Measurement of percent time spent following on two-lane highways: an exploration of spot and space-based methodologies

Abstract

Abstract Percent Time Spent Following (PTSF) is used as a key performance measure for two-lane highways as recommended by the HCM2010. PTSF is the average percent of total travel time that vehicles must travel in platoons behind slower vehicles due to inability to pass. Despite its acceptance as performance indicator, PTSF is difficult to measure directly in the field. It is therefore estimated using a surrogate measure based on the spot percent of vehicles travelling with headways less than three seconds (3 s) at a fixed point. However, the application of the 3 s headways as the sole criterion in the surrogate measure is still contentious. On the basis of definition, PTSF is both spatial and temporal variable whereas its estimation is based on fixed point. It is therefore desirable to develop a spatial measurement approach for PTSF that takes into consideration of travel time. A new method was therefore developed for spatial measurement of PTSF using a test vehicle equipped with a GPS-based speed acquisition and video recording system. This study explores the spot and spatial measurement techniques of PTSF. The indicator was then measured using the two approaches concurrently. Data for the study were sampled from 18 directional segments of two-lane two-way highways in Johor and Pahang States, Malaysia. Subsequently, the estimates from these approaches were compared. On comparing the PTSF values from the spot and space-based approaches, it was discovered the latter approach produced lower values than the former in most instances. A statistical test of difference of means at 95% confidence level indicated that the estimates from the two techniques do not differ significantly. This implies that PTSF from spot measurement do not differ much from those based spatial approach developed in this study. Finding from this effort would at least substantiate the application of the spot-based PTSF measuring technique; an issue that has been debatable. It also serves as a response to the contentious issue, which has not received the desired attention from experts previously.