The scientific basis for the legislation is reviewed in discussions of the absorption and elimination of alcohol, the distribution of alcohol in the breath and urine, and the effects of alcohol. The methods and reliability of blood and urine sampling and analysis are considered, followed by a discussion of calculations that indicate blood-alcohol levels at the time of arrest and the amount of drink consumed. A discussion of drugs and driving addresses the classification and nomenclature of drugs as well as testing and effects. A review of British attempts to set statutory limits for drug and alcohol levels in the body while driving traces the history of relevant legislation through the 1982 Transport Act. The discussion includes an analysis of relevant case law over the 17 years since the introduction of compulsory blood and urine alcohol testing. The reliability of new machines for testing blood-alcohol levels is also assessed. An examination of recent legislative changes considers problems arising from using the new printout form and the intoximeter machine. Required evidence and penalties are reviewed. Appendixes contain examples of machine printouts, the 1972 Road Traffic Act, legal and scientific references, as well as volumes and measures. Subject index.