Best Practices for Road Weather Management Version 2.0

Title:

Assessment of M-25 Automatic Fog Warning System: Final Report

Abstract:

An automatic fog warning system became operational during the summer of 1990 on the M25 London orbital motorway. The effectiveness of the system was assessed, primarily in terms of any changes in vehicle speeds which occurred when the signals were switched on as a result of the formation of fog. This report on the assessment describes briefly the automatic warning system, the method of data collection and analysis and also presents detailed results. There is an overall net reduction in mean vehicle speeds, of about 1.8 mph when the signals were switched on based on data from six test sites. Increases in speed occur when the signals are switched off. Faster vehicles slow down more; it is estimated that the 85th percentile speed falls by about 0.5 mph more than the reduction in the mean when the signals are switched on. However, the potential of the automatic fog warning system to reduce drivers' speeds is perhaps illustrated by the greatest average speed reduction at a single test site of approximately 5 mph. These speed reductions indicate that drivers are alerted to the presence of fog ahead which, coupled with the greater credibility associated with an automatic system, means that drivers are likely to respond more quickly to the hazard itself. Additional operational benefits are expected since police are notified of the presence of fog in specific locations.

Source(s):

Transport Research Laboratory (United Kingdom), Project Report 16. For an electronic copy of this resource, please direct your request to WeatherFeedback@dot.gov.

Date: 1993

Author:

Cooper, Sawyer

Keywords:

Motorist warning system
Speed
Law enforcement
Traveler information
Visibility

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