2.0 Components of a Wet Weather Crash Reduction Program
The review of state wet weather crash reduction programs identified four common program components: identification of wet pavement crash locations, friction testing procedures, investigation and remedial action of wet pavement crash locations, and project and program evaluations. This section describes the importance of each component and related FHWA guidance.
The identification of locations with a high frequency or proportion of wet pavement crashes is a key component of a wet weather crash reduction program. The FHWA Technical Advisory on Pavement Friction Management  provides three common approaches for agencies to use to analyze the data in the state crash database to identify wet weather crash locations:
- Identify locations with a wet crash ratio above a specified value as a high wet weather crash location. (The ratio of the wet weather crashes to total (wet+dry) crashes.) The specified value varies between agencies depending on geometric and climatic circumstances; typically the ratio varies between 0.25 and 0.50.
- Identify locations with a wet crash ratio above the average wet crash ratio for the corresponding functional classification of highways. If a location is above the average by a specified percentage, the location is identified as a wet weather crash location.
- Identify locations that exceed an established wet crash ratio and a minimum number of wet weather crashes within a specified segment length as a wet weather crash location. As an example, one agency uses a minimum of six wet road crashes in rural areas and a minimum of 10 in urban areas.
Segment lengths used to compute wet crash ratios vary by agency, but typically a segment length of 0.2 to 2.0 miles is used. Once sites are initially identified through an analysis of the crash database, the sites should be friction tested and further investigated for potential remedial action.
Pavement friction testing is an integral component of any wet weather crash reduction program focused on skidding crashes. Typically, as pavement friction decreases, the number of wet weather crashes will increase. The FHWA Technical Advisory on Pavement Friction Management  provides guidance to state and local agencies in managing pavement surface friction.
Two types of surface texture affect wet pavement friction: microtexture (wavelengths of 1µm to 0.5mm) and macrotexture (wavelengths of 0.5mm to 50mm). Microtexture is generally provided in asphaltic pavements by the relative roughness of the aggregate particles and in concrete surfaces by the fine aggregate. Macrotexture is generally provided in asphalt pavement by proper aggregate gradation and in concrete surfaces by a supplemental treatment such as diamond grinding or grooving, exposed aggregate texture, transverse or longitudinal tining, burlap or artificial turf dragging, and transverse brooming.
Friction Testing Methods
Four types of full-scale test equipment exist for measuring pavement friction, including locked wheel, fixed slip, side force, and variable slip. However, the recommended methods for evaluating pavement friction on U.S. highways are the locked wheel and fixed slip methods; currently side force and variable slip friction measurement systems are not widely available or used in the U.S. Table 2.1 identifies the advantages of each of these four testing methods.
Source: FHWA Technical Advisory on Pavement Friction Management .
The ribbed tire (ASTM E 501) is the most common test tire used by U.S. state highway agencies with the locked wheel method, but it is considered less sensitive to pavement macrotexture and water film depth compared to the smooth tire (ASTM E 524). However, all friction test methods can be insensitive to macrotexture under specific circumstances, so it is recommended that friction testing be complemented by a macrotexture measurement (ASTM E 1845). Macrotexture measurements can be independently used to compute the Speed Gradient (Sp). (Sp defines the relationship between measured friction and vehicle tire free rotation or slip speed.) The Sp can then be combined with friction results from most friction testers to determine the International Friction Index (IFI). The IFI can be used to directly compare friction test results using different test methods. The AASHTO Guide to Pavement Friction  provides models to use for these conversions.
To ensure reliable friction test results, it is essential to proper calibrate and maintain friction testing equipment as specified by the manufacturer.
Friction Testing Conditions
Friction test results can be impacted by various factors such as surface temperature, test speed, and ambient weather conditions. Conducting friction testing under standardized conditions helps to minimize the effects of these factors, which minimizes variability and produces repeatable measurements. Table 2.2 summarizes AASHTO’s guidance on standardized test conditions .
Source: AASHTO Guide to Pavement Friction .
Establishing Friction Thresholds
There is not a specific friction test value that represents the difference between a safe and potentially unsafe pavement surface. Each agency determines their own investigatory (or desired) friction levels or friction-level ranges for specific facility types, based upon factors such as traffic volume, geometrics (e.g., curves, grades, sight distance), potential conflicting vehicle movements, speed, and intersections. Once sites fall into the investigatory friction-level range, they are further investigated. Many states also develop intervention friction-level thresholds that represent a minimum level of pavement friction. Once sites reach the intervention level, some type of action is required. These thresholds can help an agency in prioritizing improvement projects for sites identified as wet crash locations.
The AASHTO Guide for Pavement Friction  identifies three methods for establishing investigatory and intervention-level friction thresholds. The first method establishes thresholds by examining historical pavement friction data to determine at what pavement age significant decreases in friction occur and set thresholds based on those friction values. The second method compares historical friction and crash data and establishes an investigatory level based on large changes in friction loss and an intervention level based on when there is a significant increase in crashes. Finally, the third method establishes thresholds based on friction distribution and crash rate.
A program targeting the reduction of wet weather crashes through surface friction improvements, should give consideration to the resulting friction of the surface treatment. Friction testing and friction-related specifications on new hot-mix asphalt or concrete surfaces may be justified unless historical evidence indicates that existing pavement mix-design requirements, aggregate specifications, or construction specifications have resulted in pavement surfaces that provide adequate pavement friction.
Sites identified during the crash data analysis need to be further investigated to determine potential contributing factors to the crashes. The friction number is evaluated as part of this investigation. Typically if the friction number falls below the investigatory threshold, the site is reviewed in the field to identify existing conditions and determine potential contributing factors and potential improvements. This investigation also may include a detailed analysis of the individual crash reports to identify collision patterns. The field review will also identify any site conditions that may have contributed to the crashes, including potential cross-section and pavement deficiencies. If low pavement friction is identified as a contributing factor, the next step is to identify the appropriate remedial action. The FHWA Technical Advisory on Surface Texture for Asphalt and Concrete Pavements  identifies several techniques to provide adequate surface friction on new pavements and overlays and to restore surface friction of existing pavements. These techniques are summarized in Table 2.3.
Source: FHWA Technical Advisory Surface Texture for Asphalt and Concrete Pavements .
There are several different techniques for improving pavement friction, and due to widely varying conditions of different sites, it is unlikely that one texturing method will be the optimal choice for all projects within a state. The selection of the appropriate technique should consider the existing conditions at each individual site. FHWA  identified several factors to consider when selecting a method to improve pavement friction, including:
- Splash and Spray – Reduced visibility caused by splash and spray may increase the probability of wet-weather crashes. Adequate pavement cross-slope or the use of porous surfaces will provide improved surface drainage and has been shown to reduce splash and spray.
- Climate – The increased probability of wet-weather conditions would justify a higher level of texture.
- Traffic Volume and Composition – Pavements with higher traffic volumes can justify a higher level of texture. Increased traffic would decrease the reaction/recovery time in the event of loss of control of a vehicle. Additionally, roadways with a higher composition of truck traffic typically demand a higher level of friction compared to corresponding highways comprised predominately of passenger cars.
- Speed Limit – Higher speed facilities may justify a higher level of texture. Friction test results will decrease with increasing speed, reaching a minimum at approximately 60 mph. Friction on surfaces with low texture falls more rapidly with speed than on high-textured surfaces.
- Roadway Geometry – Research has shown that curves tend to lose pavement friction at a faster rate than other roadway locations, and therefore, curves may justify a higher level of texture.
- Potential Conflicting Movements or Maneuvers (Frictional Demand) – Intersections and presence of pedestrians will justify a higher level of texture due to the increased likelihood of sudden braking movements.
- Materials Quality and Cost – The availability and cost of high-quality durable, nonpolishing materials will influence the choice of materials and techniques to provide increased friction.
- Presence of Noise-Sensitive Receptors – A pavement located near a school, hospital, or other noise-sensitive receptor may justify a higher consideration of noise effects when selecting the appropriate surface treatment for a pavement.
New and/or innovative pavement friction improvement techniques, without evidence of improved safety performance, should only be used on an experimental basis and monitored for safety performance.
The purpose of a wet weather skidding crash reduction program is to reduce the number of crashes occurring on wet pavement due to inadequate pavement friction. Conducting project and program evaluations enables an agency to determine if their efforts have met their intended purpose and provides a quantified measure of success. Individual projects can be evaluated based on the occurrence of wet pavement crashes before and after an improvement. States evaluate safety projects implemented for the Highway Safety Improvement Program (HSIP) on an annual basis, and these same methods can be used to evaluate the effectiveness of pavement improvements at wet weather crash locations.
As part of the Technical Advisory on Pavement Friction Management , FHWA identified the “wet safety factor” (WSF) as an appropriate metric for evaluating the effectiveness of a wet weather crash reduction factor. The WSF is the reciprocal of the risk of having a wet pavement crash relative to a dry pavement crash and is calculated as follows:
DC = Number of dry weather crashes;
WC = Number of wet weather crashes;
PDT = Percent of dry pavement time; and
PWT = Percent of wet pavement time.
To determine a composite statewide WSF, the network is divided into analysis areas based on similar percentages of wet and dry pavement time. The total number of dry and wet weather crashes are determined for each analysis area and used to calculate a WSF for each analysis area. Then, the WSF for each analysis area is weighted by the vehicle miles traveled (VMT) and aggregated to determine a composite statewide WSF. In a successful program one would expect the WSF to increase over time with an upper limit of 1.0. A WSF less than 0.67 suggests a potential wet weather problem. This value is based on the conservative estimate of the overall likelihood of a wet weather crash being 1.5 times greater than a dry pavement crash.