The general “legal limit” for drivers who have consumed alcohol is 0.08%. When someone has a Blood Alcohol Concentration that exceeds this level, it is automatically considered severe enough to impair their ability to drive. A breath or blood test can then be used in a drunk driving case to show impairment and lead to a conviction.
That said, the legal limit can change. Remember that people are not allowed to drink until they turn 21 years of age, but they are allowed to drive when they turn 16 years of age. You have four years in which drivers should, in theory, be completely sober. Drinking is prohibited. If they do it anyway and get pulled over, what BAC level applies to them?
Any measurable level is illegal
States have set up zero-tolerance laws for young drivers. These laws sometimes state that any measurable alcohol level — generally considered 0.02% and up — is illegal.
If alcohol can be measured, it is clear that the driver has broken the law. If they were driving, they can then get a drunk driving charge, even though they are far below the level where impairment would be assumed for an older driver.
These laws are widely used, but they can cause problems. Someone who is at 0.02% may have broken the law, but they may not feel any level of impairment. This could easily lead to accidental intoxication through a spiked drink, for instance. The driver could be arrested when they made no conscious decision to drink and drive, all because of the limits in the law.
If this happens to you, be sure you know what legal defense options you have. A drunk driving conviction can damage your future in numerous ways, so you want to do everything possible to prevent that.