Development of Adjustment Factors for HCM Sixth Edition Freeway Work Zone Capacity Methodology

Project Details
STATUS

Completed

PROJECT NUMBER

18-645, TPF-5(295)

START DATE

04/01/18

END DATE

07/14/20

FOCUS AREAS

Safety

RESEARCH CENTERS InTrans, CTRE, SWZDI
SPONSORS

Iowa Department of Transportation
Smart Work Zone Deployment Initiative

Researchers
Principal Investigator
Jing Dong

Transportation Engineer, CTRE

Co-Principal Investigator
Anuj Sharma

Research Scientist and Leader, REACTOR

About the research

This project validated the Highway Capacity Manual (HCM) work zone capacity methodology for urban and rural freeways and provides recommendations for a more accurate estimation of work zone capacity. This study collected data from 16 work zone sites across Iowa in 2018 and 2019. The free flow speeds (FFSs), capacities, and queue discharge rates (QDRs) at these work zones were calculated using the HCM method and compared to field measurements.

For the work zones considered in this study, the key findings are as follows:

  • FFSs estimated using the HCM method had a greater variance than the field-measured values. Under free flowing conditions, Iowans generally drove around the work zone speed limits, while the HCM method predicted a wide range of FFSs.
  • The field-measured prebreakdown capacities and QDRs were significantly lower than the values computed using the HCM method, indicating that traffic breakdown could happen at a much lower flow level than the capacity predicted by the HCM method.
  • With complex work zone configurations, such as narrow lanes, lane shifts, and crossovers, the observed FFS and prebreakdown capacity were significantly lower than typical work zone configurations.
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