This project investigated the basic relationship of incidents to delay on Puget Sound area freeways. The intent was to determine the amount of delay caused by incidents and the benefits obtained from the incident response actions taken by the Washington State Department of Transportation (WSDOT). The analysis was based on data from 2006 and included all days in 2006. The study area included I-5 from SR 526 in the north to S. 320th in Federal Way in the south; all of I-90 west of milepost 19.5, which is east of Front Street in Issaquah; all of I-405; SR 167 from I-405 to SR 18; and all of SR 520.
The study showed that incidents, including crashes, do not, in and of themselves, cause measurable delay. They cause delay only when the disruption they create causes functional capacity to fall below actual demand. However, the researchers calculated that the average incident that does not involve a lane closure results in 576 vehicle-minutes of delay per minute the incident is present. If the incident closes a lane, the effect of that lane closure adds 814 vehicle-minutes of delay per minute of closure.
For the 2006 study year, a conservative estimate is that crashes and other traffic incidents (including disabled vehicles, debris, and other events requiring WSDOT intervention to remove hazards) cost travelers 5,300,000 vehicle-hours of delay, in addition to typical congestion delay, on the Puget Sound region’s freeway system. That is roughly 30 percent of the total delay from all causes that occurred on these roadways. Approximately 11 percent of the total delay (1,950,000 veh-hrs) was the result of reported vehicle crashes.
The study also determined that crash rates increase substantially when delays caused in part by incidents occur. In fact, a simple summary of the available data indicated that crash rates essentially double in corridors slowed by unexpected incident-related queuing. Consequently, because reducing the duration of incidents results in faster clearance of incident-related queuing, it will also have a significant safety benefit, as measured in a reduced crash rate.