The Worst Are Always the Worst Why Congestion Threshold Speeds Do Not Matter as Decision-Making Input
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The concept of a congestion threshold is embedded in the definition of congestion. Two basic approaches exist in current practice for setting the congestion threshold. One common approach uses the free-flow or unimpeded conditions. Another approach uses a variety of targets or acceptable conditions. The limited research conducted on the congestion threshold issue has focused on operational problems or policy debates, but there has been relatively little investigation of the effect of the congestion threshold on decision making for transportation investment and resource allocation. This research investigated the differences that might be seen in the worst freeway rankings for several threshold choices by using detailed freeway data from seven metropolitan areas. Specifically, this research examined the ranking values of two congestion measures under a variety of real-world travel time distributions. The congestion performance measures of delay per mile and travel time index were evaluated; these measures included different aspects of the congestion problem. The research results showed that the rankings of freeway congestion levels held steady across the set of congestion thresholds ranging from 60 to 30 mph. The same sections were ranked as very congested for all possible threshold values. From an investment point of view, therefore, the research found that congestion threshold speed should not be a concern for funding allocation. The data analysis found that the amount of delay was decreased by 10% for every 5 mph decrease in the congestion threshold speed. This decrease confirmed that thresholds were not irrelevant for all analyses.
