Estimating quality of traffic flow on two-lane highways

Abstract

Since the publication of the 2000 Highway Capacity Manual (HCM), there have been several studies that indicate that the HCM equations for Percent Time-Spent- Following (PTSF) on two-lane highways do not correspond to field-based measurements. This discrepancy was the motivation for this research project. The purpose of this project was two-fold. First, it aimed to find an alternative performance measure to PTSF that could be measured directly in the field and could adequately describe the quality of traffic flow. Secondly, the project aimed to investigate the inter-vehicular interaction between consecutive vehicles traveling on the same lane of two-lane rural highways. Both studies were empirical in nature and utilized field data gathered from rural two-lane and four-lane highways in the state of Montana. Six performance measures for two-lane highways were investigated; they were: average travel speed, average travel speed of passenger cars, average travel speed as a percent of free-flow speed, average travel speed of passenger cars as a percent of free-flow speed of passenger cars, percent followers, and follower density. The performance measures were evaluated based on their level of association with major platooning variables. Among all performance measures investigated, follower density and percent followers exhibited the highest correlation to platooning variables, respectively. Overall, follower density was recommended as the best performance measure for two-lane highways. Based on the fact that follower density is a headway-based service measure, the second study aimed to achieve a better understanding of car-following interaction on two-lane rural highways. Car-following interaction was studied by examining headway distributions, speed-headway relationships, and percent followers and flow relationships. The study found that car-following interaction generally ceases when headways exceed a value of approximately six seconds. Also, a significant proportion of drivers choose to maintain relatively short headways while following other vehicles on two-lane highways regardless of passing restrictions.