Developing Level-of-Service Criteria for Two-Lane Rural Roads with Grades under Mixed Traffic Conditions

Abstract

Traffic operations on two-lane rural roads differ substantially from those on divided carriageways due to vehicular interactions between traffic flows in the opposite directions. With the presence of grades and mixed (heterogeneous) traffic, traffic operations on two-lane highways become even more complex and challenging. The present study developed level-of-service (LOS) criteria for assessing the performance of two-lane rural roads with grades. Eight two-lane undivided study sections with grades varying from 1% to 8% were selected. The suitability of well-established performance measures such as percent time spent following (PTSF), number of followers per capacity (NFPC), follower density (FD), average travel speed (ATS), and percent of free-flow speed (PFFS) was evaluated. The results showed that the foregoing performance measures were not practically applicable for characterizing the operational LOS for two-lane rural roads with grades. A new performance measure termed density ratio (DR) was developed in the present study. The ATS, PFFS, and FD measures for different grades revealed no significant difference when visualized at similar DR ranges. Therefore, DR can be considered an effective measure for developing LOS criteria for such roads. The criteria were first developed using DR, ATS, and FD; subsequently, a design LOS was derived.