI H05 Urban two-lane

connector ramp

19,050 

(2019)

9/11 NA 6/12, 8/13, 12/14, 3/18

IA H06 Urban freeway curve, 

EB and WB, 6 lanes

10,833 

(2017)

6/12 6/12

(NB lanes only)

9/12, 8/13, 12/14, 3/18

H07 Urban single-lane 

connector ramp

6,200 

(2017)

6/12 6/12 9/12, 8/13, 3/18

H08 Urban single-lane 

connector ramp

13,900 

(2017)

6/12 6/12 9/12, 8/13, 3/18

H09 Urban single-lane 

connector ramp

10,100 

(2017)

6/12 6/12 9/12, 8/13, 3/18

GA H10 Rural road curve, 

EB and WB lanes

1,050 

(2017)

4/14 4/14

(WB lane only)

4/14, 12/14, 4/17

MI H11 Urban two-lane exit 

ramp

4,563 

(2009)

9/10 NA 6/12, 8/13, 12/14

1

Estimated AADT by lane based on total AADT for the roadway divided by the number of lanes.

2

Ramp AADT for H03 and H04 sites was estimated using mainline traffic data and similar interchanges.

EB = eastbound; NA = not available; NB = northbound; WB = westbound.

DATA SUMMARY

HFST Sites With Historical Friction Data

Table 5 summarizes the friction change for the HFST sites with historical friction data. Because 

friction data were not available for all sites immediately after installation, this table shows the 

change in friction from the earliest to the latest HFT measurements at the site, corresponding 

initial friction, and number of months between the initial and final tests. Table 5 also shows an 

estimate of total traffic exposure on the HFST between the two tests. 

Test speeds were not always the same during the initial and final tests, as indicated in the last 

column of the table. This difference in speed may have affected the reported friction change 

because friction generally measures lower at higher speeds, particularly for conventional 

(i.e., non-HFST) pavements, and the data should be viewed with this in mind. 
15 

Table 5. Friction change for HFST sites with historical friction data. 

State Site ID

Time 

Between 

Tests 

(mo)

Estimated Traffic 

Exposure 

(Millions of Vehicles)

Initial 

HFST 

Friction 

(HFT Mu)

Friction 

Change 

(HFT Mu)

Friction 

Change

(%) 

Test Speed, 

Initial/Final 

(mph) 

OK H01, EB 43 0.84 0.90 0.10 11.1 30/40

H01, WB 43 0.84 0.82 −0.04 −4.9 30/40

H02, EB 31 0.26 1.00 0.00 0.0 30 /35

H02, WB 31 0.26 0.62 0.06 9.7 30/35

AL H03 31 10.04 1.00 0.00 0.0 30

H04 31 13.02 1.00 −0.05 −5.0 30/40

WI H05, L1 69 39.43 0.96 −0.11 −11.5 40

H05, L2 69 39.43 0.96 −0.17 −17.7 40

IA H06, NB L1 65 21.12 1.00 −0.05 −5.0 40

H06, NB L2 65 21.12 1.00 −0.10 −10.0 40

H06, NB L3 65 21.12 1.00 −0.06 −6.0 40

H06, SB L1 66 21.45 0.99 −0.09 −9.1 40

H06, SB L2 67 21.77 0.92 −0.08 −8.7 40

H06, SB L3 67 21.77 0.94 −0.07 −7.4 40

H07 66 12.28 1.00 −0.01 −1.0 40

H08 66 27.52 1.00 −0.09 −9.0 40

H09 66 20.00 1.00 −0.07 −7.0 40

GA H10, EB 36 1.13 1.00 0.00 0.0 40

H10, WB 28 0.88 1.00 0.00 0.0 40

MI H11, L1 30 4.11 0.85 0.05 5.9 30

H11, L2 30 4.11 0.84 −0.01 −1.2 30

1 mph = 1.61 km/h.

EB = eastbound; L1 = lane 1; L2 = lane 2; L3 = lane 3; NB = northbound; SB = southbound; WB = westbound.

Table 6 summarizes the estimated friction change for the underlying pavement at each site. 

Pre-HFST friction data were available (within 1–2 mo before installation) for almost all sites 

except those indicated with an asterisk. For these sites, lead-in and lead-out friction data from the 

first round of HFT testing at the site were used to estimate initial underlying pavement friction. 

For sites H05 and H10 (eastbound (EB)), these data were collected approximately 8 and 9 mo 

after HFST installation, respectively. For site H11, these data were collected approximately 

21 mo after HFST installation. For all sites, the pavement friction after HFST installation was 

estimated from lead-in and lead-out friction data. The number of months between tests, for which 

the friction change is reported, is shown for all sites regardless of whether the pavement friction 

data were collected before HFST installation. 
16 

Table 6. Estimated friction change for underlying pavement. 

State Site ID

Time 

Between 

Tests 

(mo)

Estimated Traffic 

Exposure 

(Millions of Vehicles)

Initial 

Pavement 

Friction 

(HFT Mu)

Pavement 

Friction Change 

(HFT Mu)

Friction 

Change

(%) 

Test Speed, 

Initial/Final 

(mph)

OK H01, EB 43 0.84 0.59 −0.13 −22.0 30/40

H01, WB 43 0.84 0.57 −0.10 −17.5 30/40

H02, EB 43 0.36 0.80 −0.22 −27.5 30/35

H02, WB 43 0.36 0.67 −0.08 −11.9 30/35

AL H03 39 12.64 0.59 −0.01 −1.7 30

H04 39 16.38 0.53 −0.10 −18.9 30/40

WI H05, L1* 69 39.43 0.53 −0.01 −1.9 40

H05, L2* 69 39.43 0.51 0.03 5.9 40

IA H06, NB L1 69 22.42 0.59 −0.12 −20.3 50/40

H06, NB L2 69 22.42 0.28 0.03 10.7 50/40

H06, NB L3 69 22.42 0.40 0.03 7.5 50/40

H06, SB L1 69 22.42 0.62 −0.19 −30.6 50/40

H06, SB L2* 69 22.42 0.29 −0.03 −10.3 50/40

H06, SB L3* 69 22.42 0.31 −0.03 −9.7 50/40

H07 69 12.83 0.58 −0.15 −25.9 40

H08 69 28.77 0.58 −0.19 −32.8 40

H09 69 20.91 0.65 −0.22 −33.8 40

GA H10, EB* 36 1.13 0.72 −0.05 −6.9 40

H10, WB 36 1.13 0.82 −0.15 −18.3 40

MI H11, L1* 30 4.11 0.54 −0.04 −7.4 30

H11, L2* 30 4.11 0.50 0.01 2.0 30

1 mph = 1.61 km/h.

*Sites where lead-in and lead-out friction data from the first round of HFT testing at the site were used to estimate 

initial underlying pavement friction. 

L1 = lane 1; L2 = lane 2; L3 = lane 3; NB = northbound; SB = southbound; WB = westbound.

Figure 7 and figure 8 show HFST friction values versus age and total traffic exposure, 

respectively, for the sites with historical friction data. Friction values are shown for all friction 

data collected at various dates (table 4), including multiple measurements over time at a given 

location. Therefore, one site may be represented by several data points, depending on how many 

times the friction was measured. Note that friction data in both plots are delineated by test speed

(30, 40, or 50 mph). 
17 

Source: FHWA.

Figure 7. Graph. HFST friction versus age for sites with historical friction data.
18 

Source: FHWA.

Figure 8. Graph. HFST friction versus estimated traffic exposure for sites with historical 

friction data.

HFST Sites From Safety Performance Evaluation 

The sites from the six States tested as part of the HFST safety performance evaluation were also 

evaluated for friction change over time. Although it was not possible to document actual friction 

change over time for these sites because only one set of friction data was collected, these sites 

indicate HFST friction variation with age and traffic because it is reasonable to assume that most 

of these sites would have had a friction value close to 1.0 when new. For the purpose of 

comparing friction values within a State and from State to State, only sites tested at a speed of 

40 mph (±2 mph) were evaluated. 

Table 7 summarizes the HFST sites included in this evaluation, including many that were tested 

just after HFST installation. The table shows the total number of sites by State and the number of 

those sites tested at 40 mph. For each State, the table shows the range and average values of the 

following measurements: friction, age of the HFST at the time of testing, AADT, and total 

estimated traffic exposure. Exact installation dates were not available for sites in Louisiana, but a 

range of estimated traffic exposure was provided based on an age range of 1–18 mo. 
19 

HFST sites ranged in age from 1 mo to nearly 10 yr, with the majority being less than 3 yr old. 

Estimated traffic exposure ranged from approximately 10,000 vehicles to more than 28 million. 

These variations provided a wide range of age and traffic exposure for evaluating HFST friction 

performance.

Table 7. Summary of safety performance evaluation HFST sites. 

State

Total 

Number of 

Sites

Number of 

HFT(40) Sites

HFT(40) Friction

Range and Average

Age of 

HFST at 

Testing 

(mo)

AADT for 

HFST Sites

Estimated Total 

Traffic Exposure 

(Millions of Vehicles)

AR 19 17 0.84–1.0

Avg. = 0.95

11–24

Avg. = 16

1,700–30,000

Avg. = 9,635

0.39–5.85

Avg. = 2.62

GA 127 125 0.74–1.0

Avg. = 0.97

2–27

Avg. = 7

160–15,600

Avg. = 2,540

0.01–5.35

Avg. = 0.51

KY 62 44 0.84–1.0

Avg. = 0.97

18–78

Avg. = 49

1,024–69,619

Avg. = 7,897

0.51–28.31

Avg. = 5.6

LA 27 27 0.89–1.0

Avg. = 0.97

<18 1,190–4,800

Avg. = 2,412

0.04–2.59* 

PA 112 61 0.84–1.0

Avg. = 0.95 

1–119

Avg. = 23

1,080–219,752

Avg. = 10,074

0.04–25.27

Avg. = 2.54

WV 29 11 0.77–1.0

Avg. = 0.90 

5–73

Avg. = 44

2,335–107,000

Avg. = 36,890

0.32–20.33

Avg. = 8.2

*Based on estimated age range for HFST installations because exact installation dates were not available to 

determine exact age. 

Avg. = average.

Figure 9 shows a plot of friction values by age (at the time of friction testing) for the sites in 

table 7. Figure 10 shows a plot of friction values by estimated traffic exposure (in millions of 

vehicles) for these sites, and figure 11 shows a truncated version of the data in figure 10 for sites 

with less than 3 million vehicles. The 27 sites from Louisiana are not included in figure 9 through 

figure 11 because of the unavailability of actual installation dates.
20 

Source: FHWA.

Figure 9. Graph. HFST friction versus age for safety performance evaluation sites. 
21 

Source: FHWA.

Figure 10. Graph. HFST friction versus estimated traffic exposure for all safety 

performance evaluation sites.
22 

Source: FHWA.

Figure 11. Graph. HFST friction versus estimated traffic exposure (safety performance 

evaluation sites with less than 3 million vehicles).

OBSERVATIONS

The following are some of the key observations from the evaluation of friction change over time. 

• This evaluation of HFST sites with historical friction data and HSFT sites from the safety 

performance evaluation revealed that HFST maintained a high level of friction over time 

and after exposure to high traffic volumes. 

• For the sites with historical friction data, HFST friction change varied, with some sites 

even showing a slight increase in friction. The majority of sites showed a decrease in 

friction, but most decreases were less than 10 percent over periods ranging from 2.5 to 

5.5 yr with varying levels of traffic exposure. 

• The largest decrease in HFST friction was a 17.7-percent decrease (HFT Mu value of 

−0.17) over a nearly 6-year period with exposure to more than 39 million vehicles, 

demonstrating exceptional long-term friction performance under heavy traffic.
23 

• For sites tested as part of the safety performance evaluation, HFST friction was generally 

between 0.8 and 1.0 HFT Mu from a few months after application through nearly 10 yr. 

Although there did appear to be a decrease in friction with age, this was not unexpected, 

and the latter-age friction was still very good. 

• Likewise, HFST friction based on traffic exposure remained high (between 0.8 and 

1.0 HFT Mu) in even the highest traffic exposure condition, which was more than 

28 million vehicles. 

25 

CHAPTER 4. FRICTION CHANGE WITHIN CURVES

EVALUATION OVERVIEW

This chapter discusses observations from a limited evaluation of friction change within curves for 

HFST sites tested as part of the safety performance evaluation. The purpose of this evaluation 

was to see whether any obvious change or variation in friction occurred within curves as a result

of higher wear due to higher friction demand in curves. Because the HFT provides a continuous 

measurement of friction reported every 0.30 m (1 ft), it is possible to measure friction variation 

within a curve.

A limited number of sites were evaluated for friction change within curves. The data collected 

were generally found to be inadequate to state conclusively how much friction change occurs 

within a curve, even with 91.44–152.40 m (300–500 ft) of lead-in or lead-out friction data. The 

focus of this evaluation was existing pavement tested before HFST installation because the 

lead-in or lead-out pavement friction cannot be compared to the pavement friction in the curve. In 

addition, detecting friction change within HFST sections was found to be more difficult because 

of the limited amount of data from such short sections of HFST. 

Because the majority of sites tested were low-volume rural road curves, tire-induced wear on 

these curves was likely not as significant as it could be on higher volume roadways. 

Unfortunately, pavement age and superelevation data were not available for these sites to assess

the impact of these factors on friction change.

A key observation from the HFT data was that friction measurement appeared to be influenced by 

the direction of the curve (right-hand versus left-hand). This influence may have been caused by 

the dynamics of the HFT when it traversed tight-radius curves coupled with the fact that the test 

wheel was on one side of the vehicle in the left wheel path. Further discussion on this observation 

is provided in the next section. 

For comparison, sites selected for evaluation were those tested at a consistent speed of 40 mph 

(±2 mph). 

DATA SUMMARY

Figure 12 shows an example of friction change within a curve for a site tested before HFST 

installation. This site had asphalt pavement with a curve radius of approximately 182.88 m 

(600 ft) and an AADT of 210. There appeared to be a slight drop in friction through the middle of 

the curve in both directions, but the variability of friction measurement and the limited amount of 

data outside of the curve made quantification of the friction change difficult to establish with 

certainty. Figure 13 shows friction data from this same curve after HFST installation. There was 

no obvious change in friction within the curve, although there was a slight decrease in friction 

over a short area in both directions just past the apex of the curve. The friction of the EB lane was 

highly variable, making it difficult to identify friction change within the curve. The lower friction 

in the EB lane was likely due to the lower friction of the HFST but may also have been partially 

influenced by the dynamics of the HFT device, which generally measures lower friction on 

left-hand curves. 
28 

Source: FHWA.

1 ft = 0.30 m.

SD = standard deviation; WB = westbound.

Figure 12. Graph. Friction measurement within a curve before HFST installation.
29 

Source: FHWA. 

1 ft = 0.30 m.

SD = standard deviation; WB = westbound.

Figure 13. Graph. Friction measurement within a curve after HFST installation.

Figure 14 shows an example of friction change within a curve with a slightly larger curve radius 

(approximately 304.8 m (1,000 ft)) and the same traffic volumes. This curve turned in the 

opposite direction (i.e., the left-hand curve was in the westbound (WB) direction). Any change in 

friction within the curve was less noticeable at this site, possibly because of the larger curve 

radius. Figure 15 shows this same curve after HFST installation, and there was no obvious 

change in friction within the curve. However, WB friction was lower than EB friction, which was 

likely caused by the slightly lower friction of the HFST but may also have been influenced by the 

HFT measurement being lower on left-hand curves. 
30 

Source: FHWA.

1 ft = 0.30 m.

SD = standard deviation.

Figure 14. Graph. Friction measurement within a curve (larger curve radius) before HFST 

installation.
31 

Source: FHWA.

1 ft = 0.30 m.

SD = standard deviation.

Figure 15. Graph. Friction measurement within a curve (larger curve radius) after HFST 

installation. 

Figure 16 shows an example of friction change from asphalt pavement (before HFST installation) 

within a curve with a radius of approximately 121.92 m (400 ft), which is one of the lowest curve 

radii of the sites tested, and a higher AADT of approximately 3,300 vehicles per day. There 

appeared to be a slight drop in friction within the curve that was more noticeable in the WB 

direction, but the variability of the measurements in the EB lanes made it difficult to determine 

the change in friction exactly. 
32 

Source: FHWA.

1 ft = 0.30 m.

SD = standard deviation.

Figure 16. Graph. Friction measurement within a curve (asphalt pavement) before HFST 

installation.

To further explore the effects of the HFT configuration on friction measurements, figure 17 and 

figure 18 show friction data for an S-curve before and after HFST installation, respectively. 

Figure 17 shows friction data for the underlying asphalt pavement on a low-volume (210 AADT) 

S-curve with a radius of approximately 124.97 m (410 ft). Figure 18 shows friction data for this 

curve after HFST installation. Both figures show a difference in friction based on the direction of 

travel and direction of the curve; the line of the curve reverses when transitioning from a left-hand 

to a right-hand curve or vice versa. Lower friction was measured on left-hand curves, regardless 

of the direction of travel. Figure 19 shows a similar trend for another S-curve tested before HFST 

installation. This S-curve had a higher traffic volume (6,200 AADT) and a slightly larger curve 

radius of 201.17 m (660 ft). 
33 

Source: FHWA.

1 ft = 0.30 m.

SD = standard deviation.

Figure 17. Graph. Friction measurement within an S-curve (lower AADT volume) before

HFST installation.
34 

Source: FHWA.

1 ft = 0.30 m.

SD = standard deviation.

Figure 18. Graph. Friction measurement within an S-curve (lower AADT volume) after 

HFST installation.
35 

Source: FHWA.

1 ft = 0.30 m.

SD = standard deviation.

Figure 19. Graph. Friction measurement within an S-curve (higher AADT volume) before 

HFST installation.
34 

OBSERVATIONS

The following are key observations related to friction change within a curve based on the limited 

amount of data collected with the HFT for this study. 

• The cause of the variation in friction based on direction of curvature is not known for 

certain but could be inherent to the configuration of the HFT measurement device. Side 

forces on the test wheel, located on the left side of the HFT, coupled with the HFT’s 

shifting center of gravity around tight-radius curves, likely affect friction measurement, as 

observed from analysis of friction data within S-curves. However, this observation is one 

of the reasons friction data were summarized as the average value over the curve in both 

directions in earlier chapters.

• An analysis of friction data from a select number of curve sites before HFST installation 

revealed possible variation in friction within the curve, but this variation was not 

consistent across all sites. Whether this variation in friction is caused by increased traffic 

wear in the curve cannot be determined with certainty. Unfortunately, the age of the 

asphalt pavement, and therefore the total traffic exposure of the pavement, was not 

available for inclusion in the evaluation. 

• For the limited analysis of friction data, curve radius and traffic volume did not appear to 

correlate with possible friction variation within a curve. However, superelevation and 

pavement age were not factored into the evaluation. Further investigation on roadways 

with higher volumes, varying curve radii, known superelevation, and pavements of similar 

age may reveal clearer relationships between pavement friction, geometry, and traffic 

exposure. 

• An analysis of friction data from sites after HFST installation was also inconclusive in 

terms of whether variation in friction was the result of traffic wear or lower initial friction 

of the HFST surface itself, particularly when there were differences based on direction of 

the curve. 

• In summary, although there appeared to be some variation in friction within curves, it is 

not known whether the variation is actual change in pavement friction or caused by the 

variability of friction measurements using the HFT device. Further investigation to 

compare measurements from different fixed-slip and full-slip devices and under varying 

conditions, including different grades, superelevations, curve radii, and pavement types, 

may provide more insight on friction change within curves.
35 

ACKNOWLEDGMENTS

The authors gratefully acknowledge the contributions from the various Evaluation of Low-Cost 

Safety Improvements Pooled Fund Study (ELCSI-PFS) States that provided valuable input for the 

completion of this study. The authors specifically acknowledge the following PFS and non-PFS 

States for providing data that were used or considered in the study: Arkansas, Florida, Georgia, 

Iowa, Kentucky, Louisiana, Oklahoma, Pennsylvania, South Carolina, Tennessee, Texas, 

Wisconsin, and West Virginia. 

37 

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2. ASTM International. (2011). Standard Test Method for Skid Resistance of Paved Surfaces 

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04.03, West Conshohocken, PA. 

3. ASTM International. (2006). Standard Test Method for Measuring the Skid Resistance of 

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Technique, ASTM E2340-06, Annual Book of ASTM Standards, 04.03, 

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4. ASTM International. (2016). Standard Specification for a Size 4.00-8 Smooth Tread 

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