Evaluation of Pavement Safety Performance

EXECUTIVE SUMMARY

The intent of this study was to isolate the effects of various low-cost pavement treatments on roadway safety. This was a retrospective study of pavement safety performance, looking back at crash data before and after treatments were installed. Both flexible and rigid pavement treatments were analyzed, with the majority typically used for pavement preservation or minor rehabilitation purposes. Although State highway agencies recognize that most of these treatments generally improve pavement friction, they are not typically installed explicitly for safety improvement, with one exception, high-friction surfacing (HFS), which is typically applied as a spot safety treatment.

The research was conducted as part of Phase VI of the Federal Highway Administration (FHWA) Evaluation of Low-Cost Safety Improvements Pooled Fund Study (ELCSI-PFS). This PFS was established to conduct research on the effectiveness of the safety improvements identified by the National Cooperative Highway Research Program (NCHRP) Report 500 guides as part of implementation of the American Association of State Highway and Transportation Officials (AASHTO) Strategic Highway Safety Plan. The intent of the work conducted under the various phases of the ELCSI-PFS is to provide a crash modification factor (CMF) and benefit-cost (BC) economic analysis for each of the targeted safety strategies identified as priorities by the PFS States.

With respect to pavement surfaces, NCHRP Report 500, Volume 6, which addresses reducing run-off-road (ROR) crashes, presents Strategy 15.1 A7, "Skid-Resistant Pavements," as a key to reducing ROR crashes. Volume 7, which addresses reducing collisions on horizontal curves, likewise discusses Strategy 15.2 A7, "Provide Skid-Resistant Pavement Surfaces," as a key strategy for reducing crashes at horizontal curves. The report recognizes that there had been only limited research conducted on site-specific treatments as of 2003. However, given the results of other research on general effectiveness of decreased skidding, the report places this strategy in the "proven" category. The report also recognizes that the effectiveness of friction-enhancing treatments will diminish over time; therefore, States using this strategy must conduct a dynamic program to target the appropriate sites for new treatment and to maintain the safety benefit from existing treatments.

In a similar manner, the FHWA Low-Cost Treatments for Horizontal Curve Safety guide recommends "skid-resistive pavement surface treatments" as a low-cost treatment for reducing crashes at horizontal curves. This guide specifically mentions remedial treatments such as hot-mix asphalt (HMA) overlays, surface treatments, grinding, and grooving of pavement surfaces for both concrete and asphalt pavements where friction demand is higher.

A further literature review revealed important insights to consider, including the following:

• 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.
  
  

The ultimate outcome of this effort to build on knowledge from previous work, while overcoming the shortcomings of those studies, is to gain a better understanding of the effects that various common, low-cost pavement treatments have on roadway safety. Two of the more tangible ways for quantifying this is through CMF and BC ratios for each treatment type. These products will potentially help State transportation departments in the decisionmaking process for selection of a pavement treatment for a particular project.

In achieving these outcomes, the state-of-the-art empirical Bayes (EB) before-after methodology was applied to evaluate the effects on various crash types-total, injury, wet road, dry road, wet-road ROR, and all ROR-of the following treatments, based on data from California, North Carolina, Pennsylvania, and Minnesota.

• Chip Seal (single and double layer).
  
  
• Diamond Grinding (concrete pavement only).
  
  
• Grooved Concrete Pavement.
  
  
• Microsurfacing (asphalt and concrete pavement).
  
  
• Open-Graded Friction Course (OGFC) (asphalt and concrete pavement).
  
  
• Slurry Seal (asphalt pavement).
  
  
• Thin HMA (asphalt and concrete pavement).
  
  
• Ultra-Thin Bonded Wearing Course (UTBWC) (asphalt and concrete pavement).
  
  

In addition, a simple before-after evaluation was completed for HFS treatments based on limited data from several States, including Colorado, Kansas, Kentucky, Michigan, Montana, South Carolina, Tennessee, and Wisconsin. HFS was analyzed separately because it is typically used specifically for safety improvement (through friction enhancement) and not pavement preservation, as with other treatments. Although the HFS treatment data were insufficient to apply the EB method, it still revealed tremendous crash reduction potential for this treatment.

The combined results for all treatment types subjected to the rigorous EB evaluation (except grooving, for which there were very few sites) suggest that the treatments resulted in benefits for wet-road crashes, with the exception of thin HMA for two-lane roads (for both California and North Carolina, the two States with large enough samples for a definitive result), and for OGFC for two-lane and multilane roads, for which the effect was negligible.

For dry-road crashes, crashes increased for microsurfacing on two-lane roads (except for North Carolina), thin HMA and OGFC on two-lane roads, and OGFC and chip seal on multilane roads. There were indications of a benefit for UTBWC, chip seal, and slurry seal on two-lane roads, and diamond grinding on freeways.

The CMFs for treatments by road and crash type may be considered for use in the Highway Safety Manual and the CMF Clearing House.

A thorough disaggregate analysis of the before-after evaluation data was undertaken in which regression analysis was used to investigate the effects on the CMFs of a number of variables, including traffic, precipitation, expected crash frequency before treatment, and environment (urban/rural). In the end, the crash modification functions (CMFunctions) developed were not robust enough to recommend them. Nevertheless, there were useful insights that suggest that it would be worthwhile to pursue the development of robust CMFunctions in future research. The results did suggest that there is a relationship between CMFs and average annual daily traffic (AADT) and sometimes precipitation, urban versus rural setting, and expected crash frequency. However, the direction of the effect is not always consistent, varying by crash type, site type, and treatment. Future research is needed to reconcile (i.e., explain) these apparent inconsistencies.

An economic analysis was conducted for treatments and States for which the sample size was large enough and for which there was a statistically significant (5-percent level) benefit for total crashes based on the EB evaluation. The results indicate that BC ratios larger than 2.0, considering impacts on safety only, are attainable for the following:

• Chip seal on two-lane roads (California only).
  
  
• Diamond grinding on freeways.
  
  
• Thin HMA on multilane roads (North Carolina only)
  
  
• OGFC on freeways.
  
  
• Slurry seal on two-lane roads.
  
  
• OTBWC on two-lane roads.
  
  

For other treatments/road classes/States, sample sizes were too small in some cases and, in other cases, overall safety benefits were not achieved or were statistically insignificant.

For HFS treatments, the results of the cursory before-after analysis suggest that HFS can be a highly safety- and cost-effective treatment for which implementation should continue. It is strongly recommended that additional data be collected to conduct a robust EB before study to derive a CMF that could be recommended to practitioners and for which a BC ratio could be confidently estimated.