Environmental and Traffic Effects on Incident Frequency Occurred on Urban Expressways

Traffic incidents are a major source of congestion and travel time uncertainty. Traditionally, extensive attention has been given to accidents in the view of safety when studying occurrence frequency. The regularity of incident frequency, however, deserves equal attention by practitioners and researchers, especially on urban expressways with dense ramps and high traffic volume. The objective of this study was to have a thorough exploration of environmental and traffic-related causative factors of incident rate on three urban expressways in central Shanghai City, including disability incidents and crash incidents. Incident data obtained by CCTV-monitoring system were used, which contain large quantities of minor and short-duration incidents. The disaggregation of expressway sections and time intervals of this study was rare in its scope: disability frequency is analyzed on an hourly basis and segment-hour aggregation is applied for crash frequency. To account for temporal correlation among different time intervals, Generalized Estimation Equation procedure was used in this paper. In particular, the effects of traffic interaction features on incident occurrence were analyzed by considering segment length, merging and diverging volume. Results showed that temporal correlation of crash incident occurrence was larger than that of disability occurrence. There is a significant relationship of disability rate with rain and temperature, and there was more risk of vehicle disability in dense-traffic and low-speed condition. It also pointed out that the regularity of crash incident occurrence is quite different from that of accidents on highways or rural freeways: Environmental factors exert little impact on crash occurrence except from visibility; Short segment, high merging and diverging volume increased crash rate remarkably. © 2013 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of Chinese Overseas Transportation Association (COTA).