In the United States, most states have a legislated BAC that defines inebriation? at a specific percentage (often about 0.10 %). The federal government is forcing a change to 0.08%. In some states, underage drunken drivers are considered legally impaired at lower levels (perhaps 0.02%).
In Australia, the limit is 0.05% in most states and territories, and either 0.02% or zero for inexperienced drivers (learner drivers or those on probationary licenses). It is enforced by [random breath testing]?.
See also: Alcoholic units