Evaluation of Pavement Safety Performance

CHAPTER 2. LITERATURE SEARCH

SKID-RESISTANT PAVEMENTS

The following treatment summary is taken from Volume 6 of the NCHRP 500 series guidebooks for addressing ROR collisions (pages V-27 through V-30).⁽⁷⁾

• The 1999 statistics from FARS show that for two-lane, undivided, non-interchange, non-junction roadways, 11 percent of single-vehicle ROR fatal crashes occur on wet roadways, with 3 percent more occurring on roadways with snow, slush, or ice. Accidents on wet pavements are often related to the skid resistance of the pavement. It can also happen that the pavement friction available under dry roadway conditions will be significantly less than specified for the roadway and assumed in establishing design criteria (e.g., superelevation on curves). This can also lead to crashes. However, the major problem appears to be with wet pavement crashes.

  A vehicle will skid during braking and maneuvering when frictional demand exceeds the friction force that can be developed at the tire-road interface. While this can happen on dry pavements at high speeds, friction force is greatly reduced by a wet pavement surface. In fact, a water film thickness of 0.002 inches reduces the tire pavement friction by 20 to 30 percent of the dry surface friction. Therefore, countermeasures should seek to increase the friction force at the tire-road interface and reduce water on the pavement surface. The coefficient of friction is most influenced by speed. However, many additional factors affect skid resistance, including the age of the pavement, pavement structural condition, traffic volume, road surface type and texture, aggregates used, pavement mix characteristics, tire conditions, and presence of surface water.

  There has been a large amount of research funded by the FHWA, AASHTO, and pavement associations concerning designing better pavements-pavements which are more durable and more cost-effective (e.g., the FHWA/AASHTO Strategy Highway Research Program). The FHWA has issued a series of pavement-related technical advisories on such issues as needed changes in surface finishing of Portland cement concrete pavements for increased safety (FHWA, 1996).⁽¹¹⁾ An important parameter in all this work is pavement skid resistance, perhaps the major safety-related factor along with pavement drainage design. However, most of this research and implementation effort is oriented toward policy or systemwide changes in new pavements or repaving efforts. While the best safety-related pavement design possible should be used in all paving efforts, the details of pavement design are beyond the scope of this guide.

  Instead, this section will concentrate on improvements that can be made to sites that have, or are expected to experience, skidding-related ROR crashes. These usually involve improvements to increase skid resistance (higher friction factor). Such improvements should have high initial skid resistance, durability to retain skid resistance with time and traffic, and minimum decrease in skid resistance with increasing speed. Countermeasures to improve skid resistance include asphalt mixture (type and gradation of aggregate as well as asphalt content), pavement overlays on both concrete or asphalt pavements, and pavement grooving. Water can also build up on pavement surfaces due to tire rutting, an inadequate crown, and poor shoulder maintenance. These problems can also cause skidding crashes and should be treated when present. While there is only limited research on such site-specific programs, the results of this research coupled with the results of research on the general effectiveness of decreasing skidding would place this in the "proven" category.

  Treatment will target locations where skidding is determined to be a problem, in wet or dry conditions. The ultimate target, however, is a vehicle involved in a crash due to skidding, usually on wet pavement. With respect to ROR or head-on crashes, the target vehicle is one that runs (skids) off the road due to insufficient skid resistance or becomes involved in a head-on crash either by skidding into the opposing lane or by crossing into the opposing lane after an overcorrection from an initial ROR maneuver caused by insufficient skid resistance.

  There are many different specific countermeasures that may be implemented to improve skid resistance. This may include changes to the pavement aggregates, adding overlays, or adding texture to the pavement surface. The effectiveness of the countermeasure not only depends on that measure selected, but also will vary with respect to location, traffic volume, rainfall propensity, road geometry, temperature, pavement structure, etc.

  The New York State DOT has implemented a program that identifies sites statewide that have a low skid resistance and treats them with overlays or microsurfacing as part of the maintenance program. A site is eligible for treatment if its 2-year wet accident proportion is 50 percent higher than the average wet accident proportion for roads in the same county. Between 1995 and 1997, 36 sites were treated on Long Island, resulting in a reduction of more than 800 annually recurring wet road accidents. These results and others within the state support earlier findings that treatment of wet road accident locations result in reductions of 50 percent for wet road accidents and 20 percent for total accidents. While the reductions in ROR or head-on crashes cannot be extracted from the data at this time, it appears that reductions in these types would be at least the same as for total crashes.

  While these results could be subject to some regression-to-the-mean bias, the New York staff has found that untreated sites continue to stay on the listing until treated in many cases-an indication that these reductions are clearly not totally due to regression. The New York State DOT is planning a more refined data analysis to account for possible biases in these effectiveness estimates. Based on the current knowledge, this identification/treatment strategy would be classified as "proven."

  Monitoring the skid resistance of pavement requires incremental checks of pavement conditions. Evaluation must identify ruts and the occurrence of polishing. Recent research (Galal et al., 1999) has suggested that the surface should be restored between 5 and 10 years in order to retain surface friction, but the life span is affected by site characteristics such as traffic volume.⁽¹²⁾ In addition, spot- or section-related skid accident reduction programs will be clearly most successful if targeted well. The New York State DOT program noted above provides a methodology for such targeting. In addition, in a 1980 Technical Advisory, the FHWA provided a detailed description of a "Skid Accident Reduction Program," including not only details of various treatments, but also the use of crashes and rainfall data in targeting the treatments. Skid resistance changes over time. This requires a dynamic program and strong commitment. As noted in the preceding section, it also requires good "targeting." When selecting sites for skid resistance programs, it is important to somehow control for the amount of wet-pavement exposure. This will help decrease the identification of sites that have a high wet-accident proportion or rate simply because of high wet-weather exposure with no real pavement-friction problems. Unfortunately, it is difficult or impossible for an agency to develop good wet-pavement crash rates per vehicle mile for all roadway sections due to the lack of good wet-weather exposure data for all sites. Such data would require both good rainfall data for all potential sites and good measures of traffic volume during wet and dry weather. In its Skid Accident Reduction Program, the New York State DOT uses a surrogate for such detailed data. The DOT compares the proportion of wet-weather crashes at each site with the proportion for similar roads in the same county. The assumption here is that rainfall (and thus wet-pavement exposure) would be similar across a county, a reasonable assumption.

  Data are needed on traffic crashes by roadway condition. In addition, measures of traffic exposure that identify and reflect both dry and wet periods are needed. Finally, measurements of road friction and pavement water retention should be documented both before and after implementation of a strategy.

  New York State DOT estimates that its resurfacing/microsurfacing projects are approximately 0.5 miles long, with an average treatment cost of approximately $20,000 per lane mile (1995 dollars).

  
  
  

PROVIDE GROOVED PAVEMENT

The following treatment summary is taken from Volume 7 of the NCHRP 500 series guidebooks for horizontal curves collisions (pages V-25 through V-27).⁽⁸⁾

• Pavement grooving is a technique by which longitudinal or transverse cuts are introduced on a surface to increase skid resistance and to reduce the number of wet-weather crashes. The grooves increase skid resistance by improving the drainage characteristics of the pavement and by providing a rougher pavement surface. Several studies show that grooved pavements reduce wet-weather crashes. However, some potential adverse effects should be considered before this strategy is implemented, including the potential of increased noise pollution, accelerated wearing of pavements, and negative effects on steering.

  While pavement grooving is a way to add texture to the pavement surface, its primary objective is to improve the drainage and to mitigate hydroplaning. The grooves decrease the water film thickness on a pavement surface and allow for greater tire-pavement surface interaction during adverse weather conditions. Because pavement grooving is such a unique approach to increasing the skid resistance of a pavement, it is treated separately. The section immediately following this one presents results of studies that evaluated the safety effectiveness of pavement grooving. That is followed by a section that presents attributes unique to pavement grooving that should be considered before this type of treatment is implemented.

  Numerous studies on the safety effectiveness of pavement grooving have been conducted, but none of these studied have controlled for regression to the mean so the results should be considered with caution. Wong (1990) performed a before-after evaluation of the effectiveness of pavement grooving based upon data from one site in California.⁽¹³⁾ The site was a two-lane highway with steep vertical grades and sharp horizontal curves. Based upon accident data from a 3-year before period and a 1-year after period, Wong found a 72-percent reduction in wet-pavement accidents, while only finding a reduction of about 7 percent in dry-pavement accidents. Wong concluded that pavement grooving was effective in reducing wet-pavement accidents.

  Zipkes (1976) analyzed the frequency of accidents and the percentage of accidents on wet and dry pavement surfaces during a 7-year period to evaluate the effect of pavement grooving.⁽¹⁴⁾ Accident data were obtained for a 44-km (27-mi) section of highway near Geneva, Switzerland. Transverse grooves were cut into the pavement with varying groove distances over a 2-km (1.2-mi) section of highway. Grooving of the polished road surfaces reduced the hazard of accidents when drainage conditions were unfavorable. Zipkes indicated that the advantage of grooving is the reduction of water-film thickness, which leads to better contact between the tire and the road surface for the transmission of forces.

  Smith and Elliott (1975) evaluated the safety effectiveness of grooving 518 lane-km (322 lane-mi) of freeways in Los Angeles, while 1,200 lane-km (750 lane-mi) of ungrooved pavement were used as a control.⁽¹⁵⁾ The analysis was conducted using 2 years of before data and 2 years of after data. Only fatal and injury accidents were included in the evaluation. Smith and Elliott found that longitudinal pavement grooving resulted in a 69-percent reduction of wet-pavement accident rates. Sideswipe and hit object accidents were reduced to the largest extent. Pavement grooving did not change the dry-pavement accident rates.

  Mosher (1968) synthesized results from studies conducted by state highway departments on the effects of pavement grooving.⁽¹⁶⁾ Information for the report was obtained from 17 states, including Colorado, Florida, Georgia, Idaho, Illinois, Indiana, Louisiana, Minnesota, Missouri, Nebraska, New York, Ohio, Pennsylvania, Texas, Utah, Wisconsin, and Wyoming. Some sections of highway had longitudinal grooves, while other sections had transverse grooving. Pavement grooving proved very effective, reducing crashes by 30 to 62 percent.

  Farnsworth (1968) evaluated the effects of pavement grooving on five sections of California highways.⁽¹⁷⁾ Farnsworth measured the coefficients of friction before grooving and after grooving and found that pavement grooving increased the coefficients of friction, changing the friction values from below critical to above critical. Analysis of accident data revealed a reduction in wet-pavement accidents at each of the sites.

  The NYDOT evaluated the safety effectiveness of pavement grooving based on the installation of grooves at 41 sites. NYDOT found that wet-road accidents were reduced by 55 percent, and total accidents (dry and wet) were reduced by 23 percent. The results were statistically significant at the 95th percentile. Regression to the mean was not addressed in the analysis.

  Pavement grooving involves making several shallow cuts of a uniform depth, width, and shape in the surface of the pavement (Mosher, 1968).⁽¹⁶⁾ Grooves may be cut longitudinally along the pavement (parallel to the direction of travel) or in the transverse direction (perpendicular to the direction of travel). Transverse grooving has been used to a lesser extent than longitudinal grooving, partially because most grooving equipment lends itself more readily to placing grooves parallel to the roadway. Grooves cut in the longitudinal direction have proven most effective in increasing directional control of the vehicle, while transverse grooving is most effective where vehicles make frequent stops, such as intersections, crosswalks, and toll booths. When pavements are grooved, it is important that the pavement contain nonpolishing aggregate.

  While studies have indicated that pavement grooving reduces wet-pavement accidents, there have been several concerns associated with pavement grooving (Mosher, 1968).⁽¹⁶⁾ One concern has been the effect that pavement grooving has on the durability of various pavement types. For example, one of the most frequently asked questions by engineers in northern climates is, "What will water freezing in the grooves do to the concrete pavement?" In an examination of grooved pavement in Minnesota after one winter, there appeared to be no deterioration in the grooved pavement because of the freeze-thaw cycles. Concern also has been expressed about grooves in asphalt pavement losing their effectiveness because the material can be flexible enough to "flow" back together, particularly during hot weather. This phenomenon has been observed under certain conditions with a fairly new asphalt pavement or with a pavement with low aggregate content. Concern has also been expressed over the loss of effectiveness because of grooved pavements wearing down under high-traffic conditions.

  Complaints also have been received that longitudinal grooves adversely affect the steering of certain automobiles and motorcycles. In general, no severe problems have been encountered. This finding is supported by research conducted by Martinez (1977), who studied the effects of pavement grooving on friction, braking, and vehicle control by computer simulation and full-scale testing.⁽¹⁸⁾ Martinez considered automobiles, motorcycles, and automobile and towed-vehicle combinations in his evaluation.

  In Iowa, residents living adjacent to I-380 near Cedar Rapids complained that transverse grooving was the cause of an especially annoying tonal characteristic within the traffic noise (Ridnour and Schaaf, 1987).⁽¹⁹⁾ As a result of the complaints, the surface texture of a section of I-380 was modified. The transverse grooving was replaced with longitudinal grooving. A before-after analysis of the traffic noise levels showed that the surface modification lowered overall traffic noise levels by reducing a high-frequency component of the traffic noise spectrum.

  
  
  

FURTHER LITERATURE REVIEW

A search of available literature related to the safety effects of improved skid resistance turned up few additional materials. The limited research available does indicate, as would be expected, that higher skid resistance measurements are associated with lower crash rates, particularly wet-road-related collisions. Studies comparing the safety improvement after specific skid resistance improvement treatments are particularly rare, and the data and evaluation methods typically poor. These limited studies do, however, indicate reductions in collisions following treatment. Additional literature was also identified related to low-cost pavement preservation treatments and their properties.

Neuman et al. discuss in general terms specific countermeasures that may be implemented to improve skid resistance.⁽⁷⁾ These may include changes to the pavement aggregates, adding overlays, or adding texture to the pavement surface. They state that the effectiveness of the countermeasure not only depends on the measure selected, but also varies with respect to location, traffic volume, rainfall propensity, road geometry, temperature, pavement structure, etc. They indicate that when selecting sites for skid resistance programs, it is important to somehow control for the amount of wet-pavement exposure.

Torbic et al. discuss pavement grooving.⁽⁸⁾ Pavement grooving is a technique by which longitudinal or transverse cuts are introduced on a surface to increase skid resistance and to reduce the number of wet-weather crashes. The grooves increase skid resistance by improving the drainage characteristics of the pavement and by providing a rougher pavement surface. Several studies showed that grooved pavements reduce wet-weather crashes between 55 and 72 percent although the evaluation methods applied are not considered state-of-the-art by today’s standards.

Lyon and Persaud evaluated the safety impacts of the New York Department of Transportation (NYSDOT) skid-accident reduction program.⁽²⁰⁾ In this program, sections of roadway with a high proportion of wet-road accidents are identified and are friction tested. Those locations with poor friction numbers are then treated with a 1.5-inch HMA resurfacing or a 0.5-inch microsurfacing using non-carbonate aggregates. Resurfacing is considered to be effective for 15 years while the microsurfacing is effective up to 7 years, depending on the existing pavement condition and quality of construction. Friction testing was done (using a locked-wheel skid trailer with ribbed tire), and readings under 32 were considered to warrant treatment. The EB before-after study approach was applied to several crash types and both segment and intersection locations. Results for expected accident reductions are shown in table 1 . Further results are available in the paper, disaggregated by area type and number of lanes for segments and traffic control type and number of approaches for intersections.

Table 1 . Summary of results from NYSDOT skid-accident reduction program analysis.

Ivan et al. explored the relationship between wet-pavement friction and crashes to identify whether wet-pavement friction explains significant variation in crash frequency between similar locations, and whether this is particularly significant at high crash locations such as sharp curves and intersections.⁽²¹⁾ Data for approximately 150 mi of roadway were collected. Three years of crash data were collected where available. The amount of friction at each location was measured using the locked-wheel skid trailer. Negative binomial regression models K, A, or B crashes on the KABCO scale were developed separately for divided and undivided roadways. Additional explanatory variables considered included degree of horizontal curvature, rate of change of vertical curvature, number of intersections and driveways, pavement width, area type (rural, suburban, or urban) and speed limit. Dependent variables considered included total, wet-road, segment related (sideswipe opposite direction, head-on fixed object, and moving object), and intersection related (turning same direction, turning intersecting paths, sideswipe same direction, angle, rear-end, and pedestrian) crashes. The model results indicated that wet-pavement friction is most associated with increased crashes under conditions where increased braking would be demanded, that is in curves and near driveways. Interestingly, increased wet-pavement friction was associated with more total crashes on urban undivided roads with mild curvature and on urban divided roads.

Oh et al. conducted naïve and comparison group before-after studies of three experimental types of pavements: open graded asphalt concrete (OGAC), groove pavement (GP), and rubberized open graded asphalt concrete (R-OGAC).⁽²²⁾ Wet-pavement-related crashes were the focus. The findings included a 29 and 41 percent decrease for the 13 OGAC sites using the naïve and comparison group approaches, respectively. The sample sizes were too small to draw conclusions for the GP and R‑OGAC. Calculation of crash rates included the exposure to wet weather, which was collected from the closest weather recording station. Another part of the study found that the friction numbers are dependent on seasonal effects, including temperature, average monthly precipitation, and the number of dry months prior to last precipitation.

Izevbekhai and Watson evaluated the before and after collision data for 14 concrete pavement sections where the pavement was overlaid or rebuilt and the new surface included a longitudinal turf drag, or broom drag.⁽²³⁾ Previously, transverse friction treatments (e.g., tining) had been applied but were discontinued owing to concerns regarding noise. The study sought to determine whether the new longitudinal treatments were as effective in providing adequate friction. Collisions were analyzed to see whether frequencies, collision rates (per million vehicle-mi), proportion of wet-weather collisions, or the ratio of wet to dry collision counts increased following the treatment. Differences were subjected to the Chi-squared and Mann Whitney U tests to measure the statistical significance of any differences between the before and after periods. The segments analyzed were selected to be minimally influenced by other collision risk factors such as curves, poor sight distance, poor surfaces, etc. The results found no significant differences in the various crash measures from before to after the new treatment.

Erwin conducted a naïve before-after study of resurfacing and microsurfacing projects. Results for microsurfacing indicate a 32-percent reduction in wet-weather collisions, 24-percent reduction in intersection collisions, and 29-percent reduction in rear-end collisions.⁽²⁴⁾

Reddy et al. evaluated the application of the Tyregrip™ HFS system to a 300-ft section upstream of an on-ramp in Florida.⁽²⁵⁾ The ramp was treated because a high number of wet-weather ROR collisions had occurred there. Skid testing confirmed that the available skid resistance was much higher (104) after treatment compared with 35 before. It was also observed that vehicle speeds decreased, as did vehicle encroachments to either shoulder. The limited time periods and single location did not allow for a scientific study of collisions, although they were observed to decrease from an average of 2.54 per year before treatment to 2 in a 1-year period after, a decrease of 21 percent.

Mayora and Pina studied the relationship between skid resistance and injury collisions on two-lane rural roads in Spain.⁽²⁶⁾ Segments including intersections were not included. Average sideway-force coefficient routine investigation machine (SCRIM) skid resistance measurements over a 5-year period were included in the analysis. Categories of alignment (e.g., tangent, radius > 500 m, radius 250-500 m, radius < 250 m) and categories of skid resistance (e.g., SCRIM ≤ 40, 40 < SCRIM ≤ 45, 45 < SCRIM ≤ 50, 50 < SCRIM ≤ 55, 55 < SCRIM ≤ 60, SCRIM > 60) were defined for the analysis. Statistical tests were applied to see whether the mean crash rates differed between SCRIM categories for each alignment category tested. A before-after comparison group study was also conducted to assess the benefits of skid resistance improvements. Because the comparison group crash rate was higher (0.32 to 0.29 wet-road crashes), it was concluded that the treated sites were not selected based on the crash rate and regression-to-the-mean was not a factor. A sample of 419 segments with an average SCRIM value less than 50 was treated to improve the SCRIM value to more than 60. Results of crash rate analyses showed that both wet- and dry-road crash rates decreased as skid resistance increased. Wet-road crash rates were found to be significantly higher in curves than on tangents. For dry-road crashes, no differences were found between curves and tangents. It was concluded that for tangents and curves with a radius less than 500 m, crash rates are significantly lower when the SCRIM value is greater than 55. For curves with a radius greater than 500 m, the SCRIM value cutoff is 60. The before-after study indicates the benefits of increasing the skid resistance (SCRIM value) from less than 50 to greater than 60 is a 68-percent reduction in wet-road crashes. When considering curves only, the reduction was estimated to be 84 percent.

Hughes studied the impacts of thin HMA resurfacing projects on crash performance for two-lane roads with posted speeds greater than 45 mi/h.⁽²⁷⁾ The purpose of the study was to determine whether new resurfacing projects have any impact on safety, resulting from "the improved ride quality and visual contrast created by new pavement markings on a smooth asphalt surface that could create for the driver the impression of a safer road that can be traversed at a higher speed." This is commonly referred to as a "novelty effect," which may result in more crashes initially after resurfacing before the effect wearing off over time. The study contrasted resurfacing projects that were coupled with minor or major safety improvements with those where only resurfacing was performed. Some of the key findings from this study were as follows:

• The results were inconclusive regarding the effect of resurfacing on crash rates. For some States, there was an improvement, but for others there was no benefit, or even an increase in crash rate following resurfacing. This is likely the result of highly variable conditions (and presumably paving materials) from State to State and project to project, which could not be comprehensively quantified for the study sites.
  
  
• There is no evidence to suggest that resurfacing adversely affects crash frequency downstream from the resurfaced section of the road.
  
  
• With respect to analysis methodology, selecting sites according to their pavement history (and not crash history), using a long crash history (3 to 5 years), and using a large sample size (e.g., long stretches of highway) help to mitigate regression-to-the-mean and the random nature of data.
  
  

Li et al. evaluated the long-term friction performance of pavement preservation treatments commonly used by the Indiana Department of Transportation to assist in the decisionmaking process regarding when and where to use various preservation treatments.⁽²⁸⁾ Treatments evaluated included chip seals, fog seals, microsurfacing, thin and ultra-thin asphalt overlays (including UTBWC), and diamond grinding. Key findings for the various treatments were as follows:

• For chip-sealed surfaces, the greater the friction number on the old pavement, the greater the friction on the new chip-sealed surface. Surface friction decrease occurred after 12 mo in service, and when the surface reached an age of approximately 30 mo, friction started to decrease continuously over time. Also, truck traffic was observed to affect the performance of a chip seal more significantly than AADT.
  
  
• For microsurfacing, friction increased significantly in the first 6 months and peaked after 12 mo. After 12 mo, friction tended to decrease continuously over time, but never to what might be considered an intervention level, even after 42 mo.
  
  
• For UTBWC, friction numbers tended to peak after 6 mo of service, "about 6 mo earlier than conventional HMA mixes." UTBWC provided good texture depth (macrotexture), much better than conventional HMA surfaces. However, significant friction decrease occurred over time, with up to a 34‑percent decrease after 33 mo in service. Noticeable polishing was observed in the limestone aggregate used.
  
  
• For thin, fine graded (4.75 mm) HMA overlays, friction is high after construction but decreases quickly and dramatically over time after exposure to traffic. In some sections, friction decreased by 25 percent after 6 mo and by 36 to 48 percent after 12 mo, depending on traffic volume. Friction tended to stabilize after 12 to 18 mo. The use of steel slag in these mixes is recommended for better friction performance.
  
  
• For diamond grinding on concrete surfaces, the steady-state friction should be maintained over time, but was highly dependent on the aggregate properties and grinding texture configuration.
  
  

Roe et al. examined the relationship between pavement surface texture and crashes based on an extensive analysis of texture, friction, and crash data in the United Kingdom.⁽²⁹⁾ Some of the key findings from this study included the following:

• Macrotexture has a marked effect on all accidents, whether or not they involve skidding or whether conditions are wet or dry. High macrotexture has a beneficial effect in all circumstances.
  
  
• Injury accidents are less likely to occur at high texture depths. This is possibly due to the higher hysteresis component of friction, providing drivers with additional control and maneuverability in all conditions to reduce severity of the crash.
  
  
• In terms of guidance for desirable macrotexture depth, the cross-over point falls between 0.6 mm and 0.8 mm, indicating that the risk of accidents is greater for roads with an average texture depth less than about 0.7 mm than for those above this level.
  
  
• Crash rate rises markedly with decreasing texture less than about 1 mm, but is essentially constant at a low level greater than 1 mm.
  
  
• Skidding resistance (as measured by SCRIM) is normally independent of texture depth, but may be reduced when the macrotexture is unusually low, for example on a worn surface dressing (chip seal).
  
  
• The idea of a minimum level of macrotexture does not affect in any way the existing requirements for minimum levels of skidding resistance. Both macrotexture and skidding resistance are needed for safe roads.
  
  

Davies et al. used highway data from 1997 to 2002 in New Zealand from the entire State Highway network to try to look for any correlations between crash rate and road characteristics (traffic, texture, skid resistance, curve radius, cross-fall, roughness, and rut depth).⁽³⁰⁾ Only two-lane roads were included in the analysis. Some of the key findings from this study included the following:

• As traffic volume decreases, crash rate increases. This was not unexpected because the quality of the road reflects daily traffic.
  
  
• As skid resistance increases, the greatest percent reduction in crash rate occurs for wet-road crashes.
  
  
• The percent reduction in crash rate is constant for a given absolute increase in skid resistance value, regardless of the initial skid resistance value.
  
  
• Crash risk is reduced with increasing texture, although not at a statistically significant level. The relationship between crash rate and texture is not strong.
  
  
• The primary emphasis should be on increasing skid resistance rather than texture.
  
  
• In percentage terms, increasing skid resistance (e.g., SCRIM) has a greater effect in reducing "wet-road crashes" than in reducing "all crashes."
  
  

Peshkin et al. developed guidelines for the use of pavement preservation treatments on high-volume roadways as part of a Strategic Highway Research Program 2 (SHRP2) Renewal research project.⁽³¹⁾ Agencies traditionally tend to shy away from preservation on high-volume roadways, and this project sought to provide substantial guidance for preservation practices on high-volume roadways. High-volume roadways were defined under this effort as those with an average daily traffic of at least 5,000 and 10,000 vehicles per day for rural and urban roadways, respectively. This report provided valuable information on the expected design life and cost of various pavement preservation treatments. It also addressed some of the appropriate applications and risks associated with various treatments, and should serve as a ready reference for agencies in selecting preservation treatments.

The literature review, while sparse, did reveal important insights to consider, including the following:

• Pavement friction is dependent on seasonal effects, including temperature, average monthly precipitation, and the number of dry months prior to last precipitation. The readings must be standardized across sites and years.
  
  
• Confounding factors that influence collision risk and may interact with the safety effects of skid resistance include location type (segment, intersection approach, curve, etc.), area type, speed limit, traffic volume, rainfall propensity, roadway geometry, temperature, and pavement structure.
  
  
• Expected collision reductions from friction improvements depend on both the level of friction prior to and after treatment. Reductions in wet-road crashes up to approximately 75 percent may be expected.
  
  
• Friction of various low-cost pavement treatments tends to decrease with time, and therefore the safety benefit may also be expected to decrease with time.
  
  
• Evaluations of improved skid resistance have typically not applied statistically rigorous before-after study designs.
  
  
• While good texture (depth) is important, it does not guarantee a good skid resistance or a safer pavement.