Analysis and Validation of Travel Time Reliability Metrics Derived from Intelligent Transportation System Sensors for Arterial Roadway Mobility

Travel time reliability (TTR) is a key mobility performance measure for roadway systems. Intelligent Transportation Systems have enabled transportation agencies and researchers to use large-scale travel time data for evaluating TTR. The United States Congress has directed federal, state, and metropolitan transportation agencies to use the National Performance Management Research Data Set (NPMRDS) for performance evaluation. Federal Highway Administration selected INRIX to provide probe data for NPMRDS. Previous research assessed INRIX data accuracy, but none compared NPMRDS-based TTR metrics with independent field data sources. This study addresses that gap by comparing travel time distribution characteristics and commonly used TTR metrics, including travel time index (TTI), planning time index (PTI), level of travel time reliability (LOTTR), buffer index (BI), and coefficient of variation (COV), using data from an arterial corridor in Lincoln, Nebraska. Results showed that NPMRDS generally estimated TTI, PTI, and LOTTR more accurately than BI and COV, with LOTTR being the most accurately measured metric. This is important because LOTTR is a key national mobility performance measure. However, LOTTR was less sensitive to reliability changes caused by peak-period traffic, weather, and segment characteristics, suggesting that conclusions about TTR may vary depending on the selected metric and questioning the validity of current practice.