Average Travel Time, Planning Time Index, and Buffer Time Index Thresholds for Freeway Weaving Sections, Merging Areas, and Diverging Areas

Abstract

Efforts were initiated through the Strategic Highway Research Program (SHRP) to quantify level of service (LOS) based on travel time and travel time reliability. To contribute to this goal, this study adopted a microscopic simulation-based method and researched the applicability of average travel time (ATT), planning time index (PTI), and buffer time index (BTI) to quantify freeway weaving section, merging area, and diverging area operational performance by the posted speed limit and number of lanes. A calibrated microscopic traffic simulation model was developed and validated using existing conditions data and then modified to generate density and travel time as outputs for various hypothetical analytical scenarios. Each simulation scenario included weaving sections, merging areas, and diverging areas with different posted speed limits, number of lanes, and traffic volumes. The ATT per 1.61 km (1 mi), PTI, and BTI were computed and compared with the density to examine the relationships and identify thresholds by the posted speed limit and number of lanes. The ATT per 1.61 km (1 mi) thresholds increase from LOS A through LOS F, whereas the PTI and BTI thresholds increase from LOS A through LOS D or LOS E and decrease thereafter. The ATT per 1.61 km (1 mi) thresholds increase as the posted speed limit decreases or the number of lanes decreases. However, the PTI and BTI thresholds could decrease as the posted speed limit decreases. The trends are fairly consistent for weaving sections, merging areas, and diverging areas. The differences in thresholds for merging and diverging areas, merging areas, and diverging areas indicate that separate thresholds are needed for merging areas and diverging areas, unlike what is currently provided in the Highway Capacity Manual (HCM). Caution must be exercised when applying PTI and BTI thresholds for very uncongested or congested conditions. It is, therefore, recommended to categorize the PTI and BTI thresholds as highly reliable, moderately reliable, and unreliable, unlike the standard six ATT per 1.61 km (1 mi) or density LOS A through F categories. Alternatively, they could be used in conjunction with ATT per 1.61 km (1 mi) or density thresholds to assess congestion or reliability-related affects.