Estimated and measured blood alcohol levels in the night-time driving population

The aim of this study was to compare drivers' subjective estimates of their blood alcohol concentration (BAC) with their measured BAC. As part of a night-time survey of drinking and driving in the Adelaide metropolitan area, 13 951 drivers were approached at intersections between 10 p.m. and 3 a.m. and asked to provide a breath alcohol sample. In addition, drivers were given a reply-paid mail questionnaire which sought their own estimate of their BAC. The 5765 replies were compared with the measured BACs. It was found that drivers with low measured BACs tended to over-estimate their BAC while drivers with high BACs tended to under-estimate; the error in the estimates increased as measured BAC increased. These errors were similar for males and females and did not vary with age group. These results emphasize the need for the wider adoption of reliable means by which people can assess their own BAC more accurately.