To date, studies of incidents on British motorways have been very limited. As part of a study into the role of Incident Support Units (ISUs) on the M25, the London orbital motorway, an analysis of road traffic incidents was undertaken. Motorway incident data were collected through closed circuit television (CCTV) observations at Surrey Police's Godstone Motorway Control Room. The collected data covered the Surrey Police section of the M25 motorway between Clacket Lane Services and Junction 14, for a four-week period between 5 November and 11 December 2003. Incident frequency, durations, locations, detection times, notification methods and ISU involvement were examined. It was found that the average incident frequency was 16·1 incidents per day for the study area, and the highest occurrence of incidents was found to be between 11·00 am and 12·00 pm. On average the recorded incidents lasted 1 h 9 min, with those occurring in the evening peak period lasting the longest. The distribution of incidents along the study area showed that the section between Junctions 9 and 10 had the highest rate for the study period. A comparison of incident characteristics between US and UK studies was also conducted. Incident rates between the M25 and the US were found to be substantially different, and incident durations were found to be between double and triple those of the US. Possible reasons for these differences are discussed in the text.