This policy topic is included in the APIS Highlight on Underage Drinking section.  The Highlight's overview of underage drinking policy in the United States provides additional context that may be helpful in understanding this policy topic. State-by-State summaries of the APIS underage drinking policy topics are available in the State Profiles of Underage Drinking Laws section.  Maps and charts for all of these policy topics are collected on a single page to provide a more comprehensive graphical overview of underage policies.

Policy Description

Blood alcohol concentration (BAC) is a measure of the amount of alcohol in a person's bloodstream. BAC is commonly expressed in percentage terms. For instance, having a BAC of 0.08 percent means that a person has eight parts alcohol per 10,000 parts blood in the body. State laws generally specify BAC levels in terms of grams of alcohol per 100 milliliters of blood (often abbreviated as grams per deciliter, or g/dL). BAC levels can be detected by breath, blood, or urine tests. The laws of each jurisdiction specify the preferred or required types of tests used for measurement.

BAC statutes establish criteria for determining when an operator of a vehicle is violating the law. This section provides information on State and Federal BAC laws that apply to drivers of noncommercial automobiles, trucks, and motorcycles who have not reached the legal drinking age (21 years). Laws establishing very low legal BAC limits of .02 g/dL or less for drivers under the legal drinking age of 21 have been widely referred to as "zero-tolerance laws".  Partly as a result of financial incentives established by the Federal government, all States and the District of Columbia have enacted low BAC limits for young drivers.

All jurisdictions have enacted per se BAC laws for youth operating noncommercial motor vehicles.  A per se BAC statute establishes a BAC limit for a violation. If the operator has a BAC level at or above the per se limit, a violation has occurred without regard to other evidence of intoxication or sobriety. In other words, exceeding the BAC limit established in a per se statute is itself a violation.  By limiting the use of evidence by defendants, per se laws make conviction more likely. [1].   

In the past, some States without a per se law established other standards for using BAC levels as evidence of being under the influence of alcohol. In these jurisdictions, the weight given to the BAC evidence varied. Some laws provided that a BAC at or above a particular level created a presumption of being under the influence of alcohol. Other laws provided that such evidence was prima facie evidence of being under the influence, or was admissible in making this determination. These evidentiary standards were even weaker than a presumption. Defendants in jurisdictions without a per se standard could provide evidence that, in spite of the BAC level, they were not under the influence and therefore not in violation. This differentiates other statutes from per se statutes, which provide that exceeding the BAC limit is itself a violation and only the validity of the BAC measurement is at issue.

For historical data, APIS distinguishes between per se and non-per se jurisdictions but does not distinguish the evidentiary weight of BAC evidence in jurisdictions without a per se standard.

To view the comparison tables and additional
information about this policy topic, click
on the links on the right side of this page.

[1] Cowan, J. and Joffie, S. "Proof and disproof of alcohol-induced driving impairment through blood alcohol testing," 4 Am. Jur. POF 3d 229 (July 2002).

Email a link to this page: http://alcoholpolicy.niaaa.nih.gov/BAC-Youth