Studies assessing the impairing effects of alcohol in subjects while actually operating a motor vehicle are the most relevant. A study examining the effects of alcohol in an unpredictable traffic situation, which required subjects to brake suddenly and then make an evasive manoeuvre, showed clear evidence of the degrading effects of alcohol upon driving performance in emerging situations at BACs below 50.
Studies of actual drivers who were stopped on the road have demonstrated an exponential increase in crash risk with increasing BAC. The Grand Rapids study, published by Borkenstein and colleagues in 1974, compared approximately 6,000 motor vehicle collisions to approximately 7,500 control drivers and concluded BACs over 40 are definitely associated with increased accident rate. The risk of causing an accident was increased compared to control drivers by approximately two times at 60, three times at 80, seven times at 100, and twenty-five times at 150 milligrams of alcohol in 100 millilitres of blood.
A more recent study revealed an even greater crash risk in collisions that resulted in driver fatality, which became significant at BACs of 20. When the BACs range from 50 to 79, the relative risk of a driver fatality increased from approximately three to seventeen times that of control drivers.
In conclusion, a critical assessment of the impairment literature diminishes the importance of most subject variables—that is, functional tolerance, driving ability, age, and gender. The two most relevant variables are BAC and complexity of the driving task.
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