Incorrect categorisation of ambient light level at the time of a road traffic collision

Ambient light level, one of the items recorded by police investigating a road traffic collision (RTC) was previously suggested to be incorrectly recorded in 5%–15% of cases. Significant erroneous categorisation of ambient light level, as suggested by the latter estimate, may lead to incorrect conclusions being drawn about the impact of light on RTCs which is critical where such data inform transport policy decisions. This study investigated the accuracy with which ambient light level was recorded in comparison to that determined using solar altitude at the time, date and location of the RTC. Data were drawn from the STATS19 database of RTCs in the UK for the period 2005 to 2015. Ambient light level was incorrectly reported in 103 021 (5.79%) of the 1 779 903 RTCs in that period. The percentage of errors was greater for RTCs occurring after dark than in daylight, and for RTCs where the scene was not attended by a police officer than those that were attended: ambient light level was incorrectly categorized in 8.72% unattended RTCs in darkness. The highest percentage of errors (57%) occurred within civil twilight; if these are omitted the overall percentage reduces to 2.81%, a similar error rate to that available for the determination of RTC location.