Review of Percent Time Spent Following (PTSF) as Performance Measure for Two-lane Highways

The Highway Capacity Manual (HCM) uses Percent Time Spent Following (PTSF) as key service measure for assessing the level of service of two-lane highways. However, the indicator is difficult to measure directly in the field. For this reason, its estimation to date has been based on analytical procedures using equations derived from simulations and field observations at representative location based on surrogate measure; as the percent of vehicles traveling with headway less than 3 seconds (3 s). Findings from empirical studies confirmed that the HCM analytical procedures used in estimating PTSF yield results that are inconsistent with the 3 s surrogate measure and mostly overestimate the indicator. This paper presents a review on the estimation of PTSF on two-lane highways and suggests probable approach to substantiate the application of the current practice. Further, the authors of this paper argued that the use of 3 s as surrogate for estimating PTSF based on field observation at a specific point may not represent the actual time spent following over a long segment of two-lane highway since PTSF is space related measure. Hence, the authors suggest the use of test vehicle approach over the highway segment to be evaluated to identify the variables that are required for the development of a representative PTSF measurement model. It is expected that this review and suggestion offered will contribute in advancing performance analysis of two-lane highways.