PAVEMENT FRICTION 

FOR ROAD SAFETY 

Primer on Friction Measurement and Management Methods

PUBLICATION NO. FHWA SA-23-007 
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This page has been intentionally left blank.
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FOREWORD

Pavements must have sufficient available friction to allow a driver to properly execute a maneuver, 

especially those that involve steering, braking or acceleration. In 2021, FHWA added Pavement 

Friction Management, which involves measuring, monitoring, and maintaining pavement friction, 

to the list of Proven Safety Countermeasure because it can help prevent roadway departure, 

intersection, and pedestrian-related crashes. Additionally, the integration of pavement friction 

management and safety management practices in order to achieve safety performance goals is 

consistent with the Safe System approach.

This primer is intended as a resource for road safety, maintenance, materials, and pavement 

engineers and practitioners to provide a basic understanding of pavement surface friction in order 

to integrate pavement friction into both pavement asset management and road safety 

management practices. The primer covers pavement friction characteristics such as microtexture, 

macrotexture, and pavement friction demand. It also discusses different methods of measuring 

friction and macrotexture, including using continuous pavement friction measurement for 

collecting friction data at the network-level for safety performance analysis. 

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 contents of this report reflect the

views of the contractor, who is responsible for the accuracy of the data presented herein. The

contents do not necessarily reflect the official policy of the USDOT. This report does not 

constitute a standard, specification, or regulation.

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

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

objective of this report. 

Non-Binding Content

The contents of this document do not have the force and effect of law and are not meant to bind 

the public in any way. This document is intended only to provide clarity to the public regarding 

existing requirements under the law or agency policies. While this is non-binding guidance, you 

must comply with the applicable statutes or regulations.

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. The FHWA periodically reviews quality issues and adjusts its programs and 

processes to ensure continuous quality improvement. 
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Technical Report Documentation Page

1. Report No.

FHWA-SA-23-007

2. Government Accession No.

N/A

3. Recipient's Catalog No.

N/A

4. Title and Subtitle

Pavement Friction for Road Safety: Primer on Friction Measurement 

and Management Methods

5. Report Date

February 2023

6. Performing Organization Code

N/A

7. Author(s)

Priscilla Tobias, Edgar de León Izeppi, Gerardo Flintsch, Samer 

Katicha, and Ross McCarthy

8. Performing Organization Report No.

N/A

9. Performing Organization Name and Address

Virginia Polytechnic Institute and State University

Arora and Associates, P.C.

10. Work Unit No. (TRAIS)

N/A

11. Contract or Grant No.

DTFH61-16-D-00041L

12. Sponsoring Agency Name and Address

Federal Highway Administration

Office of Safety

1200 New Jersey Ave SE

Washington, DC 20590

13. Type of Report and Period Covered

Final; August 2020–February 2023

14. Sponsoring Agency Code

N/A

15. Supplementary Notes

This report was produced under the direction of Jeffrey Shaw (FHWA Office of Safety).

16. Abstract

Pavements must have sufficient available friction to allow a driver to properly execute a maneuver, especially those that 

involve steering, braking or acceleration. In 2021, FHWA added Pavement Friction Management, which involves 

measuring, monitoring, and maintaining pavement friction to the list of FHWA Proven Safety Countermeasure because it 

can help prevent roadway departure, intersection, and pedestrian-related crashes. Additionally, integration of pavement 

friction management and safety management practices in order to achieve safety performance goals is consistent with the 

Safe System approach. 

This primer is intended as a resource for road safety, maintenance, materials, and pavement engineers and practitioners to 

provide a basic understanding of pavement surface friction in order to integrate pavement friction into both pavement asset 

management and road safety management practices. The primer covers pavement friction characteristics such as 

microtexture, macrotexture, and pavement friction demand. It also discusses different methods of measuring friction and 

macrotexture, including continuous pavement friction measurement for collecting friction data at the network-level for 

safety performance analysis. 

17. Key Words

Safe System Approach, Crashes, Friction, Pavement 

Friction Management, Microtexture, Macrotexture, 

Continuous Pavement Friction Measurement (CPFM), 

Sideway-Force Coefficient Routine Investigation 

Machine (SCRIM), Locked-wheel skid tester (LWST)

18. Distribution Statement

No restrictions. This document is available to the public.

19. Security Classification (of this report)

Unclassified

20. Security Classification (of this page)

Unclassified

21. No. of Pages

24 

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized.
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SI* (MODERN METRIC) CONVERSION FACTORS

* SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 

of ASTM E380. (Revised March 2003, Section 508-accessible version September 2009.)
v 

TABLE OF CONTENTS

LIST OF FIGURES ................................................................................................................................... vi

LIST OF ABBREVIATIONS ................................................................................................................... vii

1 INTRODUCTION ............................................................................................................................... 1

2 PAVEMENT FRICTION CHARACTERISTICS............................................................................... 2

2.1 MICROTEXTURE....................................................................................................................... 2

2.2 MACROTEXTURE ..................................................................................................................... 3

2.3 FRICTION DEMAND ................................................................................................................. 4

2.4 PAVEMENT FRICTION AND CRASHES ................................................................................ 5

3 FRICTION MEASUREMENT EQUIPMENT ................................................................................... 6

3.1 FRICTION MEASUREMENTS .................................................................................................. 6

3.2 MACROTEXTURE MEASUREMENTS ................................................................................... 8

4 PAVEMENT FRICTION MEASUREMENT TO SUPPORT SAFETY MANAGEMENT ............ 11

4.1 NON-CONTINOUS PAVEMENT FRICTION MEASUREMENT ......................................... 11

4.2 CONTINOUS PAVEMENT FRICTION MEASUREMENT (CPFM) ..................................... 11

4.3 SAFETY MANAGEMENT ....................................................................................................... 11

5 ADDITIONAL INFORMATION ..................................................................................................... 13

6 REFERENCES .................................................................................................................................. 14
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LIST OF FIGURES 

Figure 1. Illustration. Pavement surface texture characteristics that influence friction. ............................. 2

Figure 2. Diagram. “Three Zone Concept” of a wet surface. ..................................................................... 3

Figure 3. Diagram. Conceptual relationship between friction demand, speed, and friction availability. ... 4

Figure 4. Photo. Example of a slider: British Pendulum Tester. ................................................................ 6

Figure 5. Photo. Example of LFC: Fixed slip device-grip tester. ............................................................... 7

Figure 6. Photo. Example of LFC: 100% slip-LWST. ............................................................................... 7

Figure 7. Photo. Example of SFC: Sideway-force Coefficient Routine Investigation Machine (SCRIM). 8

Figure 8. Photo. Static single spot laser: Circular Track Meter. ................................................................. 9

Figure 9. Photo. Static line laser: Texture Analyzer. .................................................................................. 9

Figure 10. Photo. High-speed single spot laser. ........................................................................................ 10

Figure 11. Photo. High-speed laser macrotexture. .................................................................................... 10
vii

LIST OF ABBREVIATIONS

AASHTO American Association of State Highway and Transportation Officials

CFME continuous friction measuring equipment 

CFR Code of Federal Regulations 

CPFM continuous pavement friction measurement

CSTI Center for Sustainable Transportation Infrastructure

DGAC dense-graded asphalt 

DOT Department of Transportation 

FHWA Federal Highway Administration

GPF Guide for Pavement Friction

HFST high friction surface treatment

LFC longitudinal friction coefficient

LWST locked wheel skid tester

MPD mean profile depth

MTD mean texture depth 

NCHRP National Cooperative Highway Research Program 

NCSA National Center for Statistics and Analysis

NHTSA National Highway Traffic Safety Administration

OGFC open-graded friction course

PFM pavement friction management

RSA road safety audit 

RMS root mean square 

SFC sideway force coefficient

U.S. United States
1 

1 INTRODUCTION

Pavement friction, often referred to as “skid resistance,” is one of many factors influencing road safety 

performance. According to the National Highway Traffic Safety Administration (NHTSA), in 2021 there 

were 42,915 people killed in traffic related crashes in the United States (U.S.) (NCSA, 2022). The potential 

for crashes increases when the available friction does not allow the driver to properly execute a maneuver, 

especially those that involve steering, braking, or acceleration. Excessive speed, road characteristics, and 

pavement surface conditions can also factor into higher crash rates where friction demand is higher than 

what is available. Pavement friction is also explicitly considered in certain aspects of highway geometric 

design, such as horizontal curvature, superelevation, and stopping sight distance (Hall et al., 2009). 

Pavement friction characteristics will vary over time and location due to a variety of reasons. Therefore, 

pavement friction should be managed throughout a pavement lifecycle, up until the very end-of-life.

Pavement friction management (PFM), which involves measuring, monitoring, and maintaining pavement 

friction, is a Federal Highway administration (FHWA) Proven Safety Countermeasure (FHWA, 2021) 

because it can help prevent roadway departure, intersection, and pedestrian-related crashes. It is also 

consistent with the Safe System approach. First, it involves a shared responsibility, in this case among 

pavement, materials and safety professionals. Second, it is a proactive approach to safety where the friction 

measurement becomes the basis for treatment or intervention rather than a reactive approach that relies on 

crash occurrence. Third, it can provide system redundancy that further supplements the vehicle and tire 

systems designed to prevent crashes. The Safe System approach, along with continuing improvements to 

pavement materials and technologies, represents an opportunity to further integrate pavement friction into 

both pavement asset management and road safety management practices.

This primer is intended to provide road safety, maintenance, materials, and pavement engineers and 

practitioners a basic understanding of the design, construction, maintenance, and management of

pavement surface friction in order to meet safety performance goals. The primer covers the following 

basic topics of pavement friction:

• Pavement Friction Characteristics

• Pavement Friction Testing Methods and Equipment 

• Pavement Friction Management to Support Safety 

The FHWA pavement policy found in Part 626 of Title 23 of the 

Code of Federal Regulations (23 CFR 626) states: “Pavement shall

be designed to accommodate current and predicted traffic needs in 

a safe, durable, and cost-effective manner” (Pavement Policy, 

1996). It follows that pavement friction—skid resistance—is an 

inherent component of a pavement that accommodates traffic 

needs in a safe manner. 

“Pavement shall be 

designed to accommodate 

current and predicted traffic 

needs in a safe, durable, and 

cost-effective manner.”

(23 CFR 626 Pavement Policy) 
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2 PAVEMENT FRICTION CHARACTERISTICS

Pavement friction is defined by the interaction between the tire and the pavement; specifically, the force 

that resists the relative motion between a vehicle tire and a pavement surface (Hall et al., 2009). This 

interaction allows drivers to safely perform vehicle steering, braking, and acceleration maneuvers. It 

includes a lateral or side force that occurs as a vehicle changes direction or compensates for pavement 

cross-slope and/or crosswind effects. The two principal frictional force components are adhesion and 

hysteresis. Adhesion is the friction that results from the small-scale bonding of the vehicle tire rubber and 

the pavement surface as they come into contact with each other. Hysteresis is the friction component that 

results from the tire compressing against the pavement surface. Surface texture influences both 

components. Pavement surface characteristics that influence pavement friction include microtexture and 

macrotexture, as shown on Figure 1. 

Figure 1. Illustration. Pavement surface texture characteristics that influence friction. 

Source: CSTI

2.1 MICROTEXTURE

Microtexture is the fine-scale texture (below 0.5 mm) on the surface of the aggregate in asphalt or the 

sand in cement concrete that interacts directly with the tire rubber on a molecular scale to provide 

adhesion. In non-technical terms, this is similar to the texture felt for each individual particle when rubbing 

one’s hand across sandpaper. Microtexture is important for pavement friction on both wet and dry 

roadways and, at any speed, to allow vehicles to properly maneuver and stop. 

The gradual loss of microtexture due to the effects of traffic over time is described as “polishing.” Some 

agencies have different pavement materials requirements for “non-polishing” aggregate to be used in 

certain cases. Roads with higher friction demand may require more “non-polishing” aggregate in the 

pavement mixture than roads with lower friction demand, which recognizes the value and importance of 

microtexture. High friction surface treatment (HFST) is an example of a pavement surface with very high 

microtexture. Per American Association of State Highway and Transportation Officials (AASHTO) 

Standard Specification MP-41, HFST uses calcined bauxite, a manufactured aggregate highly resistant to 

polish. When used by road agencies in spot locations at high friction demand locations, HFST resulted in 

significant crash reductions in both wet and dry crashes (von Quintus and Mergenmeier, 2015) 
3 

2.2 MACROTEXTURE

Macrotexture is the larger scale texture (0.5 to 50 mm) on the pavement formed by the individual aggregate 

particles in the surface or by grooves cut into the surfaces. Macrotexture is influenced by either the mixture 

design (aggregate shape, size, and gradation) for asphalt pavements or method of finishing (dragging, 

tining, grooving; depth, width, spacing and orientation of the grooves/channels) for concrete pavements. 

The macrotexture provides drainage channels for water to be expelled between the tire and the pavement

which allows for better contact of the tire with the pavement, improving frictional resistance and 

preventing hydroplaning, especially at higher speeds. The level of macrotexture needed to provide 

frictional resistance depends on the speed limit of the roadway. Hall et al. (2009) found that at speeds 

above 56 mph on wet pavements, macrotexture is responsible for a large portion of the friction. Measuring 

and evaluating macrotexture is essential when analyzing the safety performance of a roadway, especially 

for higher speed facilities. 

Most highway agencies do not collect or analyze measurements of macrotexture at the network level. In 

2021, the National Cooperative Highways Research Project (NCHRP) Research Report 964 Protocols for 

Network-Level Macrotexture Measurement, developed standards that were approved by AASHTO 

(AASHTO MP 48, PP 115, PP116) related to assessing quality in the measurement of macrotexture at 

highway speeds (Flintsch et al., 2021). This study found that the mean profile depth (MPD) is currently 

the most widely used macrotexture measurement in the U.S. A project for the North Carolina Department 

of Transportation (DOT) provided post-construction MPD recommendations for roads based on posted 

speed limits; specifically, for 50 mph at least 0.80 mm, and for 70 mph (or higher) at least 1.0 mm (de 

León et al., 2017). In the U.S., normal ranges of macrotexture for a dense graded asphalt concrete (DGAC) 

have MPDs between 0.40 and 0.80 mm, whereas an open graded friction course (OGFC) will have MPDs 

between 0.80 to 1.40 mm. The value of higher macrotexture for high-speed roads is demonstrated by 

Florida DOT’s requirement to use open-graded friction courses (OGFC, FC-5) to provide frictional 

characteristics to the asphalt pavement surface on high-speed multi-lane roads with speed limits greater 

than 50 mph (Florida DOT, 2022). 

To better visualize the role of texture within the contact region of a tire on a wet pavement the “Three 

Zone Concept”, first suggested by Gough and later extended by Moore, is shown in Figure 2 (Moore, 

1966). In Zone 1, water is squeezed out by the macrotexture of the pavement surface. In Zone 2, the 

microtexture provides the drainage for the thin film of moisture. In Zone 3, the tire comes into dry contact 

with the pavement’s surface. It is in this last zone, that the forces of adhesion and hysteresis must combine 

to overcome the friction demand. 

Figure 2. Diagram. “Three Zone Concept” of a wet surface. 

Source: Moore, 1966
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2.3 FRICTION DEMAND 

Pavements must have sufficient available friction to allow a vehicle on a roadway to safely perform 

steering, braking, and acceleration maneuvers. The level of pavement friction needed will vary across the 

roadway network and at specific locations due to changing geometrics, site conditions, traffic 

characteristics, and driver/vehicle characteristics. In 1967, NCHRP Report 37 acknowledged that 

“because the intensity of the polishing process increases markedly with tread element slip, all other factors 

being equal, the lowest friction levels are found on high-speed roads, curves, and approaches to 

intersections; in short, in locations at which high friction values are needed most” (Kummer and Meyer, 

1967). Figure 3 shows the conceptual relationship between friction demand, speed, and friction supply. 

Figure 3. Diagram. Conceptual relationship between friction demand, speed, and friction availability. 

Source: Hall et al., 2009

Intersections require motorists to slow, stop, turn, and react due to changing conditions, especially where 

there may be an increased potential for crossing pedestrians. Ramps and curves involve changes in speed 

and direction that require greater lateral forces for drivers to maintain control. The amount of friction 

available at a location (i.e., friction supply) must be equal to or higher than the amount of friction needed 

(i.e., friction demand) for a vehicle to safely execute these driving maneuvers and avoid a crash.

The AASHTO Guide for Pavement Friction (GPF) states that friction demand categories should be 

established logically and systematically based on highway alignment, highway features/environment, and 

highway traffic characteristics (Hall et al., 2009). The GPF further indicates that friction demand 

categories should be established for individual highway classes, facility types, or access types, and that 

the number of demand categories should be kept reasonably small so that there are enough PFM sections 

available for each category to establish investigatory friction levels. An investigatory friction level is 

defined as the value of friction below which a detailed site investigation should be performed to determine 

if there is a need for friction enhancement. Several examples of investigatory levels used in other countries 

can be found in a recent FHWA report (de León, et. al, 2019). 
5 

2.4 PAVEMENT FRICTION AND CRASHES

Historically, PFM has been associated more with wet pavement conditions and relied on identifying 

locations with an overrepresentation of wet pavement crashes for friction enhancement. However, this 

reflects a reactive and narrow approach since wet pavement crashes generally only represent about 15 

percent of all crashes. Since dry pavement crashes can also be affected by friction, a comprehensive PFM

approach expands the opportunity to further reduce fatal and serious injury crashes through appropriate 

pavement friction enhancement strategies. In fact, as HFST implementation increased in the U.S., studies 

confirmed that all locations treated (ramps and curves) experienced significant reductions in both wet and 

dry crashes (von Quintus and Mergenmeier, 2015). 
6 

3 FRICTION MEASUREMENT EQUIPMENT

Many different devices have been developed over the years to assess the friction of a pavement surface. 

As mentioned earlier, the pavement surface characteristics that influence pavement friction are 

microtexture and macrotexture. Currently, there are no direct measurement systems of microtexture that 

relate to friction, but there are measurement systems that provide an indication of microtexture that relates 

to friction. These systems rely on the broad principle of sliding rubber over a road surface and measuring 

the reaction forces in some way. There are direct measurement systems of macrotexture, although there is 

no one macrotexture measurement that has been determined to be the most appropriate for safety analysis.

3.1 FRICTION MEASUREMENTS

There are four general methods to measure pavement friction and they all involve rubber tires or sliders. 

These methods are generally more sensitive to the pavement surface microtexture than the macrotexture, 

thus they are considered to provide more of an indication of the microtexture.

Sliders: An example of a slider device is shown in Figure 4. The slider can be attached either to the foot 

of a pendulum arm or to a rotating head and slows down on contact with the road surface. The rate of 

deceleration is used to derive a value representing the skid resistance of the road. A variant of this 

approach, used by law enforcement agencies, measures the force applied to drag a sled over the road 

surface (with sliders representing car tires).

Figure 4. Photo. Example of a slider: British Pendulum Tester. 

Source: CSTI

Longitudinal Friction Coefficient (LFC) Measurement: This type of measurement uses an 

instrumented measuring wheel mounted in line with the direction of travel. One type of high speed LFC 

system uses a fixed gear, or braking system which forces the test wheel to rotate more slowly than the 

forward speed of the vehicle. Consequently, the tire contact patch slips over the road surface and a 

frictional force that can be measured is developed. Typically, the ratio of vertical and drag forces is 

calculated (averaged over a fixed measuring length) to provide a value representing the LFC that is 

recorded. Within this category, a wide range of slip ratios may be used by individual devices (fixed-slip 

devices). The slip ratio is usually governed by the control system to a fixed proportion of the forward 
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speed, which, in turn, determines the slip speed. Figure 5 shows an example of a fixed-slip friction tester 

(TYROSAFE, 2011). 

Figure 5. Photo. Example of LFC: Fixed slip device-grip tester.

Source: CSTI

Another type of high speed LFC is the locked-wheel skid tester (LWST; see Figure 6), which measures 

the longitudinal friction by completely locking the brake of the measuring wheel (AASHTO T242). The 

LWST can use either a ribbed tire (AASHTO M261) (more sensitive to pavement microtexture) or a 

smooth tire (AASHTO M 286) (more sensitive to pavement macrotexture).

Figure 6. Photo. Example of LFC: 100% slip-LWST.

Source: CSTI

High-Speed Sideway Force Coefficient (SFC) Measurement: This type of measurement (AASHTO TP 

143) uses a freely rotating instrumented measuring wheel set at an angle to the direction of travel of the 

vehicle (see Figure 7). Due to the wheel being at an angle to the direction of travel, the tire is made to slip 

over the road surface, and the resulting force along the wheel axle (the “sideways force”) is measured. 

The ratio of vertical and side forces averaged over a defined measuring length provides the value that is 
8 

recorded to represent skid resistance. The wheel angle determines the slip ratio, and the vehicle speed 

determines the slip speed. 

Figure 7. Photo. Example of SFC: Sideway-force Coefficient Routine Investigation Machine (SCRIM). 

Source: CSTI

Decelerometer-Based Measuring Systems: These systems are mounted in a vehicle and used to measure 

the deceleration of the vehicle under emergency braking. Although, widely used by law enforcement 

agencies to assess road surface friction for collision investigations, and more recently in experimental 

naturalistic driving studies, these devices are not suitable for road network friction measurements or 

quality control purposes due to the testing variability resulting from the various vehicle, tire, and braking 

configurations. Decelerometers are only mentioned for the completeness of this primer and will not be 

addressed in any further detail.

3.2 MACROTEXTURE MEASUREMENTS

Macrotexture can be measured using both static and high-speed methods. The high-speed devices are more 

appropriate for network level data collection.

Static Devices – Volumetric Methods: The first, and oldest technique is the volumetric “patch” test 

which uses a known volume of sand, glass beads, or grease, that is placed on the road surface and spread 

evenly into a circular patch, filling the voids on the pavement surface. Operators compute the area of the 

circle and calculate the average depth below the peaks of the surface as a value known as mean texture 

depth (MTD).

Static Devices – Laser Methods: This technique uses laser technology to measure along narrow lines 

traversed by the laser (rather than across the area of a patch of sand or glass beads) to determine a surface 

profile used to compute the MPD as defined in the ASTM E1845 standard, which attempts to estimate the 

average depth below the peaks in a 100-mm segment of the surface profile. Laser technology is the most 

prevalent method to measure texture for both static and at high-speed testing and is being used by 

researchers and practitioners to also compute other parameters, such as the root mean square (RMS) of 

texture depth. Figure 8 and Figure 9 show two examples of static laser devices: the Circular Track Meter 

and the Surface Texture Analyzer.
9 

Figure 8. Photo. Static single spot laser: Circular Track Meter. 

Source: CSTI

Figure 9. Photo. Static line laser: Texture Analyzer. 

Source: CSTI

High-Speed Devices: Single-spot laser or line-laser technology is used in high-speed devices to obtain 

macrotexture measurements data for network evaluations. Vehicles, primarily used to collect texture data 

for network evaluations, use computers to record the data and process the reports for both MPD, RMS, 

and other parameters that are currently being evaluated by researchers to understand their use in PFM. 

Figure 10 and Figure 11 show the difference between a single-spot laser and a line-laser.
10

Figure 10. Photo. High-speed single spot laser. 

Source: CSTI

Figure 11. Photo. High-speed line laser.

Source: CSTI 
11

4 PAVEMENT FRICTION MEASUREMENT TO SUPPORT 

SAFETY MANAGEMENT

Currently state DOTs collect network level pavement surface condition data as part of their asset 

management program. A proactive and systemic PFM program that incorporates network level pavement 

friction data enhances asset management programs while providing many advantages to help improve the 

safety performance of the transportation system. Pavement friction varies on roadways across the network 

and even along a specific corridor, especially at curves, ramps, and intersections. High-speed testing 

equipment is the only practical alternative for collecting friction data at the network-level. There are two 

categories of high-speed test methods, non-continuous and continuous pavement friction measurement 

(CPFM).

4.1 NON-CONTINOUS PAVEMENT FRICTION MEASUREMENT

In the U.S., the standard method of measuring pavement friction used by State highway agencies is the 

LWST, which is a non-continuous method (Henry, 2000) (AASHTO T 242/ASTM E 274). LWST

measures friction by completely locking up the test wheel(s) while simultaneously wetting the surface 

with a defined water film thickness in front of the test wheel and recording the average sliding force for a 

period of 3 seconds and reporting a 1-second average after reaching the fully locked slip. Thus, with a 40-

mph test speed, a 1-second test time is equivalent to testing the pavement surface for approximately 59 

feet. In general, for agencies that collect network-level friction data using the LWST, it is customary to 

perform one or two tests per mile, which results in only 1 to 2 percent of the pavement surface being 

tested. As this device is a trailer system, it can be challenging to collect friction data on ramps, curves, or 

intersections where friction demand and the potential for polishing is higher. 

4.2 CONTINOUS PAVEMENT FRICTION MEASUREMENT (CPFM)

Continuous friction measurement equipment (CFME) tests 100 percent of the pavement surface including 

tangents, curves, and intersections. There are three general types of CFME: (1) fixed-slip (ASTM E 2340), 

(2) SFC (AASHTO TP-143), and (3) variable-slip (ASTM E 1859) (Henry, 2000). These high-speed 

methods generally operate between 30 and 50 mph, while simultaneously wetting the pavement surface 

with a user-defined, uniform water film thickness on the pavement surface in front of the test wheel(s). In 

2015, FHWA introduced to the U.S. a SFC measurement device that uses a free-rolling test wheel with a 

fixed 20-degree slip-angle to measure side-force “transverse” friction. This device records a measure of 

friction known as the sideway-force friction number (SFN; AASHTO TP-143). SFN can be reported at an 

interval as short as 4 inches (0.1 m) or averaged to up to 33 ft (20 m), normally used for network-level 

work. The FHWA device includes additional sensors to measure surface macrotexture and roadway 

surface geometry (roughness, curvature, cross-slope, and longitudinal grade), and GPS coordinates (de 

León, et. al, 2019). This helps the practitioner to more readily identify potential factors related to the 

frequency and severity of crashes.

4.3 SAFETY MANAGEMENT 

Network level pavement friction measurement using CFME provides reliable data to properly investigate 

and proactively assess the friction and the potential for crashes across the entirety of the roadway system. 

This is of particular importance to addressing locations that historically may be overlooked. Measuring 

pavement friction in locations with high friction demand (e.g., curves, ramps, approaches to intersections) 

can be safely and accurately performed using CFME.
12

CPFM data, combined with crash and road characteristic data, provide significant insight regarding 

whether increases in friction may reduce crashes. Using network level CPFM data, transportation agencies 

can identify and prioritize locations to further investigate. CFPM data, combined with various other safety 

data, enhances project level investigation and road safety audits (RSA). The practitioner can more reliably

determine factors, including friction and macrotexture, which are contributing to the frequency and 

severity of crashes, select the appropriate treatments, determine the limits of improvement, and estimate 

the potential reduction in crashes. Although installation of HFST has been used to address crashes on 

curves or at intersections, based on CPFM data, an agency can better define the area with a higher friction 

demand. This can result in extending the limits of a HFST installation to accommodate that friction 

demand and reduce crashes. Consideration of multiple locations could result in similar findings and lead 

to a systemic application or even changes in practice or policy. 

Including CPFM data as an element of safety data and integrating it into the safety analysis procedures

allows transportation agencies to create an overall PFM program anchored in safety that fully embraces a 

Safe System approach and results in the reduction of fatal and serious injury crashes. 
13

5 ADDITIONAL INFORMATION

For more information about incorporating pavement friction management and continuous pavement 

friction measurement into road safety efforts, the following FHWA resources may be of interest:

• Characterizing Road Safety Performance using Pavement Friction (FHWA-SA-23-006). 

Documents the development of crash modification factors/functions and performance 

thresholds for friction based on roadway type and category. 

• Continuous Pavement Friction Measurement for a Safe System (FHWA-SA-22-052). Short 

booklet highlighting how using CPFM for safety program purposes aligns with the Safe System 

approach. 

• Improving Pavement Friction for Safety at a Florida Signalized Intersection – Final Report

(FHWA-SA-22-10) and Project Case Study (FHWA-SA-22-09). Documents the installation of 

high friction surface treatment (HFST) at a signalized intersection in Tampa, Florida, in an 

effort to reduce crashes and improve pedestrian safety. The location was identified through a 

RSA and utilized CPFM data.

• Proven Safety Countermeasures Pavement Friction Management (FHWA-SA-21-052). Serves 

as a one-page summary handout for communicating about the value of comprehensive PFM. 

• Enhancing Safety through Continuous Pavement Friction Measurement (FHWA-SA-21-014). 

Serves as a one-page (front/back) summary handout for communicating about the value of 

CPFM. 

• Pavement Friction Management Program Utilizing Continuous Friction Measurement 

Equipment and State-of-the-practice Safety Analysis Demonstration Project - Final Report

(FHWA-RC-20-0009) Documents pilot demonstration experience of CPFM across several 

states and includes case studies on macrotexture and high-speed roads, macrotexture and 

friction at intersections, PCCP friction and macrotexture, and raveling of chip-sealed roads.

• FHWA web page on Pavement Friction, including additional information on HFST and other 

friction enhancement treatments, friction testing, case studies and noteworthy practices.
14

6 REFERENCES 

American Association of State Highway and Transportation Officials. (2018). AASHTO T242, Standard 

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