U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
| REPORT |
| This report is an archived publication and may contain dated technical, contact, and link information |
|
 |
| Publication Number: FHWA-HRT-14-065 Date: February 2015 |
Publication Number: FHWA-HRT-14-065 Date: February 2015 |
The research was broad in scope, covering several treatments in several States, and with many variations in applications, but was nevertheless groundbreaking in that there is a dearth of definitive results on the safety effects of various pavement improvement treatments. It would be beneficial for the future research to now focus on individual treatments to isolate the application types and circumstances that are most cost-effective for safety. The results of this study will be useful in guiding such future efforts.
The objective of the study was to estimate the effect of various low-cost pavement treatments on crashes by evaluating a variety of treatments from several states. The state-of-the-art 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:
A preliminary, simple before-after evaluation was completed for HFS treatments based on limited data from several States. These data were insufficient to apply the EB method.
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 for 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 Clearinghouse.
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 AADT, precipitation, expected crash frequency before treatment, environment (urban/rural), and treatment age. In the end, the 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 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 situations:
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 basic 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.
Several of the results from this analysis may not be intuitively obvious. Unfortunately, without very detailed information on the specific characteristics (friction, texture, pavement condition, etc.) of each particular pavement section included in this analysis, it is not possible to draw definitive conclusions for these observations. However, this section postulates some possible explanations for these results. It should be noted that these points of discussion are observations of the researchers and should not be construed as documented conclusions.
When looking at the impact of pavement treatments on crashes, potential changes in driver behavior or driver response must be considered in addition to the effects of the treatment on pavement surface characteristics (texture and friction). Some potential driver responses to these treatments include the following:
In the following sections, the results reached in the study and described earlier in the report appear in indented blocks, followed by the possible explanations or observations by the researchers.
For multilane roads, there are significant benefits overall for wet-road crashes, due largely to reductions in California. There was an estimated increase in dry-road crashes on these roads, which contributed to a significant (5-percent level) increase in total crashes.
For chip seal on two-lane roads, there was a small benefit overall (significant at the 10-percent level) for wet-road crashes due mainly to reductions in California and North Carolina. For dry-road crashes, there was a small benefit overall (significant at the 5-percent level) due mainly to reductions in California and Pennsylvania. These benefits contribute to an overall benefit for all crashes and States combined for chip seal on two-lane roads.
Given the aggressive nature of chip seals (good macrotexture and friction), a decrease in wet-road crashes is not surprising. However, the increase in dry-road crashes and the difference between multilane and two-lane roads is not readily explainable from a pavement perspective.
For diamond grinding, there was an overall benefit (significant at the 5-percent level) for both wet- and dry-road crashes, which resulted in a significant overall benefit for total crashes.
Concrete pavements are usually diamond ground later in their life, when the original pavement texture may be substantially worn or polished. Diamond grinding gives the pavement renewed texture and improved friction, and therefore could explain this benefit for crashes.
For thin HMA, there were benefits (significant at the 5-percent level) for wet-road crashes for multilane roads and freeways and no effect overall for dry-road crashes. (For the latter crash type, there was an increase in California and a decrease in North Carolina, both results significant at the 5-percent level).
A possible explanation for wet-road crash reduction is that the overlay may have eliminated rutting and/or flushing that existed in the old pavement. Ruts tend to hold water and can lead to more wet-weather crashes, while flushing can significantly reduce friction in the wheelpaths, particularly in wet weather. Multilane and freeways (with presumably higher AADT) would likely exhibit more rutting and flushing. This has not been formally documented in any previous research, however.
For two-lane roads, the thin HMA treatment was associated with highly significant increases overall in both wet- and dry-road crashes, a pattern that was consistent between California and North Carolina, the two States with large enough samples for a definitive result.
A new asphalt overlay, which would likely improve smoothness, could possibly lead to higher speeds and possibly more crashes in wet or dry weather. This increase in speed with improvement in pavement condition has not been formally researched, but has been postulated elsewhere.(27,49) There is also the possibility that the cumulative effect of multiple thin overlays over time could lead to a nonrecoverable side-slope at the edge of the pavement, particularly on two-lane roads without paved shoulders. Again, however, this has not been formally documented in any previous research.
For OGFC, there was a negligible effect on wet-road crashes for multilane and two-lane roads, but increases in dry-road crashes resulted in significant increases (5-percent level) in total crashes for these road types. By contrast, for freeways, there was a small but significant (5-percent level) decrease in total crashes, due in large part to highly significant and substantial reduction in wet-road crashes with no change in dry-road crashes for California and North Carolina combined.
Similar to the thin HMA overlay treatment, a smoother (and presumably quieter) OGFC may lead to higher speeds and potentially more crashes for multilane and two-lane roads. For freeways, OGFC could also possibly reduce splash and spray on heavily traveled freeways, reducing wet-weather crashes due to poor visibility.
For microsurfacing on two-lane roads, there was a decrease in wet-road crashes and an increase in dry-road crashes overall (both results significant at the 5-percent level) resulting in a net increase in total crashes that was also significant at the 5-percent level. This trend was mainly due to results from Pennsylvania, which had the largest sample. For North Carolina, the sample was small but there are weak indications of decreases on both wet- and dry-road crashes. For California, by contrast, the indication is that there was an increase in both wet- and dry-road crashes for microsurfacing on two-lane roads.
For freeways, the results for microsurfacing were inclusive, (i.e., there were no statistically significant effects), likely a result of the small sample size. For multilane roads, there was a decrease in wet-road crashes (significant at the 5-percent level) and a negligible effect on total and dry-road crashes.
The decrease in wet-road crashes is not surprising because microsurfacing is known as a treatment to help improve skid resistance. There is no readily available explanation for the increase in dry-road crashes, however.
For slurry seal, which was mostly on two-lane roads, almost all of which were in California, there were benefits for wet-road crashes and weak (i.e., statistically insignificant) indications of a benefit for dry-road crashes.
Similar to microsurfacing, slurry seal is known to help improve skid resistance and therefore would be expected to exhibit benefits for wet-road crashes.
For UTBWC treatment on freeways, there was a small and marginally significant benefit overall for wet-weather crashes, due largely to the California treatments, which had a substantial and significant benefit. There was no effect for dry weather and for total crashes when this is considered.
On two-lane roads, there was a substantial and highly significant benefit for wet-road crashes and a smaller, but significant (10-percent level) benefit for dry-road crashes.
A UTBWC is similar in nature to a thin HMA overlay, and therefore similar results might be expected. The difference in effect for two-lane roads and freeways, however, is not readily explainable.
For HFS treatments, the results of the cursory before-after analysis suggest that HFS can be a highly safety- and cost-effective treatment.
The crash reduction observed for the HFS treatment sites is not surprising as this treatment is applied specifically as a safety treatment to problem locations with high crash rates, particularly ROR crashes. The higher crash reduction for wet-road crashes over total crashes is also not surprising as this treatment provides significant improvement to both microtexture and macrotexture of existing pavement, which is particularly important for wet-road friction.
CMF estimates for all years of data and for years 1 to 3 for chip seal on two-lane roads indicate that the positive safety effect of chip seal treatment on wet-weather crashes is greatest in the first year following treatment, with a declining benefit thereafter.
Although it is not possible to say with certainty that this is the explanation of the results observed from this study, as discussed previously, the trend is consistent with the performance of chip seals over time-reduced friction as the treatment ages due to bleeding and/or raveling.
Data on single/double/triple seal were only available for North Carolina and Pennsylvania. For single applications, there is some indication that the safety benefit is greater in the first year after treatment than in later years; however, there is no such trend for double/triple seals.
This trend for double/triple seals could be the result of improved performance (e.g., reduced chip loss/bleeding over time) over a single chip seal, owing to the thickness of double/triple seals.
For freeways, there appears to be a trend of a decreasing CMF (increasing benefits) as the pavement age increases for the first 4 years. For two-lane roads, however, the trend is the opposite, and the benefits are seen to decline as the pavement ages.
The trend observed for two-lane roads is closer to what might be expected from OGFC because the treatment may clog over time, reducing its porosity and effectiveness in draining water from the surface. There is no clear explanation for the contrary effect on freeways.
