Congestion Index and Reliability-Based Freeway Level of Service
Publication: Journal of Transportation Engineering, Part A: Systems
Volume 147, Issue 6
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 () 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.
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Data Availability Statement
Some or all data, models, or code that support the findings of this research are available from the authors upon reasonable request.
Acknowledgments
The authors thank the state and local agencies in North Carolina and the private data source for their help with data for this research. They also thank the three anonymous reviewers for their thorough review and comments/suggestions to improve the clarity of our work.
Disclaimer
This paper is disseminated in the interest of information exchange. The views, opinions, findings, and conclusions reflected in this paper are the responsibility of the authors only. The authors are responsible for the facts and the accuracy of the data presented herein. This paper does not constitute a standard, specification, or regulation.
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Journal of Transportation Engineering, Part A: Systems
Volume 147 • Issue 6 • June 2021
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© 2021 American Society of Civil Engineers.
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Received: Jun 5, 2020
Accepted: Jan 28, 2021
Published online: Mar 25, 2021
Published in print: Jun 1, 2021
Discussion open until: Aug 25, 2021
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