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Continuous Pavement Friction 

Measurement for a Safe System

A Safe System Approach accommodates human 

mistakes by designing and managing road 

infrastructure to reduce incidence and severity. The 

six principles that form the basis of a Safe System 

include: deaths and injuries are unacceptable, humans 

make mistakes, humans are vulnerable, responsibility 

is shared, safety is proactive, and redundancy is 

crucial.

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, 

at locations where 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. Roadway departure and intersection crashes 

result in 75 percent of traffic fatalities across the United 

States.1

 Both wet-pavement and dry-pavement crashes 

can be mitigated by improving pavement friction and texture. Although most drivers adjust their speed to 

navigate tight curves or approaching intersections, they cannot account for pavement friction and texture 

because they simply cannot estimate these characteristics. The friction provided by a roadway surface 

affects braking and steering control, which can contribute to crashes. One action that can help agencies 

achieve a Safe System is to provide adequate friction at curves and intersections where it is needed most.

1 Fatality Analysis Reporting System, National Highway Traffic Safety Administration, https://www-fars.nhtsa.dot.gov/Main/index.aspx

Imagine if the safety performance of a specific horizontal curve or intersection were reported as an 

average of crashes observed (or expected) at locations several thousand feet or more away? 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 developing Safe Roads.

Measuring Pavement Friction for a Safe System

Why your agency should use Continuous Pavement Friction 

Measurement to Save Lives

The Safe System Approach principles and elements.

Source: FHWA
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Today, it is standard procedure for network level friction in the United States to be measured using a 

sample-based, 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 repeatable and provides point information. However, reported values that reflect averages 

across long distances through changing road conditions 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 relative to total crashes, are used to screen for 

locations that may respond favorably to friction improvement. As a result, 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 

and 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 50 

mph. This data can then be post-processed at user-defined 

increments as short 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, and 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 using LWST equipment at this 

resolution at these locations.

Methods for Measuring Pavement Friction 

Measuring Microtexture and Macrotexture

Figure 1: Visualization of CFM data through a curve with 

an intersection (presented in 30-foot averaged intervals).

There are two types of surface texture that affect pavement friction: microtexture (wavelengths of 1μm 

to 0.5mm) and macrotexture (wavelengths of 0.5mm to 50mm).2 These characteristics are controlled by 

an agency’s pavement specifications. Microtexture is the very fine texture of the pavement surface that 

is analogous to the feeling of rubbing sandpaper on skin. On pavements, the jagged edges of aggregate 

material provides the roughness needed to provide friction to the tires. Macrotexture is the visible texture 

of the surface that provides voids for water to escape. Both are important for providing an adequate 

friction supply; however, macrotexture becomes more critical at higher speeds. CPFM systems can collect 

data that is related to both microtexture and macrotexture. By being able to collect information related 

to both of these key components of pavement friction, agencies can specify, monitor, and maintain an 

appropriate level of friction to support safe roads.3

2

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Guide for Pavement Friction, American Association of State Highway and Transportation Officials

FHWA’s Technical Advisory TA 5040.36 “Surface Texture for Asphalt and Concrete Pavements” provides additional information 

on pavement surface texture.
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While aggregate testing and specifications, pavement mix designs, and rubber tire manufacturing 

have evolved in the years since TA 5040.36 “Surface Texture for Asphalt and Concrete Pavements” 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, using CPFM data can help target more efficient and effective 

installations of pavement friction treatments, including High Friction Surface Treatments (HFST).

In addition to technical considerations such as testing specifications and data management, agencies must 

plan for internal collaboration. Friction data is multi-disciplinary in terms of benefits and impacts, involving 

Safety, Maintenance, Programming, Pavements, and Materials. For example, Safety provides expertise 

on crash data and conducts data analysis, while Pavements and Materials identify and specify mixes, 

materials and treatments that provide sufficient friction where needed, including HFST. A Pavement Friction 

Management Program that is multi-disciplinary in nature and utilizes CPFM will lead to pavement surfaces 

that are designed, constructed, and maintained to minimize friction-related crashes in a cost-effective 

manner.

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 benefitcost 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.

Collaborating for Safe Roads
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In 2015, FHWA initiated a Pavement Friction Management Program demonstration project in collaboration 

with four State agencies to collect and analyze pavement friction, crash, traffic, and other geometric 

data. The research team collected, processed, and analyzed approximately 4,000 miles of data, and 

recommended a methodology to identify sections of roadway with high friction-related crash risk, 

suggesting corrective friction treatments in a cost-effective manner to reduce friction-related crashes.

The study demonstrated methods for establishing investigatory levels of friction for different friction 

demand categories and developed pilot pavement friction management plans (PFMPs) for the four States 

using proven safety analysis methods. The results of the study confirmed a strong association between 

crashes and continuously measured frictional pavement properties and that the adoption of CPFM data 

within a PFMP instead of the traditional sampling approach using LWSTs can have a significant positive 

impact on crash reductions. The analysis of the networks tested suggests that a PFMP implemented with 

CPFM data can result in high potential crash reductions, yielding significant potential economic savings 

with a favorable benefit/cost ratio. 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. 

In 2019, the Florida Department of Transportation (FDOT) conducted intersection Road Safety Audits

(RSAs) using CPFM data from the FHWA Pavement Friction Management Program demonstration project. 

The CPFM data revealed where pavement friction and macrotexture in intersection areas was significantly 

less than the adjacent pavement sections, prompting an RSA team recommendation to consider installing 

HFST, which was completed at one intersection in 2020. In the months following, traffic cameras mounted 

at the intersection facilitated analysis of stopping behavior at the intersection. It was found that improper 

stopping, defined as a vehicle crossing the stop line and resulting in an incursion into the crosswalk 

while the signal was red, was reduced by 25-30%. These results indicate that application of HFST at 

the intersection reduced stopping distance and incursions into the crosswalk, improving conditions for 

pedestrians. The CPFM data used by the RSA team was critical to the HFST recommendation that led to 

positive results.

Case Study: FHWA Demonstration Project

Case Study: Florida Department of Transportation 

Data collection methods using a sample-based approach do not adequately characterize the pavement

friction in high friction demand locations for in-depth safety analysis. A Pavement Friction Management

program utilizing CPFM can be an effective component of an agency’s Safe System Approach to save lives 

and prevent serious injuries.

To foster a safe road environment:

• Include pavement friction as a parameter in road safety performance modeling

• Establish friction performance thresholds based on context

• Proactively and systemically manage friction

• Deploy proven friction improvement treatments foster a safe road environment.

Conclusion 

For more information:

FHWA Office of Safety

Jeff Shaw

 FHWA Resource Center

Jeffrey.shaw@dot.gov Andy Mergenmeier Andy.mergenmeier@dot.gov

FHWA SA-22-052