Snow and Ice Control Performance Measurement: Comparing "Grip," Traffic Speed Distributions and Safety Outcomes During Winter Storms

Effective performance measurement provides benchmarking for transportation agencies to promote transparency, accountability, cost-effectiveness, and process improvement. Vaisala’s proprietary “Grip" measure provides an imputed measure of the condition of the road surface (Jensen et al., 2014). VTrans’ Average Distribution Deviation (ADD) measures changes in the distribution of vehicle speeds during and after winter weather events (Sullivan et al., 2016). The algorithm for the calculation of Grip was reverseengineered from Road Weather Information System (RWIS) data over the winters of 2016-2017 and 2017-2018. The resulting algorithm is consistent with research connecting snow, water and ice layer thicknesses to skidding friction. ADD and Grip were found to be relatively poorly correlated, indicating that each measure is independently useful and one cannot be used as a proxy for the other. In fact, the exploration revealed that instances when ADD and Grip diverge maybe especially useful for signaling high-risk situations, or situations when the traveling public is not correctly perceiving the road surface conditions. Finally, a review of winter storm and season severity indices concluded that the precipitation-based Accumulated Winter Season Severity Index was appropriate for use in Vermont because it was well calibrated, captured key factors influencing winter maintenance activities and calculated from data that are readily available across the state.