A New Model to Evaluate Percent-Time-Spent-Following on Two-Lane Highways

Abstract

This paper presents the evaluation of percent-time-spent-following (PTSF), one of the key service measures as indicated by Highway Capacity Manual (HCM) to assess level of service (LOS) under heterogeneous traffic conditions for two-lane highway. The PTSF is defined as “the average percentage of travel time that vehicles must travel in platoons behind slower vehicles because of their inability to pass.” Two-lane highway flow is very different from that of single-lane highways mainly because vehicles oppose traffic in the opposite lane, and as a result, they may be subjected to delaying because of their inability to pass slow-moving vehicles. Detecting from the existing studies, it is declared that measuring PTSF directly in the field is quite tedious. However, Highway Capacity Manual put forward some logical stratagem to evaluate PTSF with the 3 s surrogate measure which overestimated the field results. Because of this difficulty, the evaluation of PTSF has been based on methodical procedure which uses equations obtained from simulations and field examination at given location based on substitute measure, the percent of vehicles traveling with headway less than 3 s. In this study queuing analogy is employed to measure PTSF by measuring the headways inside and outside platoons on the two-lane highways of Himachal (India). In this analysis, middle or average range will be that where number of headways inside and outside the platoon will be equal. The difference in range of PTSF grows as volume increases. The range of PTSF measured is lesser than that accorded in the HCM. The main difference for the range of PTSF between the current study and HCM is that using queuing analogy LOS “A” has a PTSF value less than 16% while for HCM it is 35%. This study also propounds a new method of estimating PTSF which can be used as a powerful tool to provide rational idea of various measures for LOS that would otherwise be very hard to estimate.