An automatic fog warning system, designed by Traffic Control and Communications Division of the Department of Transport, became operational during the summer of 1990 on the M25 London orbital motorway. TRL were asked to assess the effectiveness of the system, 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 are switched on based on data from 6 test sites. Rather greater speed reductions occur in lanes 2 and 3 and lesser ones in lane 1. Increases in speed occur when the signals are switched off. Faster vehicles slow down more; it is estimated that the 85th percentile speed (i.e., the speed exceeded by only the fastest 15 percent of vehicles) 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 (recorded in lane 3). These speed reductions indicate that drivers are alerted to the presence of fog ahead which coupled to the greater credibility associated with an automatic system, means that drivers are likely to respond more quickly to the hazard itself. In addition operational benefits would be expected to accrue to the police. Control office staff are notified of the presence of fog, but are relieved of the difficult task of operating motorway signals in response to fog whose density and location is likely to be continuously changing. Furthermore, the mimic signal display in the control office will tell the police which signals are showing the `fog' legend and therefore where the fog is. (A)
Abstract