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Technical Report Documentation Page

1. Report No.

FHWA-SA-11-21

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

State Practices to Reduce Wet Weather Skidding Crashes

5. Report Date

 May 2011

6. Performing Organization Code

7. Author(s)

Colleen McGovern, Peter Rusch, David A. Noyce

8. Performing Organization Report No.

9. Performing Organization Name and Address

Cambridge Systematics, Inc.

100 Cambridge Park Drive, Suite 400

Cambridge, MA 02140

10. Work Unit No.

11. Contract or Grant No.

DTFH6-05-D-00026-T-10003

12. Sponsoring Agency Name and Address

U.S. Department of Transportation

Federal Highway Administration

Office of Safety

1200 New Jersey Avenue, SE

Washington, DC 20590

13. Type of Report and Period Covered

Final Report

14. Sponsoring Agency Code

15. Supplementary Notes

The Contract Manager for this report was Guan Xu (Office of Safety Technologies). Additional technical 

oversight provided by: Joseph Cheung (Office of Safety), Richard Knoblauch (Office of Research, 

Development, and Technology), Roya Amjadi (Office of Safety Research and Development), Mike Moravec 

(Office of Pavement Technology), Craig Allred (Resource Center).

16. Abstract

This report provides guidance to states for implementing a program focused on reducing wet weather 

skidding crashes and identifies the four common components of existing state programs. These 

components include identification of wet pavement crashes, friction testing procedures, investigation and 

remedial action of wet pavement crash locations, and project/program evaluations. The report identifies 

existing guidance from the Federal Highway Administration (FHWA) and the American Association of 

State Highway Transportation Officials (AASHTO) related to each of the four components. It also 

provides detailed case studies on existing state programs in California, Florida, Michigan, New York, and 

Virginia, all of which have shown success in reducing wet weather skidding crashes. The report concludes 

with a comparison of the five state programs and identifies common procedures for each of the four 

program components. This report provides states with valuable information to launch a wet weather crash 

reduction program or to improve an existing program. 

17. Key Words

Wet weather crash reduction program, pavement 

friction, pavement friction treatments

18. Distribution Statement

No restrictions. This document is available to the 

public through the National Technical Information 

Service, Springfield, VA 22161.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No of Pages

42

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed pages authorize

State Practices to Reduce Wet Weather Skidding Crashes

i

Table of Contents

Notice.........................................................................................................................i

Quality Assurance Statement.................................................................................i

1.0 Introduction and Background ..........................................................................1-1

1.1 Wet Weather Crash Problem.....................................................................1-1

1.2 Purpose of This Guide ...............................................................................1-2

1.3 Research Methodology ..............................................................................1-3

1.4 Organization of This Guide.......................................................................1-4

2.0 Components of a Wet Weather Crash Reduction Program.........................2-1

2.1 Identification of Wet Weather Crash Locations .....................................2-1

2.2 Friction Testing Procedures.......................................................................2-2

2.3 Investigation and Remedial Action of Wet Weather Crash 

Locations ......................................................................................................2-5

2.4 Project and Program Evaluations.............................................................2-7

3.0 State Practices to Reduce Wet Weather Skidding Crashes .........................3-1

3.1 California .....................................................................................................3-1

3.2 Florida ..........................................................................................................3-4

3.3 Michigan ......................................................................................................3-6

3.4 New York.....................................................................................................3-8

3.5 Virginia.......................................................................................................3-10

3.6 Additional State Practices........................................................................3-13

4.0 Summary ..............................................................................................................4-1

4.1 Identification of Wet Weather Crash Locations .....................................4-1

4.2 Friction Testing Procedures.......................................................................4-2

4.3 Investigation and Remedial Action of Wet Weather Crash 

Locations ......................................................................................................4-3

4.4 Project and Program Evaluations.............................................................4-4

A. References ...........................................................................................................A-1
State Practices to Reduce Wet Weather Skidding Crashes

ii

List of Tables

Table 2.1 Advantages of Friction Testing Methods .............................................2-2

Table 2.2 Standardized Test Conditions................................................................2-3

Table 2.3 Techniques to Provide Adequate Surface Friction..............................2-5

Table 4.1 State Analysis Procedures.......................................................................4-1

Table 4.2 State Friction Testing Procedures and Thresholds..............................4-2

Table 4.3 State Mitigation Techniques ...................................................................4-4

Table 4.4 State Project and Program Evaluations.................................................4-5

List of Figures

Figure 1.1 U.S. Total Fatal and Wet Pavement Fatal Crashes (2000-2009) .........1-1

Figure 1.2 Proportion of Fatal Crashes Occurring on Wet Pavement.................1-2
State Practices to Reduce Wet Weather Skidding Crashes

iii

Abbreviations

AASHTO – American Association of State Highway and Transportation Officials

AWI – Aggregate Wear Index

ASTM – American Society for Testing and Materials

CAR – Crash Analysis Reporting

CRASH – Crash Reduction Analysis System Hub

CRF – Crash Reduction Factor

DOT – Department of Transportation

FARS – Fatality Analysis Reporting System

FHWA – Federal Highway Administration

FN – Friction Number

HFST – High-Friction Surface Treatment

HRTIS – Highway Traffic Records Inventory System

HSIP – Highway Safety Improvement Program

IFI – International Friction Index

NCDC – National Climatic Data Center

NCHRP – National Cooperative Highway Research Program

NTSB – National Transportation Safety Board

OGAC – Open-Grade Asphalt Concrete

PCC – Portland Cement Concrete

PDO – Property Damage Only

PIL – Priority Investigation Location

PWAH – Potential Wet Accident Hotspot

SHSP – Strategic Highway Safety Plan

SKARP – Skid Accident Reduction Program

SN – Skid Number

Sp – Speed Gradient

TRB – Transportation Research Board

TASAS – Traffic Accident Surveillance and Analysis System

VMT – Vehicle Miles Traveled

VTRC – Virginia Transportation Research Council

WARP – Wet Accident Reduction Program

WSF – Wet Safety Factor
State Practices to Reduce Wet Weather Skidding Crashes

1-1

1.0 Introduction and Background

This section provides an overview of the wet weather crash problem, the 

purpose of this guide, the research methodology used, and the organization of 

this guide.

1.1 WET WEATHER CRASH PROBLEM

Between 2000 and 2009, there were a total of 371,104 fatal crashes in the United 

States; 46,811, or 12.6 percent, of these crashes occurred on wet pavement. Total 

fatal crashes and wet pavement fatal crashes1

Figure 1.1 U.S. Total Fatal and Wet Pavement Fatal Crashes (2000-2009)

for this 10-year period are 

illustrated in Figure 1.1.

Source: Fatality Analysis Reporting System [1].

 

1 Wet pavement fatal crashes in Figures 1.1 and 1.2 are based on crashes in the Fatality 

Analysis Reporting System (FARS) identified with a “wet roadway surface condition”

variable in the police crash report. Other roadway surface conditions field variables 

include: blank, dry, snow/slush, ice/frost, sand/dirt/mud/gravel, water (standing or 

moving), oil, other, and unknown. These other variables are not included in the “wet”

crash analysis.

0

1,000

2,000

3,000

4,000

5,000

6,000

30,000

32,000

34,000

36,000

38,000

40,000

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Wet Pavement Fatal Crashes

Total Fatal Crashes

Year

Total Wet Pavement
State Practices to Reduce Wet Weather Skidding Crashes

1-2

As shown in Figure 1.1, wet pavement fatal crashes have consistently decreased 

from a high of 5,308 in 2003 to a low of 3,881 in 2009, a 26.9 percent reduction. 

Total fatal crashes have also consistently decreased from a high of 39,252 in 2005 

to a low of 30,797 in 2009, a 21.5 percent reduction.

Figure 1.2 shows wet pavement fatal crashes as a percentage of all fatal crashes 

and the number of wet pavement fatal crashes. While the number of wet 

pavement fatal crashes has declined over the last several years, the percentage of 

fatal crashes occurring on wet pavement has increased the last couple of years.

Figure 1.2 Proportion of Fatal Crashes Occurring on Wet Pavement

Source: Fatality Analysis Reporting System [1].

According to research conducted by the Federal Highway Administration 

(FHWA) and the National Transportation Research Board (NTSB), a significant 

proportion of the wet pavement crashes occur on surfaces with inadequate 

pavement friction, and about 70 percent of wet pavement crashes could be 

prevented or minimized by improving pavement friction.

1.2 PURPOSE OF THIS GUIDE

Following a motorcoach crash on Interstate 35 near Hewitt, Texas on 

February 14, 2003, the NTSB issued five safety recommendations to FHWA:

• H-05-12 – Issue guidance to your field offices describing the inadequate 

stopping sight distance that could occur when poor vertical geometries exist 

at locations with low coefficients of friction and speeds higher than the 

design speed and work with states to inventory such locations.

12.2% 12.7% 13.2% 13.8% 13.7%

12.7%

11.8% 11.2%

12.1% 12.6%

0.0%

2.0%

4.0%

6.0%

8.0%

10.0%

12.0%

14.0%

16.0%

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

0

1,000

2,000

3,000

4,000

5,000

6,000

Percentage of Fatal Crashes Occuring on Wet 

Pavement

Year

Wet Pavement Fatal Crashes

Percentage Number
State Practices to Reduce Wet Weather Skidding Crashes

1-3

• H-05-13 – Once the locations in Safety Recommendations H-15-12 have been 

identified, assist the states in developing and implementing a plan for 

repaving or other roadway improvements.

• H-05-14 – Issue guidance recommending the use of variable speed limit (VSL) 

signs in wet weather at locations where the operating speed exceeds the 

design speed and the stopping distance exceeds the available sight distance.

• H-05-15 – Conduct research on commercial vehicle tire and wet pavement 

surface interaction to determine minimum frictional quality standards for 

commercial tires on wet pavement; once completed, 1) revise the tire 

requirements for commercial vehicle operating on wet pavement at highway 

speeds, and 2) develop minimum acceptable pavement coefficients of friction 

and maximum permissible pavement rut depths as part of roadway 

maintenance requirements, as appropriate.

• H-05-16 – Review state programs that identify and eliminate locations with a 

high risk of wet weather accidents and develop and issue a best practice 

guide on wet weather accident reduction.

This project was conducted in response to NTSB recommendation H-05-16. The 

project has two main objectives:

• Identification of state Department of Transportation (DOT) practices and 

procedures for improving pavement friction to reduce the occurrence of wet 

weather crashes; and

• Preparation of a best practice guide.

This report represents a guide that covers identification of wet pavement crashes, 

friction testing procedures, investigation, and remedial action of wet pavement 

crash locations, and project/program evaluations. It is based on the practices 

and procedures documented in five of those states, selected because of their wellestablished programs.

1.3 RESEARCH METHODOLOGY

A three-tiered approach was used to identify states with formal wet weather 

crash reduction programs. This approach involved outreach to FHWA Division 

Offices, review of the literature, and analysis of crash data.

Outreach

A message was posted on the FHWA Safety Exchange web site requesting 

FHWA Division Offices to respond if any of the states in their division had a 

formal program to address wet weather skidding crashes. Select FHWA 

Division Office personnel and colleagues were contacted directly to inquire 

about existing programs.
State Practices to Reduce Wet Weather Skidding Crashes

1-4

Literature Search

A literature scan was conducted to identify states with existing programs for 

reducing wet weather crashes. The review included reports published by 

FHWA, Transportation Research Board (TRB), National Cooperative Highway 

Research Program (NCHRP), American Association of State Highway and 

Transportation Officials (AASHTO), as well as individual state DOT documents

such as Strategic Highway Safety Plans (SHSPs) and DOT web sites. A full scan 

of the literature found few examples of state programs focused on wet weather 

issues. Although most states describe some elements of a program to improve 

transportation safety by focusing on countermeasures for locations with wet 

weather skidding crashes, few states have comprehensively documented their 

policy or methods for identifying wet weather “hot spot” locations and 

evaluating the countermeasures applied.

Review the Data

Finally, wet weather fatal crashes provided in the Fatality Analysis Reporting 

System (FARS) and precipitation data provided by the National Climatic Data 

Center (NCDC) were reviewed. This information was used to identify states that 

appear to have made progress in reducing the number of wet weather crashes 

from a purely numerical standpoint. These states were considered for 

interviews. Many other and more complex factors may have had an impact on 

the reduction in wet weather crashes but are beyond the scope of this effort.

Interviews

Phone interviews were conducted with the following eight states identified with 

formal wet weather crash reduction programs:

• California;

• Florida;

• Kentucky;

• Maryland;

• Michigan;

• New Jersey;

• New York; and

• Virginia.

Each state provided information on when and why they began their wet weather 

crash reduction program, their program structure, procedures for identifying 

sites, mitigation techniques, and program results. The results of this research 

and the state interviews were documented for one of the tasks in this project.

1.4 ORGANIZATION OF THIS GUIDE

This guide is based on the four common components of wet weather crash 

reduction programs focused on friction improvements. These components 

include identification of wet pavement crashes, friction testing procedures, 
State Practices to Reduce Wet Weather Skidding Crashes

1-5

investigation and remedial action of wet pavement crash locations, and project/

program evaluations. Section 2.0 describes each of these four components in 

detail and provides suggested procedures based on FHWA and AASHTO 

guidance on friction management programs. Section 3.0 provides case studies 

which outline the four common program components in California, Florida, 

Michigan, New York, and Virginia. Finally Section 4.0 provides a summary of 

the findings of this research effort.
State Practices to Reduce Wet Weather Skidding Crashes

2-1

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.

2.1 IDENTIFICATION OF WET WEATHER CRASH 

LOCATIONS

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 [2] 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 ratio2

• 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.

above a specified value as a high 

wet weather crash location. The specified value varies between agencies 

depending on geometric and climatic circumstances; typically the ratio varies 

between 0.25 and 0.50.

• 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.

 

2 The ratio of the wet weather crashes to total (wet+dry) crashes.
State Practices to Reduce Wet Weather Skidding Crashes

2-2

2.2 FRICTION TESTING PROCEDURES

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 [2] 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.

Table 2.1 Advantages of Friction Testing Methods

Method Advantage

Locked wheel 

(ASTM E 274)

• Simulates emergency braking without anti-lock brakes.

• Can be used with either the ribbed tire (ASTM E 501) or the smooth tire 

(ASTM E 524).

Fixed slip • Relates to braking with anti-lock brakes.

• Ability to operate continuously over a test section.

Side force • Measures the ability to maintain control on curves.

Variable slip • Relates to braking with anti-lock brakes

Source: FHWA Technical Advisory on Pavement Friction Management [2].

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).
State Practices to Reduce Wet Weather Skidding Crashes

2-3

Macrotexture measurements can be independently used to compute the Speed 

Gradient (Sp).3

To ensure reliable friction test results, it is essential to proper calibrate and 

maintain friction testing equipment as specified by the manufacturer.

 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 [3] provides models to use for these

conversions.

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 [3].

Table 2.2 Standardized Test Conditions

Factors Consideration

Season Limit friction testing to a specific season or time of year when friction is typically the lowest 

to maintain year-to-year consistency and reduce variability in measured data. When this is 

not possible, correction factors can be developed to normalize raw friction test data to a 

common baseline season or coordinate initial and subsequent section testing to occur 

during a specific season.

Test speed The standard speed recommended by AASHTO T 242 for pavement friction tests (locked 

wheel) is 40 miles per hour. However, since most agencies conduct friction tests without 

traffic control and posted or operational speeds vary throughout the roadway network, it is 

difficult for the operator to conduct testing at just this speed. Therefore, results of friction 

testing conducted at speeds other than 40 miles per hour should be adjusted to the 

40 miles per hour baseline to make friction measurements comparable. This requires 

establishing correlations between friction measurements taken at 40 miles per hour and 

those taken at other speeds.

Test lane and 

line

Friction testing must be done in the most heavily trafficked lane, since it expected to have 

the highest rate of friction loss due to wear. Two-lane highways with a near 50-50 

directional distribution of traffic only need to be tested in one lane; otherwise, the lane 

direction with the higher traffic volumes should be tested. For multilane highways, the 

outermost lane in both directions is typically the most heavily traveled and should be 

tested.

Testing must be carried out within the wheel path, since this is the location where friction 

loss is the greatest. Testing should be carried out in the same lane and wheel path to 

maintain consistency between test results and reduce variability. If it is necessary to 

deviate from typically practice, the test data should be marked accordingly.

 

3 Sp defines the relationship between measured friction and vehicle tire free rotation or 

slip speed.
State Practices to Reduce Wet Weather Skidding Crashes

2-4

Factors Consideration

Ambient 

conditions

It is important to standardize and document ambient test conditions, as they can have an 

effect on friction test results. Avoid testing in extremely strong side winds as these can 

create turbulence under the vehicle causing the water jet to divert from the correct line and 

affect the measurement results. Avoid testing in heavy rainfall or where there is standing 

water on the pavement surface. Excess water on the surface can affect the drag forces at 

the pavement-tire interface and influence the measurements. Do not conduct testing when 

the air temperature is below 41oF (5oC).

Contamination Avoid testing locations where the pavement surface is contaminated by mud, oil, grit, or 

other contaminants. 

Source: AASHTO Guide to Pavement Friction [3].

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 [3] 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.

Resurfacing Projects

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 hotmix 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.
State Practices to Reduce Wet Weather Skidding Crashes

2-5

2.3 INVESTIGATION AND REMEDIAL ACTION OF WET 

WEATHER CRASH LOCATIONS

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 [4] 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.

Table 2.3 Techniques to Provide Adequate Surface Friction

Technique Description

Concrete Surfaces

Transverse tining Achieved by a mechanical device equipped with a tining head that 

moves laterally across the width of the pavement surface. A width of 

3mm (±0.5mm) and a maximum depth of 3mm are recommended. 

Recommended random spacing average tine spacing of either 13 mm 

with a tine spacing pattern of 10/14/16/11/10/13/15/16/11/10/21/13/

10 mm or 26 mm with a time spacing pattern of 24/27/23/31/21/34 mm.

Longitudinal tining Achieved by a mechanical device equipped with a tining head that 

moves parallel to the pavement centerline. A width of 3mm (±0.5mm) 

and a maximum depth of 3mm are recommended. Narrower, deeper 

grooves are better than wider, shallower grooves (within the limits) for 

minimizing noise. Straight, uniformly spaced grooves at 19mm have 

been shown to provide adequate handing characteristics for small 

vehicles and motorcycles. 

Exposed aggregate Normally constructed in two layers. The top layer consists of 30% 

siliceous sand of 0-1 mm and 70% high-quality chips of 4-8 mm. A 

water cement ratio of 0.38 and a mean texture depth of 0.77 mm are 

recommended.

Diamond grinding Typically provides grooves of approximately 3mm width, spaced at 

5-6 mm intervals. Specific groove depth and spacing is dependent on 

hardness of aggregate.

Diamond grooving Transverse or longitudinal can provide adequate friction 

characteristics. Groove geometry should be consistent with 

recommendations for tining.
State Practices to Reduce Wet Weather Skidding Crashes

2-6

Technique Description

Burlap drag Typically produced by trailing a moistened, course burlap from a 

construction bridge that spans the pavement. Striations of 1.5-3 mm 

depth are typical.

Artificial turf drag Typically produced by trailing an inverted section of artificial turf from a 

construction bridge that spans the pavement. Striations of 1.5-3 mm 

depth are typical when using turf with 77,500 blades per square meter.

Transverse broom Typically obtained using a hand broom or mechanical broom device 

that lightly drags stiff bristles across the surface. Striations of 

1.5-3 mm depth are typical.

Thin epoxy laminates Typically aggregates are 4-6 mm.

Asphalt-based surface treatments May include micro-surfacing.

Asphalt surfaces

Surface treatments or thin asphalt 

overlays

Generally, hot-mixed asphalt pavements designed in conformance with 

Superpave mix design will provide adequate macrotexture and 

microtexture without supplemental treatments. When supplies of 

durable nonpolishing aggregate are limited, an agency may choose to 

construct an asphalt pavement using high-durability aggregates 

optimized for friction properties only in the top layer.

Concrete overlays May be considered as an option to restore adequate surface texture on 

asphalt pavements.

Source: FHWA Technical Advisory Surface Texture for Asphalt and Concrete Pavements [4].

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 [4] 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 crossslope 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.
State Practices to Reduce Wet Weather Skidding Crashes

2-7

• 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.

2.4 PROJECT AND PROGRAM EVALUATIONS

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 [2], 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:
State Practices to Reduce Wet Weather Skidding Crashes

2-8

Where,

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.
State Practices to Reduce Wet Weather Skidding Crashes

3-1

3.0 State Practices to Reduce Wet 

Weather Skidding Crashes

This section describes the current state practices for reducing wet weather

skidding crashes used in California, Florida, Michigan, New York, and Virginia

and provides some additional state practices used to identify and mitigate wet 

crash locations.

3.1 CALIFORNIA

In 1972, Caltrans developed their Traffic Accident Surveillance and Analysis 

System (TASAS) to identify high-collision concentration locations. Included in 

this system was a methodology for identifying locations with a high 

concentration of wet crashes known as Wet Table C.

Identification of Wet Weather Crash Locations

Caltrans State Office of Traffic Safety Program analyzes the crash data to develop 

the Wet Table C on an annual basis. The Wet Table C identifies locations with a 

minimum of 9, 6, or 3 wet crashes within a 36-, 24-, or 12-month period, 

respectively and are significantly higher than the statewide average. Wet 

collisions are identified by those with a road surface coded as “wet.”

A significance test is conducted to determine if the defined highway segments, 

ramps, or intersections have a wet crash count significantly higher than the 

number of crashes required for significance (NR). For a segment to have 

significantly high crashes, the segment crash count must be greater than or equal 

to NR. NR was derived from the Poisson’s distribution one-tail test for a 

99.5 percent confidence interval and is defined by the following equation:

Where,

NR = Number of crashes required for significance; and

NE = Average number of crashes.

NE is calculated using the following equation:
State Practices to Reduce Wet Weather Skidding Crashes

3-2

Where,

ADT = Average daily traffic (vehicles per day);

t = Analysis period (days);

L = Length (miles); and

RE(wet) = Average wet crash rate (wet crashes/million vehicles or wet crashes/

million vehicle miles).

RE(Wet) is calculated as follows:

Where,

wt% = Percentage of wet time (decimal).

RE is calculated as:

Where

Base rate = Base crash rate for specific highway segment type (provided by 

TASAS);

ADT factor = Adjustment factor for specific highway segment type (provided 

by Caltrans tables); and

Total ADT = Average daily traffic for the specific segment (provided by 

TASAS).

The percent of wet time is determined for each county in the State. The percent 

of wet time was initially developed in 1972 using 11 years of data from 1957 to 

1967. In 2008, Caltrans conducted a study with the assistance of the Western 

Transportation Institute at Montana State University to update the percent of wet 

time values to reflect current climatic trends. The 2008 values were not found to 

be significantly different than the 1972 values [5].

The analysis is based on crash data from July 1 to June 30, so that the entire wet 

season is included in each analysis period. The analysis starts at the beginning of 

a route and evaluates one 0.2-mile segment at a time using a sliding scale of 

0.02 miles. If the first 0.2-mile segment is not found to be significant, the analysis 

moves ahead 0.02 miles and evaluates the next 0.2-mile segment. If a segment is 

found to be significant it is added to the output table. This process is repeated 

for the entire roadway length.

The results of the analysis are documented in an annual report. The report is 

sent out to the 12 districts in August or September; this time period corresponds 

with the wet season and is appropriate for conducting field reviews. The 
State Practices to Reduce Wet Weather Skidding Crashes

3-3

districts conduct data analysis of the locations and perform field reviews to 

identify recommended safety improvements.

Friction Testing Procedures

As part of their highway inventory, Caltrans maintains an active friction data set 

for the outer two lanes of the highway network with one test per lane-mile. The 

program tries to maintain a regular rotating schedule of every three years; 

however, staffing and equipment issues have had a profound impact on the 

schedule. Skid testing is also conducted at locations identified with wet highcollision concentration locations; however, an active file of these testing locations 

is not kept. For their highway inventory and wet weather crash investigations, 

Caltrans uses a locked wheel towed trailer tester with ASTM standard ribbed 

tires according to ASTM E 274 procedures.

Investigation and Remedial Action of Wet Weather Crash Locations

The list of locations identified in the Wet Table C is provided to the District 

Traffic Safety Engineers to follow up with a safety investigation. Locations 

identified in the Wet Table C are evaluated within a 12-month period. For each 

location, the investigator gathers and analyzes relevant data in an effort to 

identify contributing factors and potential countermeasures. The Caltrans Traffic 

Safety Program [6] provides recommended procedures for conducting site 

investigations. Several processes are used to identify the most effective 

improvement strategy. This includes a detailed analysis of the collision history, 

collision diagrams, friction test results, field investigations, and a review of 

roadway geometrics at the site to assist in identifying collision patterns that may 

susceptible to correction. Additional data elements such as time of day, direction 

of travel, movement preceding collision, or other factors may help in 

investigating crash patterns.

To mitigate the locations identified in the Wet Table C where improvements are 

recommended, Caltrans typically implements pavement improvements such as

superelevation changes, open-grade asphaltic concrete (OGAC) overlays, 

pavement grooving, high-friction surface treatments, or drainage improvements. 

Caltrans also installs “Slippery When Wet” signs if recommended by an 

investigation and will consider a reduction in the posted speed limit, if justified 

by a spot speed study.

Project and Program Evaluations

Caltrans evaluates wet weather crash location improvements that are funded 

through the HSIP program. These evaluations compare three years of both 

before and after improvement crash data using total, fatal, injury, and property 

damage only (PDO) crashes. However, Caltrans does not evaluate individual 

projects or subprograms; the safety program is evaluated as a whole.
State Practices to Reduce Wet Weather Skidding Crashes

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3.2 FLORIDA

The State Safety Office conducts an analysis to identify wet weather crash 

locations on the state roadway network using Florida’s Crash Analysis Reporting 

(CAR) System. The State Safety Office provides a report to the District Safety 

Engineers (DSEs) on an annual basis. However, the CAR system also provides 

the DSEs with the ability to conduct analyses of wet weather crashes in their 

districts at their discretion.

Identification of Wet Weather Crash Locations

The State Safety Office conducts an analysis of wet weather crashes on the state 

roadway network using five years of crash data through the CAR system. The 

analysis identifies sections with either a minimum of four wet weather crashes 

with 25 percent or more wet weather crashes or 50 percent or more wet weather 

crashes during a five-year period. The analysis uses a sliding window with 

0.3-mile segments and increments of 0.1 miles. Intersections are included in the 

segment analysis. The State Safety Office conducts the analysis on an annual 

basis and provides a report to the DSEs. The CAR system also allows the DSEs

to run an analysis to identify wet weather crash locations in their districts at any 

given time. For example, to identify wet weather crash locations, District 5 

(Orlando/Daytona Beach area) runs an analysis on a quarterly basis, and 

District 7 (Tampa area) runs an analysis on a monthly basis. The DSEs then 

make requests to the State Materials Office to conduct friction tests at the 

locations identified through the crash analysis.

Friction Testing Procedures

The Florida DOT State Materials Office maintains a database of skid test results 

and conducts skid tests on all state roadways on a three-year rotating schedule. 

The friction testing procedures are outlined in the Skid Hazard Reporting System 

User Documentation [7]. Skid tests are typically conducted at a speed of 40 miles 

per hour in the left wheel path using a standard two-wheel trailer towed by a 

one-ton pick-up truck conforming to ASTM E 274 requirements. Testing

equipment is calibrated in-house at intervals of 30 to 45 days per ASTM 

specifications. The normal testing procedure is to conduct three tests per mile or 

section (if less than a mile). A mean friction value is determined based on the 

arithmetic average of the tests conducted on a section of roadway. In addition to 

the routine test schedule, tests are also conducted when:

• Initiated by District Safety Engineer based on crash data;

• After construction or resurfacing;

• There is a special request (e.g., research project, county/city roadways); or

• There are roadway surfaces approaching the questionable range of friction 

values.
State Practices to Reduce Wet Weather Skidding Crashes

3-5

The DSEs review the most recent friction test results of the identified segments to 

determine if the friction number is low, 28 (FN40R) or less for posted speed limit 

of 45 miles per hour or less and 30 (FN40R) or less for posted speed greater than 

45 miles per hour.

Investigation and Remedial Action of Wet Weather Crash Locations

If the friction number is low for locations identified through the crash analysis, 

the DSEs review the work program to determine if the roadway is programmed 

for resurfacing. When the location is not included in the work program, the DSE 

must further investigate the site to identify potential contributing crash factors. 

The DSE reviews the traffic crash reports and field conditions (e.g., geometrics, 

surface condition, drainage, etc.). If inadequate pavement friction is identified as 

a contributing factor, the DSEs identify the appropriate mitigation techniques.

Florida has developed high-friction surface treatments for ramps, curves, or 

other locations with wet weather crashes. A specification for asphalt concrete 

friction courses has been developed to support this effort. Specific provisions are 

provided for using different aggregates, including granite. However, the 

specification does not specifically address the required friction value at the 

conclusion of compaction and rolling. Some of the districts use granite in the 

friction course if a skid hazard exists. The percent of granite changes in different 

parts of the State. Higher percentages of limestone aggregate are used in the 

southern part of the State, where the source mines are locally available. Florida 

is currently working on specifications for hybrid mixes with granite and 

limestone. As a temporary improvement, the District Maintenance Engineer will 

install warning signs at locations with a low friction number until a project can 

be implemented to improve the pavement surface friction.

The District 4 office (Broward County) has experimented with the use of highfriction surface treatments (HFST) in areas where friction-based crashes are a 

concern [8]. The HFST used a modified exothermic epoxy resin as a binder 

material and was top dressed with specific aggregates. Evaluation data found 

significant friction number increases after the application of the HFST. For 

example, one location reported a friction number of 35 (FN40R) before the 

treatment and a friction number of 104 after application. Evaluations concluded 

the HFST was effective in increasing the friction value of the roadway.

Typically it takes about two and one-half to three years to implement a 

resurfacing project (one year for programming, one year for design, and one year 

for construction). Currently, District 7 is piloting a design build or “push button”

program. With this program, multiple projects are programmed at one time; 

allowing design and construction to occur within nine months.

Project and Program Evaluations

Florida DOT developed a web-based database application called the Crash 

Reduction Analysis System Hub (CRASH), which is used to record and maintain 
State Practices to Reduce Wet Weather Skidding Crashes

3-6

safety improvement projects, update crash reduction factors (CRF), and apply 

CRFs to conduct a benefit/cost analysis of proposed safety improvement 

projects. The DSEs input all HSIP-funded projects, including improvements for 

wet weather crash locations, into CRASH. CRASH uses before and after crash 

counts to evaluate the safety projects and develop CRFs for the countermeasures 

implemented. The CRFs are typically calculated based on five years of both 

before and after data, but the system administrator may specify any specific time 

period for the calculation.

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.

3.3 MICHIGAN

Michigan DOT has a long-standing program for addressing wet weather crashes 

that has been in place for approximately 25 years. The Safety Programs 

Section leads the program and develops a list of locations for the regions to 

investigate.

Identification of Wet Weather Crash Locations

The identification of potential wet weather crash locations is carried out by 

Michigan DOT’s Safety Programs Section. The identification of locations is 

based on the results of the statewide friction testing program. Annually, the 

Safety Programs Section develops a list of locations with a skid number of less 

than 30 (SN40R). This list is then provided to each of the regions to investigate 

further and evaluate the need for action.

Friction Testing Procedures

Michigan DOT conducts friction testing on each lane of all state-maintained 

roadways on a three-year cycle (approximately one-third of roads each cycle). 

The tests are conducted using a Dynatest 1295 (locked wheel) friction tester 

according to ASTM E 274 requirements. Tests are conducted using an ASTM 

E 501 ribbed tire for level testing and an ASTM E 524 smooth tire for special 

testing. Friction tests are also conducted on corridors, spot locations, or 

intersections identified with a potential rear-end wet weather crash problem. 

The skid test results are available on the Michigan DOT network for regions to 

review.

Typically friction tests are not conducted on new construction or resurfacing 

projects, unless by special request. To ensure adequate surface friction, Michigan 

developed and implemented a wear track-based polishing test called the 

Michigan Aggregate Wear Index (AWI). The AWI is used to evaluate aggregates 
State Practices to Reduce Wet Weather Skidding Crashes

3-7

used in the top surface of hot mixed asphalt pavements. Each source of 

aggregate is tested and assigned AWI values which are updated as quarrying 

progresses into new areas. There is also a process to assign an AWI value to 

blended aggregates.

Investigation and Remedial Action of Wet Weather Crash Locations

The investigation of the sites identified by the Safety Programs Section with a 

skid number of less than 30 (SN40R) is carried out by the individual regions. The 

regions consider four factors in the evaluation:

1. Wet surface friction tests result is less than 30 (SN40R);

2. Estimated reduction in wet crashes is equal to at least three crashes per year 

per spot (intersection approach) or 0.5-mile segment location;

3. A field review to identify factors not related to surface friction qualities, such 

as “wheel tracking” or a clogged drainage structure that may contribute to a 

higher percentage of wet crashes; and

4. The time-of-return on the investment is five years or less.

Michigan DOT developed a tool in Microsoft Excel for determining if the location 

warrants action. The spreadsheet contains the Region’s wet crashes, wet crash 

percentages, and total crashes. The analyst must enter data for each of the 

locations identified based on the friction number, including project location, 

limits, number of nonwet crashes, number of wet crashes, and analysis period. 

The spreadsheet then calculates the expected reduction factor used in the timeof-return (benefit/cost) analysis.

If the location warrants action, the regions review the five-year plan to see if a 

project is scheduled for resurfacing. If the location is included in the schedule,

they might be able to schedule the resurfacing at a sooner date. If it is not in the 

plan, typically the regions will do an overlay, ultra thin overlay, mill and 

resurface, microsurfacing, paver placed surface seal, chip seal, or diamond 

grinding. The regions can submit these improvements as part of the annual call 

for safety projects. Once the regions have decided on a course of action, they 

must report it and the results of the analysis to the Safety Programs Section.

Michigan DOT is currently working with the FHWA Office of Pavement 

Technology on a pilot project on surface treatments. The project is focused on 

freeways ramps with curves. They are currently pilot testing Tyregrip on curves.

Project and Program Evaluations

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.
State Practices to Reduce Wet Weather Skidding Crashes

3-8

3.4 NEW YORK

New York established their Skid Accident Reduction Program (SKARP) in the 

mid 1990s to address a problem with the aggregate (dolomite) becoming 

polished. The program identifies and treats wet road crash locations on stateowned roadways and arterials (data is not available for local roads). Program 

requirements are outlined in Engineering Instruction 02-007, Skid Accident 

Reduction Program [9].

Identification of Wet Weather Crash Locations

The Office of Modal Safety and Security is responsible for the identification of 

wet weather crash locations. During April of each year, the Office of Modal 

Safety and Security analyzes the crash data to identify locations with an 

unusually high proportion of wet road crashes and develops a Wet Road 

Accident Priority Investigation Location (PIL) listing. The two primary 

considerations for identifying high wet road crash locations are the proportion of 

wet road crashes compared to total crashes and the occurrence of a specified 

minimum number of wet road crashes over a two-year period. Locations are 

identified if there are at least six wet road crashes during a two-year period in 

rural areas and at least 10 in urban areas with at least 35 percent of the total 

crashes occurring on wet road conditions. The PIL listing is sent to the Office of 

Technical Services to schedule friction testing for the locations identified on the 

list.

Friction Testing Procedures

Generally, the Office of Technical Services conducts friction tests from April to 

November at all locations included on the PIL list. However, there are a couple 

exceptions:

• Locations tested during the previous two years are not retested unless the 

regional offices specifically request the test; and

• Locations previously tested and found to exhibit one or more friction test 

results below an FN40R of 32 are not retested, even if more than two years 

has lapsed.

The tests are conducted with a skid trailer according to ASTM E 274 

requirements using a ribbed tire meeting ASTM E 501 requirements. Tests are 

conducted at 0.1-mile intervals. The Materials Bureau maintains a file of all 

friction testing data.

The Wet Road Accident PILs are categorized into two classes depending on the 

friction test results:

• Class 1 – Locations where one or more friction test results are below 32

(FN40R); or

• Class 2 – Friction test results are greater than or equal to 32 (FN40R).
State Practices to Reduce Wet Weather Skidding Crashes

3-9

Following the friction testing, the results are transmitted to the regions for 

review and consideration in the regions’ capital programming and preventive 

maintenance paving activities. The regions must investigate all locations with a 

friction number below 32. All sites with a friction number less than 26 must be 

remediated immediately. A friction number of 32 provides a stopping distance 

consistent with AASHTO design standards for highway sight distance and is 

consistent with design requirements for curves. A friction number of 26 was 

identified as a threshold coefficient of crash frequency based on analysis of wet 

weather PIL locations that had been friction tested.

Friction tests are not routinely conducted on resurfacing projects but are done 

occasionally. To provide for adequate surface friction, New York evaluates every 

aggregate source for both asphalt and concrete pavements. Petrographic 

evaluations are conducted on a biennial basis to qualify aggregates for the

Approved List of Fine and Coarse Aggregates and on plant samples to provide 

quality assurance (QA) of aggregates during production for state contracts. 

Friction performance is monitored annually on selected sections in the Pavement 

Friction Inventory.

Investigation and Remedial Action of Wet Weather Crash Locations

The Wet Road Accident PIL listing and the results of the friction tests are 

forwarded to the Regional Offices for consideration in the regions’ capital 

programming and preventive maintenance paving activities. The region’s 

investigate the locations on the PIL listing and are responsible for identifying and 

implementing remedial actions.

The remedial actions of wet road crash locations 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.

The regions are required to report on pending and completed remedial actions 

for all locations including in the PIL listing to the Office of Modal Safety and 

Security in May each year.

Project and Program Evaluations

New York conducts evaluations of SKARP projects funded through the HSIP. 

The evaluations are based on three 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 its 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 but has not 

since evaluated the overall program. The previous evaluation developed crash 

modification factors for resurfacing with high wet road crash locations based on 
State Practices to Reduce Wet Weather Skidding Crashes

3-10

the evaluation of 40 improved sites. The results indicated that resurfacing 

treatments at wet road crash locations are expected to reduce total crashes by 

approximately 20 percent, total wet road crashes by approximately 60 percent, 

and severe (fatal and injury) wet road crashes by approximately 70 percent [10].

Overall, the frictional quality of the State’s pavements has improved since the

program’s inception. A summary of 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.

3.5 VIRGINIA

In 1976, the Virginia Transportation Research Council (VTRC) developed a 

procedure for systematically identifying and evaluating wet crash sites or low 

skid number sites and established the Wet Accident Reduction Program 

(WARP). The program procedures are outlined in Virginia’s Wet Accident 

Reduction Program: A User’s Manual [11].

Identification of Wet Weather Crash Locations

The Traffic Engineering Division conducts an analysis of the crash data on an 

annual basis to identify Potential Wet Accident Hotspots (PWAH). Crashes are 

located at 0.1-mile intervals and serve as the principal database for identifying 

PWAHs. The identification process is as follows:

1. Crashes involving snow and ice are discarded.

2. Crash files are scanned by district, county, route, and mile point.

3. When a wet weather crash is registered, an additional 0.2 miles on either side 

of the site is scanned for additional wet weather crashes.

4. If one or more wet weather crashes are found, an additional 0.2 miles of the 

road is scanned for wet weather crashes.

5. Locations are classified as PWAHs when:

a. There are a minimum of three wet weather crashes, each separated by 

less than 0.2 miles; and

b. The proportion of wet weather crashes (wet/(wet+dry)) is at least 

20 percent higher than the ratio for all roads in the area.

Sites meeting these criteria are then friction tested by the Materials Division and 

locations. Virginia uses the conservative requirement of three wet weather 

crashes to designate a PWAH so as not to overlook a potentially dangerous 

condition. While the crash data is the primary source for identifying PWAHs, 

locations may also be identified based on field requests or Virginia’s ongoing 

program of annual skid tests of the primary and Interstate systems.
State Practices to Reduce Wet Weather Skidding Crashes

3-11

Friction Testing Procedures

The Materials Division conducts skid tests of the PWAH locations. Tests are 

conducted using an ASTM E 274 trailer unit on a wetted pavement at a speed of 

40 miles per hour. Each unit includes force and speed transducers, a control 

system, a record system, and a pavement wetting system. Test wheels are 

incorporated into the trailer. The test tire is a standard smooth tire.

Skid units are routinely (weekly, monthly, and yearly) calibrated following the 

manufacturer’s recommendations. Virginia DOT has a comprehensive 

calibration and verification program to ensure quality test results.

PWAH locations are tested using the following guidelines:

• Tests are conducted at a minimum frequency of one test for every tenth of a 

mile; for sites less than 1 mile in length, as many tests as possible are 

conducted with up to one test for every 0.05 mile.

• Unless a jurisdictional or construction project interferes, each section should 

have a minimum of three tests evenly spaced at 0.1-mile intervals beyond the 

limits of the referenced site (both before and after). If the skid number is less 

than 24 (SN40S), the sections are extended until three consecutive skid 

numbers greater than 24 (SN40S) are recorded. This ensures that any 

questionable areas are accurately identified.

• If possible, a minimum of one test is conducted within 200 feet prior to an 

intersection with a stoplight or stop sign.

• Data is reported by county-relative mileposts. Straight line diagrams from 

the Highway Traffic Records Inventory System (HRTIS) are used by the 

operator for reference and locating starting nodes in the field.

• Friction test results are uploaded into the HRTIS at the completion of testing 

for a district.

Locations with a friction number less than 20 (SN40S) are flagged for review by 

the districts. A friction number of 20 (SN40S) was selected as the threshold, to be 

consistent with other agencies. However, a more recent study conducted by 

VTRC and Virginia Tech has recommended use of a higher value (25-30).

The Virginia DOT conducted a study to identify and characterize Virginia’s 

nonpolishing aggregates in terms of their wet skid resistance for pre-evaluating 

the skid resistance of nonpolishing aggregates [12]. This was done by comparing

the effect of different asphalt mixtures on pavement surface macrotexture using 

the ASTM E 965-87 sand patch method and considering the effect of texturing on 

pavement surface friction. Fifty-seven sources of nonpolishing aggregates, 

representing 18 lithologies, were studied. Using the ASTM E 274-90 procedure 

skid testing of the 18 lithologies was carried out on 1,246 bituminous pavement

sections. Aggregate groups with the highest and lowest wet friction rankings

were identified.
State Practices to Reduce Wet Weather Skidding Crashes

3-12

Investigation and Remedial Action of Wet Weather Crash Locations

After the PWAHs are friction tested, the districts are responsible for reviewing 

the sites with a skid number of 20 (SN40S) or less and making decisions on 

possible remediation. The State does not mandate resurfacing if the friction is 

less than 20 (SN40S) without further review. Typically the districts will first 

check to see if the location is included in the maintenance schedule for 

resurfacing.

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. Pavement preservation activities also provide an opportunity to 

improve both the pavement condition and surface characteristics in a very costeffective manner. Depending on the pavement distress condition, the section 

could also be overlaid.

For Portland Cement Concrete (PCC) pavements, diamond grinding can be used 

to increase concrete pavement friction by enhancing surface macro texture. 

Adequate macro texture reduces the potential for hydroplaning. Saw cut 

grooving is used traditionally to restore adequate frictional characteristics of 

PCC pavements. Grooving can be either longitudinal or transverse.

If no action can be taken immediately, the district may temporarily put up 

“Slippery When Wet” signs until a more permanent improvement can be made. 

Districts have the option to apply for safety improvement program funding to 

improve these sites. HSIP projects are typically implemented within a three-year 

timeframe.

Project and Program Evaluations

Project evaluations are conducted for all safety improvements, including WARP 

projects funded through the HSIP. The evaluations are conducted using three 

years of both before and after improvement data.

Virginia DOT’s 2007 Wet Accident Reduction Program Report [13] provides a 

comparison of the 2007 PWAH sites to historical results. The findings include:

• Between 2002 and 2007, the total number of traffic crashes and the number of 

dry crashes increased in the State by almost 13 percent and 17 percent 

respectively, while the number of wet weather crashes decreased by almost 

7 percent. During this time period, the wet to dry crash ratio also decreased 

from 0.182 to 0.145.

• Between 2002 and 2007, the total number of PWAH sites reduced by almost 

27 percent. The number of PWAH sites in terms of million vehicle-miles 

traveled also reduced from 0.009 to 0.006.

• From 2006 to 2007, there was a 23 percent decrease in the number of low 

friction (less than or equal to 20) sites.
State Practices to Reduce Wet Weather Skidding Crashes

3-13

• As wet weather crashes are directly related to pavement friction, this

decrease in the number of wet weather crashes, the number of PWAHs 

identified, and the number of low skid number sites, could probably be 

indicative of improving pavement friction on Virginia’s primary and 

Interstate routes. All these reductions occurred despite an increase in overall 

crashes.

3.6 ADDITIONAL STATE PRACTICES

While not all of the interviewed states procedures are included in this guide, this 

section provides some additional notable practices and additional 

countermeasures being used to address wet weather crashes.

Identification of Wet Weather Crash Locations

New Jersey was the only State to incorporate crash severity into the initial site 

identification process. New Jersey originally identifies sites with an 

overrepresentation of wet weather crashes, based on statewide averages, and 

then ranks the locations by weighting the crash frequency by the crash severity. 

However, it should be noted that other states may incorporate severity into the 

process by prioritizing projects using of a benefit/cost analysis.

Kentucky did not have a program specifically focused on wet weather crashes; 

however, the State recently developed a Roadway Departure Safety 

Implementation Plan [14], which included the identification of roadway 

segments for implementing friction treatments to reduce wet pavement crashes. 

The analysis used to identify roadway sections for friction improvements 

evaluated 3,000 feet roadway segments and identified sections that met a 

minimum criterion of eight or more crashes and 35 percent or more occurring on 

wet pavement during the five-year period between 2004 and 2008. The analysis 

identified 227 segments for potential friction improvements. Kentucky started to 

implement the pavement friction improvements during the 2010 construction 

season, and it is anticipated the plan will be implemented over a five-year 

period.

Additional Countermeasures to Address Wet Weather Crashes

While the focus of this guide is on surface treatments to reduce wet weather 

skidding crashes, many of the states identified additional countermeasures to 

reduce wet weather crashes as a whole.

Florida is currently looking into the development of specifications for wet 

weather pavement markings to address nighttime, wet weather, and lane 

departure crashes. Florida also implemented a policy to use audible and 

vibratory markings on rural roadways (excluding limited access facilities) to 

address lane departure crashes, but the policy also indicates audible and 

vibratory pavement marking can be used in areas with a history of wet weather 
State Practices to Reduce Wet Weather Skidding Crashes

3-14

crashes. It is implied that the marking will be more visible and conspicuous 

during wet weather conditions. Similarly, Florida also uses inverted rib profile 

markings to improve the conspicuity of lane markings under all weather 

conditions. As a temporary improvement to address wet weather crashes 

occurring on the ramps at the interchange of two Interstate routes in FDOT 

District 5, optical speed bars and variable speed advisory signs were installed 

until the project can be completed.

In the Detroit metropolitan area, wet weather tape has become the standard 

pavement marking for freeways to provide for better reflectivity. It is being

installed systematically as part of overlay projects.

New York is currently investigating grooved in epoxy pavement markings with 

wet-night visibility elements mixed with standard glass beads as an option. 

They have done a few trial placements. The New York State Thruway Authority 

is now moving to all grooved in epoxy with special retroreflective elements.

New York has also been implementing Type 9 sheeting on signs for more 

visibility and is looking into a policy on rumble strips.
State Practices to Reduce Wet Weather Skidding Crashes

4-1

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
State Practices to Reduce Wet Weather Skidding Crashes

4-2

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 wetcollision 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)
State Practices to Reduce Wet Weather Skidding Crashes

4-3

State Testing Frequency Testing Equipment and Procedures

Friction 

Threshold

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.
State Practices to Reduce Wet Weather Skidding Crashes

4-4

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.
State Practices to Reduce Wet Weather Skidding Crashes

4-5

Table 4.4 State Project and Program Evaluations

State Project Evaluations Program Evaluations

California 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 Florida conducts evaluations on all HSIPfunded 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 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.

Virginia 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.
State Practices to Reduce Wet Weather Skidding Crashes

4-6

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.
State Practices to Reduce Wet Weather Skidding Crashes

Appendix

A-1

A. References

1. Fatality Analysis Reporting System web site: www-fars.nhtsa.dot.gov.

2. Federal Highway Administration. Technical Advisory, Pavement Friction 

Management, T 5040.38, June 17, 2010. http://www.fhwa.dot.gov/pavement/

t504038.cfm.

3. American Association of State Highway Transportation Officials (AASHTO).

Guide for Pavement Friction. Washington, D.C., 2008.

4. Federal Highway Administration. Technical Advisory, Surface Texture for 

Asphalt and Concrete Pavements, T 5040.36, June 17, 2005. 

http://www.fhwa.dot.gov/pavement/t504036.cfm.

5. Wang, S. et al. Estimating Wet-Pavement Exposure with Precipitation Data: Final 

Report. Contract 65A0226, California Department of Transportation, March 

2008. http://www.dot.ca.gov/research/researchreports/reports/2008/

estimating_wet_pavement_exposure.pdf.

6. California Department of Transportation (Caltrans) Traffic Safety Program. 

December 2010.

7. Skid Hazard Reporting (SHR) System User Documentation. Florida Department 

of Transportation. October 2008.

8. HNTB Corporation, University of Central Florida, and Wayne State 

University. Evaluation of Innovative Safety Treatments, Final Report. Contract 

B-D500, Florida Department of Transportation, January 2008. 

http://www.dot.state.fl.us/research-Center/Completed_Proj/Summary_SF/

BD500/BD500_v3_rpt.pdf.

9. New York State Department of Transportation. Skid Accident Reduction 

Program, Engineering Instruction EI 02-007, 2002. https://www.nysdot.gov/

portal/pls/portal/mexis_app.pa_ei_eb_admin_app.show_pdf?id=1679.

10. New York State Department of Transportation. 2008 Annual Evaluation 

Report, Highway Safety Improvement Program, Office of Modal Safety and 

Security, Albany, New York, 2008.

11. Mahone, D.C. and W.C. Sherwood. Virginia’s Wet Accident Reduction 

Program: A User’s Manual. Virginia Transportation Research Council, 

Charlottesville, Virginia, May 1996.

12. Mahone, D.C. and W.C. Sherwood. The Effect of Aggregate Type and Mix 

Design on the Wet Skid Resistance of Bituminous Pavement: Recommendations for 

Virginia’s Wet Accident reduction Program. Virginia Transportation Research 

Council, Charlottesville, Virginia, October 1995. http://www.virginiadot.org/

vtrc/main/online_reports/pdf/96-r10.pdf.
State Practices to Reduce Wet Weather Skidding Crashes

Appendix

A-2

13. Virginia Department of Transportation 2007 Wet Accident Reduction 

Program Report. Virginia Department of Transportation, Pavement Design 

and Evaluation Section, Central Office, Materials Division, October 2009. 

http://www.virginiadot.org/business/resources/Materials/2007_WARP_

Report.pdf.

14. Kentucky Roadway Departure Safety Implementation Plan. Commonwealth of 

Kentucky Transportation Cabinet. July 2010.
U.S. Department of Transportation

Federal Highway Administration

Office of Safety

1200 New Jersey Avenue, SE

Washington, DC 20590

May 20, 2011

Publication Number: FHWA-SA-11-21