Every state's DMV permit test asks at least one question about alcohol and at least one about implied consent. The numbers are easy if you memorize them now, and the underlying rules are even simpler than they sound.
The 0.08 BAC limit
The legal blood-alcohol limit for drivers age 21 and older is 0.08% in 49 states. Utah lowered theirs to 0.05% in 2018, the first state to do so. A driver can be convicted of DUI below those numbers if test results or officer observation show actual impairment — the BAC threshold is a per-se floor, not a safe harbor.
Zero tolerance under 21
The BAC limit for drivers under 21 is 0.02% in nearly every state — effectively zero. That number is low enough that even a single drink can trigger it. A zero-tolerance suspension does not require a DUI arrest; a roadside breath test alone can produce an immediate administrative suspension that runs concurrently with any criminal case.
Commercial drivers
The BAC limit for any driver operating a commercial vehicle is 0.04%, regardless of age. This is a federal standard, and a commercial driver convicted of DUI loses their CDL for at least one year on the first offense and for life on the second.
Implied consent
By accepting a state driver license or learner's permit, you give implied consent to a breath, blood, or urine test if you are lawfully arrested for DUI. [Recommended driving resource] Refusing the test triggers an automatic license suspension separate from any criminal DUI penalty. The arresting officer is required to inform you of the consequences before you decide.
Marijuana
Marijuana DUI is enforced under the same impaired-driving statute as alcohol in every state. There is no per-se THC limit you can rely on, and the active-THC level that produces impairment varies wildly between users — any amount that impairs is illegal. State legalization of recreational marijuana does not change driving law.
Prescription drugs
Prescription drugs are not a defense to DUI. If a medication impairs your ability to drive — drowsiness, slowed reaction time, blurred vision — you can be charged exactly as you would for alcohol. Read every prescription label, and don't drive on a new medication until you know how it affects you.
Open containers
Open containers of alcohol in the passenger area of a vehicle are illegal in 49 states. Mississippi is the lone holdout, and even there the driver may not exceed the BAC limit. A handful of states extend the open-container ban to the entire vehicle, including the trunk.