Influence of the longitudinal inclination on the speed on highways

Abstract

The main objective of the present paper is to investigate the influence of the longitudinal inclination of grades on the speed behavior on highways. For assessing this dependence, all uphill and downhill highway sections were considered separately in the two travel directions and the mean values of both 85th and 15th percentile speeds were calculated for each section. In order to take into account the free speed selection, all speed limited highway sections were excluded from the study. To investigate the speed changes as a function of the longitudinal inclination, the speed differences V85 between the speeds at the end and the beginning of each individual ascending and descending section were calculated as well. The dependencies were assessed also using the “moving average” over the grades. Both methodological approaches show that the magnitude of the longitudinal inclination on uphill sections has a clear influence on the speed and on the speed differences. A significant reduction of the speed and a significant increase of the speed difference were established on ascending sections with slopes over 4%. Furthermore, it was found that the length of the ascending grades also affects the speed differences ΔV85, which increase considerably on longer uphill sections.