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FHWA Highway Safety Programs

4.0 Summary - State Practices to Reduce Wet Weather Skidding Crashes

4.0 Summary

This section provides a comparison of the five state programs and general findings on the practices used for crash reduction programs focused on wet weather skidding crashes.

4.1 Identification of Wet Weather Crash Locations

All states reviewed conduct a statewide analysis on an annual basis and consider both concrete and asphalt pavements. Table 4.1 provides a summary of the procedures and factors used by each of the five states for identifying locations with wet weather crashes to investigate further.

Table 4.1 State Analysis Procedures
State Factors for Identifying Locations Screening Frequency
California A wet crash count significantly higher than statewide average and includes a factor which represents the percent wet time for each county in the State. Analysis evaluates 0.2‑mile segments using a 95% confidence interval. Annually
Florida Minimum of 4 wet weather crashes with 25% or more wet weather crashes or 50% or more wet weather crashes during a 5-year period on 0.3‑mile segments. Statewide – annually
Districts – at their discretion
Michigan Identifies locations with a friction number less than 30 (SN40R). Annually
New York At least six wet road crashes during a 2‑year period in rural areas and at least 10 in urban areas with 35 percent or more of the total crashes occurring on wet road conditions. Annually
Virginia Three or more wet weather crashes in the previous year and if the location has a wet to wet plus dry ratio 20 percent greater than the ratio for all roads in the area (excluding ice and snow crashes). Annually

The review of state analysis procedures for identifying wet weather crash locations identified three distinct methods. The three methods are as follows:

  1. Sites exceeding a set threshold of the number of wet weather crashes and a threshold proportion of wet crashes are identified as potential problem locations;
  2. Sites with a friction test result below a set threshold are identified as potential problem locations; and
  3. Sites with the number of wet weather crashes exceeding the statewide average by a particular significance level are identified as potential problem locations.

While any one of these three methods is appropriate for identifying potential wet weather crash locations, the first method is the most commonly used by states.

4.2 Friction Testing Procedures

Friction test results are a consideration in all of the states programs, and all of the states maintain an active pavement friction database. All of the five states conduct friction tests using the locked wheel method, and with the exception of Virginia, they all use a ribbed wheel for the testing. Table 4.2 summarizes the friction testing procedures and thresholds.

Table 4.2 State Friction Testing Procedures and Thresholds
State Testing Frequency Testing Equipment and Procedures Friction Threshold
California Conducted on a 3‑year rotating schedule for the highway inventory program. Also conducted at locations identified with high wet-collision concentrations (locations tested based on wet weather crash experience are not included in the State’s friction database). Uses a locked wheel towed trailer tester with ASTM standard ribbed tires according to ASTM E 274 procedures. Caltrans maintains an active friction data set for the outer two lanes of the highway network with one test per lane-mile as part of their highway inventory. 30 (SN40R)
Florida Conducted on a 3‑year rotating basis on all state roadways as well as on overlays and new construction, locations with a high frequency of wet crashes, and special requests. Standard two-wheel trailer towed by a one-ton pick-up conforming to ASTM E 274 requirements. Conducted in the left wheel path at 40 mph. Normal testing procedure is to conduct 3-5 tests per mile or section (if less than a mile). A mean skid value is determined based on the arithmetic average of the tests conducted on each section of roadway. Posted speed >45 mph = 30 (FN40R);
Posted speed < 45 mph = 28 (FN40R)
Michigan Conducted on all state-maintained roads on a 3‑year rotating schedule or at locations identified with a high crash experience. Intersections tested only if potential issues are identified. Conducted using a Dynatest 1295 friction tester according to ASTM E 274 requirements using an ASTM E 501 ribbed tire for level testing and an ASTM E 524 smooth tire for special testing. 30 (SN40R)
New York Conducted at wet road crash locations and select locations for the Pavement Friction Inventory. Conducted from April to November according to ASTM E 274 requirements using a ribbed tire meeting ASTM E 501 requirements. 32 (FN40R)
Virginia Conducted at locations with three or more wet weather crashes in the previous year with a 20 percent or greater wet to dry ratio. Conducted using ASTM E 274 trailer units on a wetted pavement at a speed of 40 miles per hour with a standard smooth tire. Tests are conducted at a minimum frequency of one test for every 0.1 mile; for sites less than 1 mile, as many tests as possible are conducted with up to one test for every 0.05 mile. 20 (SN40S)

Florida was the only State that conducts friction testing on new construction or resurfacing projects. However, most of the states indicated that they evaluated the friction characteristics of the aggregates used for pavement surfaces.

Most states utilize the locked wheel method according to ASTM E 274 requirements to conduct friction tests. The ribbed tire is most commonly used with friction thresholds ranging from 28 to 32.

4.3 Investigation and Remedial Action of Wet Weather Crash Locations

To date, the states have been focusing on spot improvements. While many of the states are implementing systematic improvements, such as rumble strips and raised pavement markings, no state has implemented systemic improvements focused specifically on addressing skidding-related wet weather crashes.

Table 4.3 provides a summary of the mitigation techniques used by the interviewed states to improve pavement friction.

Table 4.3 State Mitigation Techniques
State Mitigation Techniques
California Improvements include superelevation changes, open-grade asphalt concrete (OGAC) overlays, pavement grooving, high-friction surface treatments, or drainage improvements.
Florida A specification for asphalt concrete friction courses has been developed for ramps, curves, or other locations with wet weather crashes. Specific provisions are provided for different aggregates usages, including the use of granite. Florida is currently working on specifications for hybrid mixes with granite and limestone. The District 4 office (Broward County) has experimented with the use of high-friction surface treatments in areas were friction-based crashes are a concern.
Michigan If an identified location is not in the current work plan for resurfacing, typically the regions will do an overlay, ultra thin overlay, mill and resurface, microsurfacing, paver placed surface seal, chip seal, or diamond grinding. Signing is used only as a short-term solution.
New York Treatments typically include resurfacing with one and one-half inches of hot mix asphalt using the appropriate friction aggregates, or a thin cold emulsion microsurfacing (using noncarbonate aggregates). Superpave hot mix asphalt is the standard for New York State contracts.
Virginia For asphalt pavement, micro surface treatments are widely used to restore pavement with inadequate friction characteristics. Seal coats or chip seals are also used to restore pavement friction characteristics and extend the life of pavements. Depending on the pavement distress condition, the section could also be overlaid. For Portland Cement Concrete (PCC) pavements, diamond grinding increases concrete pavement friction. Saw cut grooving (longitudinal or transverse) is used traditionally to restore adequate frictional characteristics of PCC pavements.

Site investigations are a key element for identifying appropriate mitigation techniques for locations identified as potential wet weather crash locations. All states include a field investigation in their process for identifying appropriate mitigation.

4.4 Project and Program Evaluations

Evaluations are a critical element of any safety program. Projects should be evaluated to verify the desired results have been achieved and to ensure the investments have been worthwhile. Evaluating the program helps measure its impact on the entire system. All of the states indicated they used before and after studies to evaluate improvements at wet weather crash locations funded through the HSIP. However, projects funded through other funding sources are generally not being evaluated for their safety effectiveness. While all of the states indicated they were identifying fewer sites year after year, only a couple indicated they had conducted a program evaluation. New York evaluated their program back in 2002, but has not conducted a formal evaluation since. Virginia’s 2007 Wet Accident Reduction Program Report [13] provided an evaluation of the program for the years of 2002 to 2007 and demonstrated positive results. Table 4.4 summarizes the project and program evaluations conducted by the states.

Table 4.4 State Project and Program Evaluations
State Project Evaluations Program Evaluations

Caltrans evaluates wet weather crash location improvements funded through the HSIP program. The evaluation compares 3 years of both before and after improvement crash data based on total, fatal, injury, and PDO crashes.

Caltrans does not evaluate individual subprograms; the safety program is evaluated as a whole.


Florida conducts evaluations on all HSIP-funded projects using their CRASH database. The evaluations are used to calculate CRFs for various countermeasures. The CRFs are typically calculated based on 5 years of both before and after data.

Florida has not conducted any recent formal evaluations of their Skid Hazard Elimination Program but anticipates conducting an evaluation over the next year. Although not a formal process, typically the status of previously identified wet pavement crash locations are investigated to determine how many remain from year to year.


Michigan conducts before and after evaluations of all safety projects.

Michigan has not conducted a recent evaluation of their wet weather crash reduction program; however, the overall number of wet weather crashes has gone down. In addition, fewer locations are being identified each year.

New York

New York conducts evaluations of all safety projects funded through the HSIP. The evaluations are based on 3 years of before improvement fatal and injury crash data and three years of after improvement fatal and injury crash data. As part of the analysis, a benefit/cost ratio is calculated to determine if the project achieved it purpose. If the projects are not part of the HSIP,evaluations are conducted at the discretion of the region.

New York conducted a formal evaluation of the program in 2002. The evaluation developed crash modification factors for resurfacing with high wet road crash locations and found resurfacing treatments at wet road crash locations are expected to reduce total crashes by approximately 20%, total wet road crashes by approximately 60%, and severe (fatal and injury) wet road crashes by approximately 70%.

A summary of Priority Investigation Locations (PIL) testing from 1996 through 2006 shows a steady decline in the number of sites requiring treatment from 91 sites in 1996 to 19 sites in 2006. In 2007, 14 sites required treatment.


Project evaluations are conducted for all safety improvement projects funded through the HSIP. The evaluations are conducted using 3 years of both before and after improvement data.

Between 2002 and 2007, the total number of traffic crashes and the number of dry crashes increased in the State by almost 13% and 17% respectively. The number of wet weather crashes decreased by almost 7%. During this time period, the wet to dry crash ratio also decreased from 0.182 to 0.145. During this same time period, the number of Potential Wet Accident Hotspot (PWAH) sites reduced by almost 27%, and the number of PWAH sites in million vehicle-miles traveled reduced from 0.009 to 0.006.

From 2006 to 2007 there was a 23% decrease in the number of low friction sites identified in the crash analysis.

Evaluations of improved wet weather crash locations are typically conducted through a before and after study. Before and after studies compare the number of crashes at a site before an improvement to the number of crashes at a site after an improvement for a corresponding time period. The evaluation should focus specifically on wet weather crashes. A reduction in crashes during the after period provides an indication the surface treatment contributed to the reduction in wet weather crashes.

Program evaluations can be conducted in a number of ways. A program evaluation might compare the trend in overall crashes, wet weather crashes, and dry crashes over a specified time period. It could evaluate the trend in the number of sites identified for treatment on an annual basis over a long-term period. Finally a program evaluation could be conducted by calculating the wet safety factor, as outlined in Section 2.4.