Variable speed limits effective along Virginia’s foggy highways

By Chris McCahill

Fog can create deadly driving conditions, particularly in mountainous areas like those along I-77 near Virginia’s southern border. Fog along the 12-mile stretch of highway has led to hundreds of crashed vehicles and several deaths over the last couple of decades. In 2016, VDOT launched a system of weather sensors, variable speed limits (VSLs), and dynamic message signs (DMS) meant to slow down drivers during unsafe conditions. This system lowered speeds by an additional 2 to 5 mph, on average, and the number of fog-related crashes seems to have dropped by more than 50 percent.

The system cost $7.5 million, including new power lines and fiber optic cables, and installation started in 2014. The speed limit along the highway is 65 mph during good weather conditions, but that can drop to as low as 30 mph when fog rolls in. As conditions worsen, speeds can drop by as much as 15 mph every five minutes. The signs are spaced 1.5 miles apart and no two successive signs drop by more than 15 mph, allowing drivers to gradually slow down. The system is a variation of several other similar ones, including Tennessee’s fog warning system.

Drivers naturally slow down in foggy conditions, but typically not enough to give them safe stopping sight distance. That’s where the VSLs come in. The system seems to be most effective in lighter fog, when safe speeds are still above 45 mph. In those cases, it brings average speeds down by around 5 mph and reduces the standard deviation by around 2-4 mph (meaning speeds are more uniform). Drivers rarely drop below 50 mph, however, even if that leaves them without enough time to stop in the case of an obstacle or crash ahead of them. VDOT will continue to monitor the corridor, but the Virginia Transportation Research Council says similar systems should be effective for more widespread use.

Chris McCahill is the Deputy Director at SSTI.