Assessment of Level-of-Service of Two-Lane Highways with Heterogeneous Traffic

The recent edition of Highway Capacity Manual (HCM 2010) classified two-lane highways that serve moderately developed areas as Class III and recommends use of percent free-flow speed (PFFS) as performance measure to define level-of-service (LOS). However, a few researchers have observed some deficiencies in this approach for highways having heterogeneous traffic with large speed differential as normally observed in developing countries. On the basis of the field data collected in a study conducted on a two-lane National Highway in India, it has been observed that the parameter suggested by HCM for the determination of LOS do not capture the traffic characteristics in mixed traffic situation correctly. The current study, therefore, identified two alternative performance measures, that are responsible for platooning of traffic stream which in turn affects the LOS; they are the variation of free-speed characteristics of different types of vehicles and the limiting speed that would differentiate a slow-moving vehicle from other vehicles plying on the highway. Accordingly, percent speed- reduction (PSR) and percentage of slower-vehicles (PSV) in the traffic stream were selected to define the LOS criteria. The data collected at wide range of flow characteristics was grouped into six clusters, namely A, B, C, D, E and F representing the levels of service using partitional clustering algorithms. It was also concluded that different kind of vehicles might experience different levels of service under the same traffic volume under mixed-traffic situations.