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.