Vehicle Deceleration Levels during Wet-to-Dry Transitions on an Asphalt Surface with a High Proportion of Aggregate

The braking performance of a vehicle at varying levels of road wetness is an important factor in collision reconstruction. Here we quantify the deceleration levels of two modern vehicles equipped with antilock brake systems (ABS) on a wetted asphalt surface with a high proportion of exposed, large-sized aggregates as the road naturally dried over time. We also compare our current results to prior tests on asphalt with a small proportion of small-sized aggregate. Two ABS-equipped vehicles were maximally braked on an asphalt road surface as the road naturally transitioned from a saturated wet state to a completely dry state. Road wetness was visually categorized from photographs taken during testing. Overall, we found that deceleration levels on wet asphalt were significantly less than deceleration levels on dry asphalt (average dry: 0.902g and 0.962g; average wet: 0.787g and 0.818g for the two vehicles). Within the wetness categories we used, there was either no significant difference or only a small significant difference between the three wettest categories (road surface >75% wet, 100% wet with a matte surface, and saturated with a glossy surface). Similarly, there were no significant differences between the two driest conditions (road surface <25% wet and completely dry). This pattern of findings is similar to our prior tests, although the absolute deceleration levels in the current study were 0.028g lower on dry asphalt and 0.076g lower on wet asphalt than in our prior study on asphalt with a lower proportion of smaller-sized aggregate but a similar mean texture depth. These findings provide two important insights: first, the current data support our prior proposition of a readily identifiable boundary in ABS deceleration levels between dry and wet road surfaces, and second, ABS deceleration levels measured using the current test methods may be more sensitive to changes in road surface friction than the standardized measure of road surface macrotexture we used here.