Selection of an Appropriate Reference Speed for the Calculation of Highway Performance Measures

About the report:

This study examines the impact of various reference speed estimation methods on interstate highway performance measures, alongside an exploration of factors influencing reference speed on arterials. Using probe speed data from the National Performance Management Research Data Set and INRIX Inc., datasets, several reference speed estimation methods are evaluated to assess how these different estimates affect key performance metrics, including travel delay and the Planning Time Index. Findings indicate that reference speeds derived from the 85th-percentile speed during low-traffic-volume periods are consistent across methods and provide reliable measurement for delay and Planning Time Index calculations. In addition to interstate analysis, the study assesses the probe data quality on arterial networks, investigating factors such as segment length and traffic signal density to understand their impacts on arterial reference speed. Results suggest that although reference speed estimation methods are well-established for interstates, estimating reference speed on arterials requires further investigation and exploration due to inherent variability. This study recommends that the Traffic Operations Division of the Virginia Department of Transportation adopt the 85th-percentile-based reference speed calculation method as a best practice for interstate performance measurement purposes. This study also recommends further research to develop a reference speed estimation method for arterial performance measures. Implementing these recommendations would help improve the consistency of performance measurement, thus enhancing support for data-driven policy decisions.

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