[Absolute driving incapacity at 1.0 promille--an erroneous limit]

Following a well known decision of the Bundesgerichtshof (BGH, German Federal Court) a driver with a blood alcohol concentration (BAC) of more than 0.13% is seen as "absolutely unfit for driving" and will be prosecuted by criminal law. This decision was based on the results of a comprehensive review of the literature about the effects of alcohol done 1966 by the Bundesgesundheitsamt (BGA, National Bureau of Health). There, severe impairments were described at levels above 0.1%, a value which the BGH "rounded up" to 0.11%. The possible errors of measurement were estimated as 0.02%. Adding both values yielded the legal limit of 0.13%. Most recently Salger (vice-president of the BGH) in a decisive manner pleaded for a reduction of this limit to 0.11% both by lowering the assumed measurement error to 0.01% and by taking back the rounding-up operation. Explicitely he based these intended corrections of the former decision on two arguments he derived from the scientific research on alcohol and driving: 1. Significant impairments are linked with BACs higher than 0.1%. 2. This was demonstrated by road-side studies, studies with instrumented cars, and driving simulators which are more valid methods to predict driving behavior than studies conducted in the laboratory. We proved these assumptions by analyzing the literature about driving studies as well as the literature from laboratory studies on alcohol. This article mainly concentrates on the empirical results coming from driving studies. It demonstrates a clear-cut empirical evidence that no single limit can be determined from where alcohol-induced impairment begins. Quality and quantity of this impairment strongly depends on how difficult the driving situation actually is. Therefore, BAC and situational difficulty act together in a synergistic way. A driver only confronted with standard traffic situations which are easy to handle may react correctly despite a BAC of 0.1% and quite more. If difficulty is slightly augmented to situations where a more controlled and/or precision-oriented non-automatic driving is required, significant impairments were seen at levels of 0.07%. In traffic situations with unpredictable events and/or extreme driving conditions severe impairments are found even at levels below 0.05%. These results perfectly match with the results from the laboratory studies. As a consequence, any public policy decision as to the acceptable legal limit of alcohol has to include a decision as to the difficulty of traffic situations which has to be mastered by the driver. Claiming that a normal driver has to deal efficiently with a medium level of difficulty the scientific evidence demands a legal limit of about 0.07%.