Congestion Index and Reliability-Based Freeway Level of Service

Abstract

Examining the relationship between congestion and travel time reliability (TTR) can help in gaining a broader perspective on the development of congestion mitigation strategies. Several performance measures have been proposed and used by researchers and practitioners to assess congestion levels on a freeway in the past. These performance measures identify congestion when the space mean speed of a segment is below a critical static threshold and does not vary with changes in road geometry, land use, area type, and presence of acceleration or deceleration lane. This research integrates travel time data and demonstrates the applicability of a probabilistic approach based on hazard function to quantify congestion, characterize the congestion levels, and generate reliability-based level-of-service (LOS) thresholds for freeways. Toward this purpose, travel time data for 44 geographically distributed freeway segments in an urban area with varying speed limits and road characteristics were used to derive the travel time threshold (Tc) representing the lower boundary limit of congested condition using a probabilistic approach. A new measure called congestion index (CI) was then proposed to assist in quantifying both the intensity and duration of congestion. The CI accounted for the variation in road characteristics, spatial location, and traffic condition. Further, the CI was observed to be correlated between continuous segments, highlighting that the CI could characterize recurring congestion. The CI was correlated with the TTR measures to generate reliability-based LOS thresholds for freeways. A strong positive correlation (correlation coefficient greater than 0.80) was observed between the congestion level and TTR measures. This research demonstrates the applicability of the reliability-based LOS for prioritizing segments and effectively allocating funds. The generated reliability-based LOS thresholds can help practitioners assess the performance of freeways in terms of CI and TTR, which can be followed up with microlevel analysis to identify congestion mitigation strategies.