Validation and Calibration of Freeway Reliability Methodology in the Highway Capacity Manual: Method and Case Studies

Abstract

Traffic congestion costs drivers an average of $1,200 a year in wasted fuel and time, with most travelers becoming less tolerant of unexpected delays. Substantial efforts have been made to account for the impact of non-recurring sources of congestion on travel time reliability. The 6th edition of the Highway Capacity Manual (HCM) provides a structured guidance on a step-by-step analysis to estimate reliability performance measures on freeway facilities. However, practical implementation of these methods poses its own challenges. Performing these analyses requires assimilation of data scattered in different platforms, and this assimilation is complicated further by the fact that data and data platforms differ from state to state. This paper focuses on practical calibration and validation methods of the core and reliability analyses described in the HCM. The main objective is to provide HCM users with guidance on collecting data for freeway reliability analysis as well as validating the reliability performance measures predictions of the HCM methodology. A real-world case study on three routes on Interstate 40 in the Raleigh-Durham area in North Carolina is used to describe the steps required for conducting this analysis. The travel time index (TTI) distribution, reported by the HCM models, was found to match those from probe-based travel time data closely up to the 80th percentile values. However, because of a mismatch between the actual and HCM estimated incident allocation patterns both spatially and temporally, and the fact that traffic demands in the HCM methods are by default insensitive to the occurrence of major incidents, the HCM approach tended to generate larger travel time values in the upper regions of the travel time distribution.

Evaluating the Effect of Incidents on Freeway Facilities and Updating the Incident Capacity Adjustments for HCM

“…There are other studies on the details of the incident's effect on freeway capacity (11). Several agencies have tested the travel time reliability method on several facilities (12)(13)(14)(15)(16). A study in North Carolina modeled three routes on I-40 in the Raleigh-Durham area and predicted the travel time indices (TTIs) reported by HCM modeling within the 80th percentile of true probe-based data (16).…”

“…Several agencies have tested the travel time reliability method on several facilities (12)(13)(14)(15)(16). A study in North Carolina modeled three routes on I-40 in the Raleigh-Durham area and predicted the travel time indices (TTIs) reported by HCM modeling within the 80th percentile of true probe-based data (16). A major drawback of the HCM methods was that traffic demands were assumed not to be affected by the occurrence of major incidents, in the process generating a longer tail of the travel time distribution than was warranted based on empirical observations.…”