The selection of a per se BAC limit for drivers has often been, and continues to be, a contentious and inherently political issue. In Canada, the House of Commons Standing Committee on Justice and Human Rights recently considered changes to federal impaired driving legislation, including a reduction in the per se BAC limit from 80 mg per cent to 50 mg per cent. This paper provides a brief history of per se laws in Canada, an examination of the strength of the research evidence on lowering the per se BAC limit, and a discussion of why lower BAC limits have not been more effective in reducing the alcohol-crash problem.