Pavement Friction Management
June 17, 2010
- What is the purpose of this Technical Advisory?
- Does this Technical Advisory supersede another Technical Advisory?
- What is the background on pavement friction management?
- What is the purpose of a pavement friction management program?
- What are the elements of pavement friction management?
- What design, construction, and maintenance practices will provide good friction characteristics?
- How are locations with elevated crash rates identified and classified?
- How does an owner-agency ensure that pavement surfaces with good friction characteristics are constructed?
- How are projects to improve pavement friction prioritized?
- How is pavement friction measured?
- What is the appropriate frequency and extent for network friction testing?
- Is there a friction test value that represents the difference between a safe and unsafe pavement surface?
- How can program effectiveness be determined?
- Are there any reference materials on pavement friction measurement and management?
What is the purpose of this Technical Advisory? This Technical Advisory issues guidance to State and local highway agencies in management of pavement surface friction on roadways.
Does this Technical Advisory supersede another Technical Advisory? Yes. This Technical Advisory supersedes Federal Highway Administration (FHWA) Technical Advisory 5040.17, "Skid Accident Reduction Program" dated December 23, 1980.
What is the background on pavement friction management? The FHWA pavement policy is contained in 23 Code of Federal Regulations (CFR) 626.3, and it states: "Pavement shall be designed to accommodate current and predicted traffic needs in a safe, durable, and cost effective manner." Further, FHWA policy related to highway safety improvement programs is contained in 23 CFR 924.5. It states: "Each State shall develop, implement, and evaluate on an annual basis a HSIP that has the overall objective of significantly reducing the occurrence of and the potential for fatalities and serious injuries resulting from crashes on all public roads." The highway safety improvement program shall be "data -driven" [ 23 CFR 924.7(a)] and shall incorporate "A process for collecting and maintaining a record of... ...roadway... data on all public roads." [23 CFR 924.9(a)(1)].The highway safety improvement program shall also incorporate a process for analyzing available safety data that... Identifies highway safety improvement projects on the basis of crash experience, crash potential, or other data supported means as identified by the State, and establishes the relative severity of those locations." [23CFR 924.9 (a)(3)(i)(A)]. To provide roadway data that will establish the relative severity of locations identified for highway safety improvement projects, a state highway agency should implement a program to manage pavement friction on its public roads.
What is the purpose of a pavement friction management program? The main purpose of a pavement friction management program is to minimize friction-related vehicle crashes by ensuring that new pavement surfaces are designed, constructed, and maintained to provide adequate and durable friction properties, identifying and correcting sections of roadways that have elevated friction-related crash rates, and prioritizing use of resources to reduce friction-related vehicle crashes in a cost-effective manner.
What are the elements of pavement friction management? Pavement friction management includes engineering practices to provide surfaces with adequate and durable friction properties plus data collection and analysis to ensure the effectiveness of the engineering practices. The data collection and analysis component of an effective friction management program shall utilize pavement friction and friction-related data, crash data, and traffic data to:
- Evaluate pavement design, construction, and maintenance practices to ensure pavement surfaces with good friction characteristics are provided.
- Identify and investigate locations with elevated wet-weather crash rates relative to comparable locations for the purposes of minimizing locations with elevated friction-related crash rates.
- Provide data for use in prioritizing projects to improve highway safety.
What design, construction, and maintenance practices will provide good friction characteristics? FHWA Technical Advisory TA 5040.36 "Surface Texture for Asphalt and Concrete Pavements" provides information of state-of-the-practice techniques for providing surface texture/friction on pavements and issues guidance for selecting techniques that will provide adequate wet pavement friction. The TA 5040.36 is available at http://www.fhwa.dot.gov/legsregs/directives/techadvs/t504036.htm.
How are locations with elevated crash rates identified and classified? Wet weather crash locations are identified using the data available in the crash database. The common procedure to identify locations is to calculate the ratio between wet weather crashes and total crashes (wet+ dry) crashes, which will hereafter be referred to as wet crash ratio. Usually, one of the following approaches is used to determine if a location has elevated wet-weather crash rates.
- Agencies may use a specific value for the wet crash ratio (e.g. 0.35 or 0.40) above which a location will be identified as an elevated wet-weather crash location. The value of this ratio generally varies from 0.25 to 0.50 depending on the agency and the geographic and climatic circumstances.
- Agencies may compare the computed wet crash ratio with the average ratio for that functional class of highway in that area. If the computed ratio is above the average by a specified percentage (e.g., 20 percent) that location is identified as an elevated wet-weather crash location.
- In addition to the wet-weather crash ratio, a minimum number of wet-weather or total crashes within a segment is another criterion that some agencies use in order for a segment to be identified as an elevated wet-weather crash location. One state uses a minimum number of six wet-road crashes in rural areas and a minimum of ten wet-road crashes in urban areas. Segment lengths used to compute wet crash ratios can vary, but is typically 0.2 to 2.0 miles.
How does an owner-agency ensure that pavement surfaces with good friction characteristics are constructed? Positive evidence of the provision of adequate pavement friction on new pavement surfaces should be demonstrated. Friction testing and related friction specifications on new hot-mix asphalt or concrete surfaces may be justified unless historical evidence indicates that pavement mix-design requirements, aggregate specifications, or construction specifications have resulted in pavement surfaces that provide adequate pavement friction after construction.
How are projects to improve pavement friction prioritized? Once network-level friction testing is completed, sites are evaluated for additional investigation or possible treatment. The approach recommend by the AASHTO Guide for Pavement Friction is to establish Investigatory Level and Intervention Level values for pavement friction and texture. These values are established based upon the specific needs of a facility (friction demand) and may be based in part on costs and benefits of providing specific friction levels on the network. Friction demand should be determined by the owner-agency for each road segment and be based upon factors such as traffic volume, geometrics (curves, grades, sight distance, etc.), potential for conflicting vehicle movements, and intersections. Research has shown that curves and intersections tend to lose friction at a faster rate than other roadway locations and thus justify a higher friction demand. Typically once a pavement section falls below the investigatory threshold value the specific pavement section is evaluated for friction-related crash potential.. Action, if justified, is performed. This Investigatory Level can also be considered a "desirable" level for pavement friction based upon site requirements. Pavements that fall below the Intervention Level require the performance of some type of action. This intervention level can also be considered a "minimum" level for pavement friction based upon site requirements.
How is pavement friction measured? There are four types of full-scale friction test equipment: locked wheel, fixed slip, side force, and variable slip. Each method of measuring friction has advantages. The locked wheel method simulates emergency braking without anti-lock brakes, the side force method measures the ability to maintain control on curves, and the fixed slip and variable slip methods relate to braking with anti-lock brakes. Side force friction measurement systems and variable slip friction measurement systems are not currently widely available or used on US highways. The locked wheel method (ASTM E-274) can be used with either the ribbed tire (ASTM E-501) or the smooth tire (ASTM E-524) but, the ribbed tire is the most common test tire used by US state highway agencies. The ribbed tire is considered less sensitive to pavement macrotexture and water film depth than the smooth tire. The locked wheel method and the fixed slip method are recommended as appropriate methods for evaluating pavement friction on US highways. The advantage of the fixed slip method over the locked wheel method is the ability to operate continuously over a test section, and the better relationship to braking with anti-lock brakes. Because all friction test methods can be insensitive to macrotexture under specific circumstances, it is recommended that friction testing be complemented by macrotexture measurement (ASTM E-1845). Macrotexture measurements can be used independently to compute the Speed Gradient (Sp) which can be combined with friction results from most friction testers to determine International Friction Index (IFI). IFI can be used to directly compare friction test results using different test methods.
What is the appropriate frequency and extent for network friction testing? Agencies should utilize a risk-based approach to determining the frequency and extent of friction testing on the highway network. The facilities with the highest traffic volumes, the highest likelihood of changes in friction over time, and the highest friction demand (the level of friction needed to safely perform braking, steering, and acceleration maneuvers) justify the most frequent monitoring of friction. Many agencies monitor friction on the most important parts of their network on an annual basis. Portions of the network that are lower-risk may justify friction monitoring on a 2 or 3 year cycle. The spatial interval for friction tests is typically 1-2 tests per mile with some US highway agencies performing 3-5 friction tests per mile. The left wheel path is generally considered to have the most traffic due to passing maneuvers and is the most frequently tested. Network friction monitoring is generally not necessary in both wheel paths. Friction test results also have a seasonal variability. If friction testing is performed throughout the year, seasonal variation should be established and the test results normalized to the influence of normal seasonal variation.
Is there a friction test value that represents the difference between a safe and unsafe pavement surface? No. Results obtained with any friction test equipment represent the frictional properties obtained when using the specific equipment and procedures and do not necessarily agree or correlate with other friction measurement methods. The values obtained are intended for use in evaluating friction characteristics of a pavement relative to other pavements or to evaluate changes of one pavement over time. The values are not sufficient to determine the distance required to stop a vehicle or the driving speed at which control would be lost on either wet or dry pavement.
How can program effectiveness be determined? The purpose of a pavement friction management program is to minimize friction-related vehicle crashes. Changes to numbers of crashes or crash rates may be influenced by factors other than changes to pavement friction. To monitor the effectiveness of the agency's pavement friction management program a suitable metric is "wet safety factor" (WSF). The WSF may be determined as follows:
- WSF = (DC)(PWT)/(WC)(PDT)
- DC = number of dry weather crashes
- WC = number of wet weather crashes
- PDT = percent of dry pavement time
- PWT = percent of wet pavement time
This factor is the reciprocal of the risk of having a wet pavement accident relative to having a dry pavement accident. Within analysis areas (similar PDT and PWT) the DC and WC are summed to determine the WSF for the analysis area. The WSF for each analysis area is weighted by VMT and aggregated to determine a composite statewide WSF. A desirable trend is increasing with an upper limit of 1.0. A WSF less than 0.67 suggests a potential wet weather problem. This criteria is based upon the conservative estimate of the overall likelihood of a wet weather crash being 1 1/2 as great as a dry pavement crash.
Are there any reference materials on pavement friction measurement and management? Yes. The following references apply to pavement friction measurement and management.
- American Association of State Highway Transportation Officials ( AASHTO) "Guide for Pavement Friction" AASHTO, Washington, DC, 2008
- J.J. Henry, ": Evaluation of Pavement Friction Characteristics", NCHRP Synthesis 291," Transportation Research Board, Washington, DC, 2000
- C. Lyon and B. Persaud, "Safety Effects of a Targeted Skid Resistance Improvement Program," Transportation Research Board 2008 Annual Meeting," TRB, Washington DC, 2008 also NCHRP Report 617, Appendix D: "Safety Effects of Improved Skid Resistance" Transportation Research Board, Washington, DC, 2008
- J. Bray, "Skid Accident Reduction Program (SKARP): Targeted Crash Reductions," Institute of Transportation Engineers (ITE) 2003 Technical Conference and Exhibit
- World Road Association (PIARC), "Managing Surface Condition to Limit Skidding Accidents" Report of the Technical Committee 4.2, Working Group C2, XXIII World Road Congress, Durban South Africa, 2003
- R. Larson and K. Smith, "Relationship Between Skid Resistance Numbers Measured with Ribbed and Smooth Tire and Wet-Accident Locations", Ohio Department of Transportation, Columbus, OH 2008.
- Federal Highway Administration, "Surface Texture for Asphalt and Concrete Pavements" Technical Advisory 5040.36, Federal Highway Administration, Washington, DC, 2005
- M. Pratt, and J. Bonneson, "Assessing Curve Severity and Design Consistency using Energy- and Friction-Based Measures", Transportation Research Record: Journal of the Transportation Research Board No. 2075, Transportation Research Board of the National Academies, Washington, DC, 2008, pp 8-15.
/s/ David A Nicol for
Joseph S. Toole
Associate Administrator for Safety
King W. Gee
Associate Administrator for Infrastructure