Application of Traffic Thermostat for I-30 Managed Lanes in Dallas, Texas

Abstract

Managed lanes have historically experienced an evolution in operational policies. Over time, speeds on a managed lane may degrade because of oversubscribed demand. Operators have strategies to mitigate problems and maintain optimal performance; these strategies include changing the occupancy requirement, varying tolls (if the facility is priced), and altering transit service. Assessing the potential impact of operational strategies is a challenge. The traffic thermostat tool is a software-based guide that helps select specific strategies projected to influence the metrics for each goal. This paper reports on how the tool was adapted for use on the I-30 managed lanes in Dallas, Texas, a high-occupancy-toll lane with reduced toll rates for qualifying carpools. An example scenario shows the consideration of travel speed and throughput goals, with acceptable performance thresholds of 50 mph and 5,700 persons per hour for the entire facility. The thermostat estimated values of projected speed and throughput for selected operational fixes by using the calibrated speed–flow relationship from the regional travel demand model. Results from a quantitative travel survey helped to derive mode shift elasticities for use by the tool. Overall, the dynamic nature of demand, diversity of user groups, ambiguity with exogenous factors (e.g., regional unemployment and fuel prices), and need for extensive data on the lanes led to uncertainty and difficulty with prediction capability. However, use of the traffic thermostat can show policy makers and others the inherent challenge of performance management for managed-lane facilities.

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