Modeling basic freeway section level-of-service based on travel time and reliability

Abstract

The assessment of operational performance is a challenge for the state, regional and local transportation agencies due to the complex and dynamic nature of traffic movement. A qualitative classification of operational performance is often done in the form of a six letter-grade, A to –F level-of-service (LOS) scale, defined in the Highway Capacity Manual (HCM). It is based on density for basic freeway sections, weaving sections, and merging/diverging areas. Presently, speed and travel time are two performance measures used by the practitioners when making decisions. Further, travel time reliability is becoming an important concept for modern and urban transportation system managers, planners and engineers. Efforts have been initiated through Strategic Highway Research Program (SHRP 2) to quantify LOS based on travel time reliability. To contribute to the body of knowledge, this paper adopts a microscopic simulation based method, and researches the validity of travel time and travel time reliability indices to quantify LOS thresholds for basic freeway sections. Findings indicate that the average travel time per mile threshold values for respective LOS letter-grades increase as the speed limit decreases, until the saturation point where the speed limit on the freeway does not have any influence on the operational performance. As the posted speed limit decreases, the percent difference between two adjacent travel time per mile threshold values also decreases. Further, the average travel time reliability LOS threshold values for the respective LOS letter-grades decrease as the speed limit decreases.