Enhancing Safety through Continuous 

Pavement Friction Measurement

Pavement friction can save lives in your state.

The friction provided by a roadway surface affects how vehicles interact 

with the roadway. Measuring, monitoring, and maintaining pavement 

friction – especially at locations where vehicles are frequently turning, 

slowing, and stopping – can prevent many roadway departure and 

intersection related crashes, resulting in fewer serious injuries and 

fatalities. Best practices and proven technology in use for several 

decades in other countries present an exciting opportunity for the U.S. 

road safety community.

Roadway departure and intersection 

crashes account for 75 percent of traffic 

fatalities across the United States.

Source: Fatality Analysis Reporting System

Experience with High Friction Surface 

Treatment (HFST) in the U.S. has revealed 

that friction is an important safety 

performance parameter.

Source: FHWA HFST Website

Why Continuous Pavement Friction Measurement is Better

To characterize the safety performance of a specific horizontal curve or intersection, it would not make sense to 

report it as an average of the crashes observed (or expected) at locations several thousand feet or more away. And 

yet, this is usually how friction is reported for most locations. Furthermore, pavement friction is not currently a 

parameter used in crash-based safety modeling in the same way as other roadway characteristics, such as number 

and width of travel lanes; presence, width, and type of shoulder; degree of curvature, etc. For these reasons, 

Continuous Pavement Friction Measurement (CPFM) offers a two-fold opportunity for enhancing road safety.

Today, it is standard procedure for network level friction in the United States to be measured using a samplebased, discrete (i.e., not continuous) measurement called the Locked-Wheel Skid Trailer (LWST) test, in which a 

measurement is taken over a 60-foot distance by locking a wheel on a tow-behind trailer. This method is highly 

reliable and does provide useful point information. However, reported values reflect averages across long distances 

through changing road conditions, and do not effectively differentiate the changes in friction along the route 

corridor. Furthermore, LWST equipment is difficult to utilize in critical high friction demand locations, such as 

horizontal curves or intersections, which tend to experience greater tire scrubbing and polishing that lead to loss of 

pavement friction. For this reason, surrogate safety metrics, such as the number or ratio of wet weather crashes, 

are used to screen for locations that may respond to friction improvement. Unfortunately, opportunities to improve 

friction and enhance safety at locations below the wet weather crash threshold may be overlooked.

Fortunately, CPFM is an established and proven approach 

that has been used for several decades in other countries that 

could revolutionize the role of pavement friction in framing our 

understanding and management of the safety performance 

of our Nation’s roads. CPFM equipment is able to measure 

pavement friction continuously, through tangents, curves and 

intersections, at speeds as high as 50MPH. This data can then 

be post-processed at user-defined increments as small as 1-foot. 

This approach is commonly used by road authorities in many 

European countries, Australia, and New Zealand, and even by 

airport authorities in the U.S. to measure friction on runways. 

Figure 1 presents CPFM data acquired at one U.S. location that 

was part of a recent FHWA pilot project, where it was found 

that pavement friction varied throughout the curve; it was 

considerably less through the curve and intersection area than 

on the tangent approaches. It would have been very difficult, if 

not impossible, to measure pavement friction at this resolution 

in these locations using LWST equipment.

Figure 1: Visualization of CFM data through a curve with 

an intersection (presented in 30-foot averaged intervals).
Managing Friction for Safety

More than 50 years ago, National Cooperative 

Highway Research Program (NCHRP) Report 37

stated that “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.” Essentially, this 

research recognized that a clear friction “supply 

and demand” relationship exists, and is a factor in 

determining the safety performance of a road. While 

aggregate testing and specifications, pavement mix 

designs, and rubber tire manufacturing have evolved 

in the years since that report was published, the 

basic friction supply and demand relationship is still 

relevant. Research conducted in other countries has 

consistently found a relationship between pavement 

friction levels and safety, and programs that 

subsequently established maintenance values for 

friction that are grounded in safety performance rely 

upon CPFM for monitoring. Furthermore, pavement 

friction treatments, including HFST, can be better 

targeted for installations that are more efficient and 

effective when using CPFM data.

In 2015, the Federal Highway Administration (FHWA) 

began collaborating with four State departments 

of transportation on a pilot study to demonstrate 

CPFM equipment technologies and compare 

results to each State’s LWST equipment. The study 

confirmed that CPFM data, combined with crash 

data, provides significant insight regarding whether 

friction improvements reduce crashes. Based on 

the pilot results, FHWA encourages the use of CPFM 

to provide comprehensive pavement friction data, 

combined with existing safety data and analysis, to 

create an overall pavement friction management 

program anchored in safety.

CPFM: An International Best Practice

United Kingdom

Since the 1980s, pavement friction of the English 

Strategic Road Network has been managed through 

a requirement to provide specific levels of skid 

resistance and texture depth, using CPFM as the basis 

for monitoring. A 1991 paper by Rogers and Gargett 

referenced a National Skidding Resistance Survey 

report that estimated this approach would result in 6 

percent fewer casualties per year on trunk roads, and 

a benefit-cost ratio of 5.5-to-1. In 2016, the Transport 

Research Laboratory published PPR 806, which further 

reviewed the relationship between crash risk and 

skid resistance. The study found that for curves and 

steep grades, roadways with higher skid resistance 

have a lower risk of collisions, even in wet conditions, 

and recommended that enhanced skid resistance 

treatments be prioritized for those sites.

New Zealand

Throughout the 1990s, the New Zealand Transport 

Agency (NZTA) sponsored road surface friction 

research and development and established their 

first skid resistance policy and specification in 1997, 

which required CPFM equipment be used for network 

skid resistance measurement. Consistent with UK 

experience, the 1998 Transfund New Zealand Research 

Report 141 documented a statistically significant 

relationship between crashes and skid resistance at 

junctions, curves and steep grades, and indicated that 

wet road crashes could be reduced 45-61% at these 

locations with targeted enhanced skid resistance. 

Finally, a 2011 paper by Whitehead, et al, reviewed 11 

years of experience with the NZTA policy and found the 

benefit-cost ratio ranged between 13:1 and 35:1.

Including pavement friction as a parameter in road safety performance modeling, establishing friction 

performance thresholds based on context, and proactively and systemically managing friction can help your 

agency achieve its road safety goals to save lives and prevent serious injuries.

For more information:

FHWA Office of Safety

Jeff Shaw Jeffrey.shaw@dot.gov

FHWA Resource Center

Andy Mergenmeier Andy.mergenmeier@dot.gov

FHWA-SA-21-014