Freeway Link-Level Travel Time and Reliability Thresholds

The average travel time (expected travel time), 95th percentile travel time (planning time), planning time index (PTI), and buffer time index (BTI) are being widely explored and used by practitioners and researchers, as performance measures in the field of transportation engineering. However, literature documents limited efforts on travel time and reliability thresholds for use in transportation planning and operational analysis. The focus of this paper is to examine the inter-relationships between currently used macroscopic parameters such as density, volume-to-capacity ratio (V/C), and speed with the selected travel time based performance measures to establish level-of-service (LOS) thresholds, from planning perspective, for freeway links. The city of Charlotte, North Carolina, USA, was used as the study area to gather data, perform the analysis, examine the inter-relationships, and establish the thresholds. The findings indicate that density and V/C, computed from a calibrated regional network model, are non-linearly related to the average travel time, 95th percentile travel time, PTI, and BTI. The threshold values for different LOS letter-grades, for all travel time based measures, vary based on whether density or V/C is used to establish the relationships. Therefore, the applicability of fewer LOS or reliability threshold categories (for example, reliable, moderately reliable, and unreliable) based on statistical parameters were explored for possible implementation.