DESLIZAMIENTO EN LA LOCALIDAD DE RIPAFRATTA

Traffic volume

As expected, traffic volume, AADT for each specific road segment, was found to be significant with a positive sign, suggesting that an increase in traffic volume would result in an increase in the mean number of weather-related crashes expected to occur on the road segment during the winter season. The value of the coefficient associated with the traffic volume is 0.65, which is less than one and suggests the moderating effect of traffic volume is non-linear with a decreasing trend. A Similar value was found for road segments belonging to multilane

speed and weather-related crash frequency (Monsere et al., 2008). Traffic volume represented by AADT in the current study has a considerable impact on safety, as an increase in traffic volume (ranging from 90 to 113,600 during the winter seasons) by 1% would cause the mean number of crashes to increase by 6.4% on interstate/freeways. The elasticity value for volume reveals that a 1% increase in AADT (ranging from 50 to 34,225 during the winter seasons) will result in 6.74% increase in the mean crash frequency on multilane divided/undivided roadway segments.

Air temperature

Air temperature was found to be significant with a negative sign suggesting that the mean number of weather-related crashes will increase with the decrease in the air temperature. The elasticity value for the air temperature reveals that a 1% increase in air temperature during the winter season would decrease the mean number of crashes by 0.43% on interstate/freeway roads for air temperature ranging from -25 to 37 degree Fahrenheit. This result is in agreement with some of the previous findings (Fu et al. 2006). The elasticity value for multilane

divided/undivided roadway indicates a 0.12% decrease in the mean number of crashes with a 1% increase in air temperature (from -18 to 38 degree Fahrenheit) during the winter season.

Pavement temperature

Modeling results reveal a significant relationship between average pavement temperature and mean number of crashes during the winter season on interstate/freeway road segments. The elasticity value for the pavement temperature shows that a 1% increase in pavement temperature would cause the mean number of weather-related crashes to increase by 0.38% when pavement temperature ranges from -9 to 38.8 degree Fahrenheit. Though the finding seem counterintuitive,

it is possible that the increase in pavement temperature might result in different level of variation in road surface condition affecting crash frequency. Pavement temperature was not found

statistically significant with crash frequency occurring on multilane divided/undivided roadway segments.

Road surface width (in feet)

Road surface width (measured from edge line to edge line) was found to be statistically significant and the positive sign indicates that roadway segments with wider surface were associated with a higher number of weather-related crashes during the winter seasons. Results revealed that a 1% increase in the roadway surface (from 16 to 90 feet) width would result in 0.58% increase in the mean number of crashes on interstate/freeway and 0.71% on multilane divided/undivided road segments (from 12 to 72 feet) during the winter seasons. On

interstate/freeway roadways, wider roadways might make drivers feel safer and the number of crashes might increase due to the risk compensating behaviors of drivers during winter seasons. It might also suggest that drivers slow down during winter weather on narrower roadways. Drivers are also prone to changing lanes on a multilane roadway segment and this might have increased potential for greater number of crashes on road segments having larger surface width. Previous studies have also found similar results while developing crash frequency models for speed and winter-weather related crashes (Monsere et al. 2006).

Visibility (in mile)

mean number of weather-related crashes during winter seasons will increase with better visibility. Although, this finding might seem counterintuitive, the increase in the frequency of crashes during winter seasons might be attributed to the risk compensating behaviors of the drivers due to increased visibility. Previous research findings showed a decrease in average vehicle speeds during winter weather with a decrease in visibility below 0.4 kilometer (Knapp and Smithson, 2001). The elasticity value for visibility indicates that a 1% increase in visibility (ranging from 0 to 5 mile) during winter seasons will increase the mean number of weater- related crashes expected along interstate/freeway road segments by 0.06%. The elasticity value for visibility (ranging from 0 to 5 mile) was found to be 0.13 for multilane divided/undivided roadway segments. Note that these results are similar with that of a past study Hermans et al. (2006a) using data from 37 sites. However, our results are different from the study conducted by Usman (2011) that found a negative relationship between visibility and crash frequency during a storm event. Note that the models developed in this research are not winter-storm event based models but rather consider all the weather-related crashes that occurred during the winter

seasons. Large aggregation of data at the temporal level may have masked the effect of visibility in the current model.

Posted speed limit

Posted speed limit was found to be significant with a negative sign suggesting that the mean number of weather-related crashes would increase with a decrease in posted speed limits along roadway segment during winter weather seasons. This finding is in agreement with a previous study (Monsere et al., 2006). The elasticity value for the posted speed limit variable reveals that a 1% increase in posted speed limit will (35-70 mph for interstate and 20-65 mph for

multilane roadways) result in a 0.66% and 0.53% decrease in the mean number of winter weather-related crashes on interstate/freeway road segments and multilane divided/undivided roadways segments, respectively. The greater variability of vehicle speeds during winter weather conditions compared to non-winter conditions (Knapp and Smithson, 2001) might contribute to this finding. Literature also shows evidence of decrease in the average winter weather speed compared to the typical average speed at the same location during non-winter weather conditions (Knapp and Smithson, 2001).