The Grand Rapids dip revisited.

This analysis addresses an issue that has concerned road safety authorities for some 28 years: the celebrated "Grand Rapids Dip." This, most readers will recognise, is the below-baseline excursion, which occurs in the blood alcohol concentration (BAC) interval of .01%-.04%, of the relative risk curve for accident risk versus blood alcohol, derived from the 1964 Grand Rapids Study data. The present analysis has its starting place in the explanation advanced by Allsop, who noted that the case/control comparisons were biased due to the disproportionate representation of demographic subgroups in different blood alcohol concentration class intervals. Indeed, when relative-risk curves are derived separately for subgroups of differing drinking habits, the resulting separate risk curves all show monotonic increases at all blood alcohol concentration ranges. Such separate relative risk curves are unpopular, and most of the road safety community pays them little heed. Thus, the original concept of the "dip" remains with us. For this reason, we have derived, using a simple but realistic statistical model, a single relative-hazard curve from the Grand Rapids data, one that is free from the distortion introduced by unequal representation of different demographic subgroups in different blood alcohol concentration class intervals. This curve indicates that accident risk increases with increased blood alcohol concentration regardless of self-reported drinking frequency. However more frequent drinkers have less risk at all blood alcohol concentration levels, including zero, than less frequent drinkers at the times and places sampled.