Percent Time Spent Following Research Articles

Derivation of ATS and PTSF Functions for Two-lane, Rural Highways in Brazil

The HCM2000 uses average travel speed (ATS) and percent time spent following (PTSF) as measures of effectiveness to assess the level of service on two-lane highways. While ATS is difficult to observe directly, it is virtually impossible to obtain PTSF from traffic observations; thus, the HCM2000's procedure uses ATS-flow and PTSF-flow functions derived from simulation to estimate the level of service. The highway environment in Brazil is sufficiently different from the North-American one to invalidate the use of the original HCM2000 ATS and PTSF functions. The objective of this study was to adapt HCM2000's ATS and PTSF functions for two-lane rural highways in Brazil. Traffic data were collected in 11 locations, capturing a wide range of road and traffic conditions. These data were used to calibrate and validate TWOPAS, using a genetic algorithm (GA). Synthetic traffic data, similar to those collected by in-road traffic detectors, were generated for a wide range of conditions. The generation of synthetic data uses solutions created by the GA during the calibration of TWOPAS that were just marginally worse than the optimal calibration parameter set. The synthetic data set obtained consists of distributions of flow-ATS-PTSF observations for a hypothetical road segment representing a two-lane rural highway with ideal conditions in Brazil. The HCM2000 functions were then recalibrated using these synthetic data. New mathematical relationships were also investigated: a concave ATS-flow function and an exponential function for PTSF-flow. Besides being capable of representing several curve shapes (including those in the HCM2000), these new models are more compatible with the experimental data and are thus able to better represent the behavior traffic streams on Brazilian roads. These models could potentially be used to develop an adaptation of the HCM for Brazil.

Evaluating new methodologies for estimating performance on two-lane highways

This paper examines two new methodologies for field estimation of percent time spent following (PTSF) on two-lane highways. Using speed and time headway data, the two methodologies estimate the percentage of vehicles trapped in platoons, unable to pass slow-moving vehicles. A total of 236 h of field data, including individual speeds and headways, were collected at three study sites in the state of Montana. The study examined the sensitivity of PTSF estimates to some important platooning variables. The study also included a comparison of PTSF estimates from the new methods versus those found using the current United States National Research Council Highway capacity manual (HCM) analytical procedures. Study results showed that the two new methodologies were successful in capturing the effect of platooning variables on PTSF estimates and outperformed the HCM procedures in this regard. Further, the results strongly attest to the promise of the proposed methods in advancing the current analytical procedures for estimating performance on two-lane highways.

Field Evaluation of Highway Capacity Manual 2000 Analysis Procedures for Two-Lane Highways

Highway Capacity Manual (HCM) 2000 two-lane rural highway directional, two-way, and passing-lane analysis procedures based on field data and simulation were evaluated. Detailed field data were collected from two highway sections in northern Idaho, one with and one without passing lanes. The TWOPAS simulation model was used to provide additional insights. Particular attention was given to the differences in estimates for percent time spent following (PTSF) produced by the twoway and directional analysis procedures. It was found that the two-way analysis procedure was more accurate, although both procedures produced estimates that were too high. The passing-lane analysis procedure was also evaluated, and the HCM 2000 procedure was found to be conservative in its estimates of PTSF reductions due to a passing lane.

Percent Time-Spent-Following as Performance Measure for Two-Lane Highways

The 2000 Highway Capacity Manual (HCM) uses percent time-spent-following (PTSF) as the primary level-of-service (LOS) measure for two-lane highways. On Class I highways, average travel speed is used as an auxiliary measure. The new HCM provides methods for both twoway and directional estimation of PTSF, although the methods are not totally compatible. On Finnish two-lane highways, PTSF is lower than that estimated by the HCM 2000 methodology. Also, the impact of directional distribution is greater, whereas the impact of no-passing zones is less. From the beginning of the motorized transportation era, traffic engineers have searched for methods to define the traffic capacity of highways. Capacity estimation is not enough, however; traffic engineers and road users are both interested in the quality of traffic flow. After almost a century, this search is not yet over. In fact, transportation telematics has increased the need for reliable descriptions of quality traffic flow by using performance measures and concepts that are easily understood by road users. Several performance and service measures (to assign a LOS) suggested for two-lane highways are discussed. The properties of PTSF are discussed in more detail because PTSF has been the most important service measure for two-lane highways since HCM 1985. The estimation of PTSF and its adjustment for prevailing conditions in HCM 2000 are compared by the use of data from Finland. Finally, the applicability of PTSF as a service measure is evaluated against a set of qualifications.