Introduction to the Special Issue on Calibration and Validation of the Highway Capacity Manual Models

This special issue of the journal is the result of cooperative efforts of the Transportation and Development Institute (T&DI) Committee on Transportation Operations and the Transportation Research Board (TRB) Committee on Highway Capacity and Quality of Service. Primary credit for organizing the issue goes to Robert Bryson of the T&DI committee and Andrew Tarko of the TRB committee, who were responsible for soliciting papers, getting them reviewed, and having revisions made. I want to recognize them, along with the dedicated group of volunteers who served as reviewers.

While the Highway Capacity Manual (HCM) has been the primary tool for analyzing highway operation, recent evaluations have raised concerns about the ability of some of the HCM procedures to adequately replicate field conditions. It was believed that a robust validation effort could shed light into the real-world implementation of HCM procedures. Such an effort has long been hampered by the lack of readily accessible data, and this special issue brings together the results of validation efforts being made by researchers.

The special issue includes nine technical papers and three technical notes. Most of the contributions involve traffic operation at intersections, with many of them dealing with signalized intersections. The issues range from the potential effect of weather conditions to saturation flow rate to cycle length and capacity parameters. Although the emphasis is on field data that can be used for validation, there are cases where well-accepted simulation models have been employed to generate validation data. Data variability issues are also discussed, with suggestions about how to impute missing values. The accuracy of the HCM queue and delay models for interrupted facilities is examined, and a new level-of-service standard for signalized pedestrian crosswalks is presented. Three contributions are from outside North America; they discuss the use of HCM in developing a free-flow model for the first freeway in Chile, determination of the capacity of single-lane roundabouts in Turkey, and the capacity of unsignalized intersections in Poland.

We are pleased that the Transportation and Development Institute has been able to partner with a TRB committee to put together this special issue. We hope those who use the Highway Capacity Manual will find this special issue of value, both for further research and for practice.