Traffic incidents are a major source of congestion in Virginia. Secondary incidents comprise a relatively small but important portion of all incidents, and relatively little is known about their occurrence, characteristics, and associated delays. The main objectives of this study were to define secondary incidents, understand and analyze the occurrence and nature of such incidents, and develop tools that can comprehensively and continuously analyze primary and secondary incidents at the planning and operational levels, ultimately contributing to congestion management. The scope of the study is limited to freeway incidents in the Hampton Roads (HR) area.
The study found that secondary incidents account for nearly 2% of TOC-recorded incidents, using the 2006 data. Of all accidents, 7.5% had associated secondary incidents, 1.5% of disabled vehicles had secondary incidents, and 0.9% of abandoned vehicles had secondary incidents. Despite their relatively low percentages, on average, two to three secondary incidents occur daily in the HR area. Further, the average durations of secondary incidents in HR are 18 minutes, which is 4 minutes longer than the mean duration of other (independent) incidents, indicating that secondary incidents are not necessarily minor “fender benders.” The study also found that a 10-minute increase in primary incident duration is associated with 15% higher odds of secondary incidents.
This study developed and applied a dynamic queue-based tool (SiT) to identify primary and secondary incidents from historical incident data and incorporated the models developed for incident duration, secondary incident occurrence, and associated delays in an online prediction tool (iMiT). Although the tools developed in this study (SiT and iMiT) are currently calibrated using HR data, the methodology is transferable to other regions of Virginia.
The study recommends that (1)VDOT TOC analysts (where available) use primary and secondary incidents as additional performance measures; (2) VDOT TOC analysts (where available) identify secondary incident hot-spots; (3) VDOT’s Regional Traffic Operations Managers give priority (in terms of monitoring, patrol coverage, and traveler information dissemination) to secondary incident hot-spots; (4) TOC managers and their staff use the online prediction tool, iMiT; (5) VDOT TOCs continue and expand the use of service patrols to implement aggressive incident clearance procedures (where appropriate), continue and strengthen their outreach to other response agencies using the RCTO or similar mechanisms, and improve incident scene management to avoid distractions from both the same and opposite directions; and (6) VDOT Operations and Security Division staff work to reconstitute the Statewide Incident Management Committee.
The benefit of reducing the number of secondary incidents by 25% (an implication of the stated goal of the HR RCTO) was calculated using two methods. The first method resulted in a benefit in terms of reduced incident delay estimated at $1.11 million per year. The second method used slightly different assumptions and resulted in an estimated delay savings of $1.23 million.