Investigating the Effect of Freeway Congestion Thresholds on Decision-Making Inputs

Project Description

Although the congestion problem has been studied extensively, there has not been a consensus on when congestion technically begins. Policy discussions about the size of the congestion problem and the need for solutions are often side-tracked by this threshold issue. This research specifically examined:

1. the ranking values of congestion measure for different congestion thresholds under a variety of real-world travel time distributions,
2. the relationship between change of congestion threshold and change of performance measure, and
3. the appropriateness of using speed limit as a congestion threshold choice by evaluating the peak and off-peak average speed changes in relation to a speed limit change in Houston, Texas.

Researchers found that the rankings of congestion measures for freeway segments hold steady across the congestion thresholds ranging from 60 mph to 30 mph and across the congestion measures. From an investment point of view, the congestion threshold speed used is not a concern for funding allocation. The relationship between the delay values for an alternative threshold and the 60 mph threshold has a quadratic form. As the alternative threshold decreases further away from 60 mph, the increment is larger. The more congested a section is, the less the threshold affects measured congestion. However, if the speed limit or a percentage of speed limit is used to estimate the congestion, the amount of congestion may be underestimated because the free flow speed is higher than the speed limit.

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Link: Final Report

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