To the Editor.— Because many drivers with a blood alcohol concentration (BAC) above 0.05% but below the "usual" legal limit of 0.10% are significantly impaired, we agree with the American Medical Association's proposal that a BAC of 0.05% be established as "per se illegal for driving."1 However, we wish to point out some of the practical issues related to enforcing such a policy. Lowering the legal BAC will increase the number of drivers who should be identified and tested by police officers. Because there are no good measurements (ie, data from roadblocks) to estimate the number of persons who drive with BACs of 0.05% or higher, the best estimate may be self-reports of driving while impaired. Study.— We estimated the prevalence of alcohol-impaired driving in Michigan by analyzing a random telephone survey of 1492 Michigan adults (18 years old or older) conducted during June and July 1982 as part