Design Criteria for Minimum Passing Zone Lengths: Operational Efficiency and Safety Considerations

Abstract

Passing zones are provided to improve operational efficiency of two-lane highways where passes can be performed safely. Minimum passing zone lengths of 120 m were established in MUTCD and Green Book, although some studies indicate a potential need to increase them. However, no changes have been recommended pending further research on the safety of short passing zones. The objective of this study is to develop design and marking criteria for minimum passing zone lengths that consider traffic operational efficiency and safety. In the first part of this study, a traffic microsimulation was carried out with Aimsun software. The calibration and validation included the observation of 1,750 passing maneuvers in Spain. The results indicate that passing zones shorter than 250 m add very little to operational efficiency. In the second part of the study, a reliability analysis was applied. It quantified the probability that a passing maneuver was completed beyond the end of the passing zone (noncompliant passing maneuvers). Afterwards, the number of non-compliant passing maneuvers was calculated. Traffic flow as well as passing zone length were contributing factors.. Findings from the analysis indicate that the minimum passing zone length should be increased to a minimum of 275 m, for high traffic volumes, 300 m for medium traffic volumes and 350 m for low traffic volumes. From this length, the number of non-compliant passing maneuvers decreases. The marginal increase in the minimum length of passing zones can potentially improve safety without significantly reducing the operational efficiency. The results can be directly used by practitioners to establish the minimum passing zone length based on the range of hourly volumes and the level of risk willing to assume.