Investigation of Driver Speed Choice and Crash Characteristics During Low Visibility Events

Fog can create a significant safety hazard for motorists. If motorists drive faster than current visibility permits, severe multiple-vehicle crashes may occur. In Virginia, sections of I-77 and I-64 in mountainous parts of the state experience significant reoccurring fog events. These locations have also experienced several chain reaction crashes involving more than 50 vehicles during fog. These crashes were typically caused by drivers traveling too fast for available visibility conditions.

In order to improve safety on the I-77 corridor, the Virginia Department of Transportation has constructed a variable speed limit (VSL) system which will post dynamic speed limits based on the available visibility. Before the system is activated, it is important to understand existing driver speed choice behavior during low visibility conditions. It is possible that posting a VSL speed based only on stopping sight distance (SSD) could create significant speed variance and decrease safety if drivers are currently driving much faster than conditions warrant. In this study, crash, speed, and visibility data were examined at several locations on I-64 and I-77 where there were recurring fog events.

Crash history at I-77 revealed that crashes in low visibility were more likely to be severe and involve more than 2 vehicles than crashes during clear conditions. Mean speed analysis found that observed mean speeds exceed safe speeds in all low visibility conditions and at all sites. In the worst visibility conditions, drivers are exceeding the safe speed by more than 20 mph. Standard deviation analysis found that speed variance did not increase as visibility decreased on I-77, but at several locations in I-64 standard deviation was different during low visibility compared to clear conditions.

Models were developed better understand the relationship between speed and visibility. The models developed showed that while motorists are slowing down in low visibility, there is a still a significant differential between observed speeds and the safe speed calculated from the SSD. The models show that on I-64 speeds are much less sensitive to changes in visibility compared to I-77. A possible explanation for the differences between I-77 and I-64 are the presence of illuminated in-pavement markers on I-64 that provides motorists with and sense of safety that causes them to driver faster than visibility conditions dictate. It’s also possible that mean speeds in low visibility are higher on I-64 because of the regular commuter traffic who are more comfortable driving during foggy conditions.

The model for I-77 was used to develop the initial VSL control algorithm. A primary concern of the VSL system operators was that it will not be respected by motorists and result in increased speed variance in foggy conditions. The model was used to help bridge the gap between current driver behavior and safe speed. It is recommended that future VSL system deployments use existing driver behavior in the initial algorithms as well. Speed and crash data on I-77 should be analyzed after the VSL system is deployed to determine the operational and safety effects. If the system on I-77 is deemed to be successful, a similar system should be developed for I-64 using the current driver behavior models as part of the initial algorithm.