An automatic fog warning system became operational during the summer of 1990 on the M25 London orbital motorway, a freeway facility circling London. The system automatically detects the presence of fog and posts warning messages on dynamic message signs (DMS) upstream of the fog detector.
Detectors are located at 54 fog prone areas along the M25, including river valleys and high ground prone to hill fog. When a fog detector senses visibility below 250 meters, the system posts a warning message (the word "fog" ) on existing DMS ranging from 0.8 to 2.2 km upstream from the activated detectors. During periods of heavy fog additional warnings are posted farther upstream of the detector at signs ranging from 1.8 km to 3.8 km upstream.
An independent evaluation assessed the effectiveness of the system, primarily in terms of changes in vehicle speeds which occurred when the messages were posted as a result of the formation of fog. The intent of the evaluation was to determine if a statistically significant reduction in vehicle speeds occurs in the presence of a fog warning system.
The study measured vehicle speeds, comparing the change in mean speeds during observation periods before and after the fog messages were displayed on the DMS. The observation periods ranged in length from 1 to 10 minutes. Similar measurements were taken each time the fog messages were deactivated. These speed changes were also compared to speed changes at the same times during non-foggy days.
When the fog messages were switched on, there was a statistically significant overall net reduction in mean vehicle speeds of about 1.8 mi/h. This conclusion is drawn from analysis of data from the 10 sign locations which experienced enough fog over the evaluation period to yield useful data. Greater speed reductions occurred in the inner/faster lanes, and lesser reductions occurred the outer/slow lane. Faster vehicles slowed down more when the fog signals switched on, with the average change in the 85th percentile speed being 0.5 mi/h greater than the average change in the overall mean speed. Increases in speed occurred when the signals were switched off.
These speed reductions indicate that the fog warning messages do alert drivers to the presence of fog ahead. Once aware that they are about to enter a foggy area, drivers are prepared to drive more safely through the fog.