Highway Traffic Response to Dynamic Fog Warning and Speed Advisory Messages

Abstract

The response of traffic to messages displayed on a changeable message sign (CMS) warning of fog ahead and advising specific speeds at increasingly lower visibility levels is examined. Over a 2-year period, the speed, length, and time of detection were individually recorded for all vehicles at four sites: two before and two after a CMS. Mean speed, speed variance, and potential collision speed (PCS), a new metric, were calculated with a 45-s moving window and averaged over each constant message state. PCS is a predictor of impact speed in a potential chain collision that considers visibility as well as speed and separation between individual vehicles. When the CMS was activated, mean traffic speed decreased 1.1 mph on average over all fog events in comparison to mean speed in the absence of a message. However, average PCS increased differentially by 8 mph because of platoon compression and the sensitivity of this metric to increasingly worse visibility after the CMS during most activations. Speed standard deviation among proximate vehicles did not appear to be significantly affected—within 5 to 7 mph across all lanes in most cases. The warning messages evoked a measurable effect on driver behavior but well below design expectations. Drivers appeared to respond predominantly to their own perceptions and reduced speeds in fog, but not nearly enough to compensate for the reduced visibility. When the advised speed was 30 mph in dense fog, mean speeds averaged 61 mph.