The impact of road grade on vehicle accelerations behavior, PM2.5 emissions, and dispersion modeling

This study explores the impact of road grade on the acceleration behavior of light-duty vehicles and heavy-duty transit buses on arterials. A piecewise linear model was constructed to estimate the impact of grade on acceleration at various speeds across field-collected second-by-second driving traces. The model results indicate that light-duty vehicles are only slightly impacted by road grade, while a significant decrease of average acceleration is observed for transit bus operations at higher grades, resulting in longer accelerating distances and times for buses to reach a target speed at these higher grades. Given the observed influence of road grade on acceleration behavior, a comparative analysis based on MOVES-Matrix and AERMOD was conducted to examine the impact of ignoring grade, and especially ignoring the grade-acceleration correlation, on the distribution of PM2.5 emissions and dispersion modeling results. Ignoring grade results in a significant under-estimation of PM2.5 emissions on uphill segments, and over-estimation on downhill segments. More interestingly, for heavy-duty buses, ignoring the grade-operation correlation introduces biases in opposite directions at the beginning versus the middle versus ending stages of the acceleration period. Results from this study help improve the understanding of how grade interacts with on-road operations for different vehicle types and supports policy guidance to potentially improve transportation air quality conformity and hot spot analyses, especially at signalized intersections for transit buses, and most likely for heavy-duty trucks.