An illegal per se law makes it an offense in and of itself to drive with a blood alcohol concentration (BAC) measured at or above the established legal level, whether or not the driver or operator exhibits visible signs of intoxication. Lowering the BAC limit to .08 sets the legal limit to a point at which driving skills are proven to be compromised. At .08 BAC, all drivers, even experienced drinkers, show impairment in driving ability. For the great majority, there is serious deterioration in driving performance at .08 BAC.
Measuring Impairment The amount of alcohol in a person’s body is measured by the weight of the alcohol in a certain volume of blood. This is the blood alcohol concentration, or “BAC.” The measurement is based on grams per deciliter (g/dl), and in most states a person is considered legally intoxicated if his or her BAC is .10g/dl (.10) or greater; that is, alcohol makes up one-tenth of one percent of the person’s blood.
With each drink consumed, a person’s BAC increases. Although outward appearances vary, virtually all drivers and motorcycle operators are substantially impaired at .08 BAC. Laboratory and on-road research shows that the vast majority of drivers, even experienced drinkers, are significantly impaired at .08 with regard to critical driving tasks such as braking, steering, changing lanes, divided attention tasks, and judgement. The risk of being in a crash rises gradually with each BAC level, but then rises very rapidly after a driver reaches or exceeds .08 BAC, compared to drivers with no alcohol in their system.
While there is no “safe” amount of alcohol for drivers, most people can drink moderately and drive legally when the illegal per se limit is set at .08 BAC. A 170-pound male typically would have to consume more than four drinks in one hour on an empty stomach to reach a BAC of .08. A 135-pound female typically would have to consume three drinks in the same time frame.
.08 Laws Work The National Highway Traffic Safety Administration (NHTSA) examined the effects of lowering the legal BAC limit from .10 to .08 and implementing an administrative per se law in California. The two new laws, and the associated publicity, reduced the number of expected alcohol-related fatalities by 12 percent in 1990.
Another study* on the effects of lowering BAC levels to .08 compared the first five states to lower their BAC limit to .08 (California, Maine, Oregon, Utah, and Vermont) with five nearby states that retained the .10 limit. Overall, the .08 states experienced a 16 percent reduction in the proportion of fatal crashes with a fatally injured driver whose BAC was .08 or higher, as well as an 18 percent reduction in such crashes with a fatally injured driver whose BAC was .15 or higher.
* Hingson, R., Heeren, T., and Winter, M. (1996) “Lowering State Legal Blood Alcohol Limits to 0.08%: The Effect on Fatal Motor Vehicle Crashes,” American Journal of Public Health, Vol. 86 (9) pp. 1297-1299.
State Laws As of July 1997, 48 states have a per se law (the exceptions are Massachusetts and South Carolina). In 35 of those states, the legal limit is .10 BAC per se. Fourteen other states have .08 as the legal limit.
States With "Per Se" Laws
