Impaired driving means operating a vehicle when alcohol, drugs, or another substance has reduced your ability to drive safely. In every state, a blood alcohol concentration of 0.08% is enough on its own to convict an adult driver, and lower limits apply to commercial drivers and anyone under 21. The offense goes by different names in different states — DUI, DWI, OWI — but the core idea is the same: if a substance has compromised your ability to drive, you can be charged.
Impairment Is a Functional Standard
You don’t have to be visibly drunk to be impaired in the legal sense. The question courts ask is whether your coordination, focus, or judgment has slipped far enough that you can no longer drive with the caution of a sober person using ordinary care. Swerving between lanes, delayed reactions at traffic signals, and trouble maintaining a steady speed are the kinds of observable behavior that signal impairment.
That standard applies no matter what caused the impairment. Alcohol is the most common cause, but prescription medications, illegal drugs, over-the-counter cold medicine, and combinations of otherwise legal substances can all support an impaired driving charge. The law cares about your condition behind the wheel, not whether the substance was legal to buy or legal to take.
The 0.08% Per Se Rule
Blood alcohol concentration, or BAC, is the percentage of alcohol in your bloodstream. Federal law pushes every state to treat a BAC of 0.08% or higher as a “per se” offense, meaning the test result by itself proves the violation without any further evidence of bad driving.1Office of the Law Revision Counsel. 23 USC 163 – Safety Incentives to Prevent Operation of Motor Vehicles by Intoxicated Persons All 50 states and the District of Columbia use 0.08% or lower, and one state has set the limit at 0.05%.
The per se rule simplifies prosecution. If your breath or blood reading is 0.08% or above, prosecutors don’t need dashcam footage or testimony about slurred speech. The number is the evidence. But the threshold is a floor, not a safe harbor. You can still be charged below 0.08% if an officer observes signs that your actual driving ability is impaired.
Lower Limits for Commercial and Underage Drivers
Two groups face stricter BAC thresholds.
Commercial Drivers
If you hold a commercial driver’s license, the limit drops to 0.04%. A first conviction at or above that level while operating a commercial vehicle disqualifies you from driving any commercial vehicle for one year. A second offense in a separate incident brings a lifetime disqualification.2eCFR. 49 CFR 383.51 – Disqualification of Drivers A first offense while hauling hazardous materials triggers a three-year disqualification instead of one. Federal regulations also prohibit commercial drivers from drinking within four hours of going on duty or having any detectable alcohol while operating a commercial vehicle.3eCFR. 49 CFR 392.5 – Alcohol Prohibition
Drivers Under 21
Federal law requires every state to enforce zero tolerance for drivers under 21, with a per se BAC limit of 0.02% or lower.4Office of the Law Revision Counsel. 23 USC 161 – Operation of Motor Vehicles by Intoxicated Minors About half of states set the underage limit at 0.02%; the rest use 0.00% or 0.01%. Since it’s already illegal for anyone under 21 to drink, any measurable alcohol while driving is enough.
Drugs, Prescriptions, and the Missing Breathalyzer
Drug-impaired driving is harder to prosecute than alcohol cases because no equivalent to the 0.08% breath standard exists for most drugs. A blood test can confirm the presence of THC, opioids, or benzodiazepines, but the relationship between blood concentration and actual impairment varies by substance, tolerance, timing, and metabolism.
Some states have responded with zero tolerance rules for certain controlled substances, making any detectable amount illegal while driving.5National Institute on Drug Abuse. DrugFacts Drugged Driving Others require prosecutors to prove actual impairment regardless of what the blood test shows.
This creates a trap for people taking legal medications. Prescription painkillers, sleep aids, anti-anxiety drugs, and some antihistamines can all impair driving. A valid prescription is not a defense if the medication made you an unsafe driver.
DUI, DWI, OWI, and the “Operating” Trap
The acronyms shift by state, but they describe the same core offense.
- DUI — Driving Under the Influence. The most widely used term, covering impairment from alcohol, drugs, or both.
- DWI — Driving While Intoxicated or Driving While Impaired. Used in some states, and in a few states DWI and DUI distinguish between higher and lower levels of impairment.
- OWI — Operating While Intoxicated. Used in states that frame the offense around “operating” a vehicle rather than “driving” it.
The difference between “driving” and “operating” matters. In states that use “operating” or “actual physical control” language, you can be charged when the vehicle wasn’t moving. Sitting in the driver’s seat with the keys in the ignition while intoxicated has been enough in many jurisdictions, even if the engine was never started. The reasoning is that a person in physical control of a vehicle while impaired poses an imminent risk of driving it.
How Impairment Gets Proved
Before an officer can request a breath or blood test, they generally need probable cause from field observations. The National Highway Traffic Safety Administration developed three standardized field sobriety tests — the Horizontal Gaze Nystagmus eye test, the Walk and Turn, and the One Leg Stand — that form the backbone of most roadside investigations.6National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Test Full Manual When drug impairment is suspected and the breath test shows little alcohol, the case often shifts to a Drug Recognition Expert who runs a systematic evaluation and sends a blood or urine sample to a lab.7National Highway Traffic Safety Administration. Drug Evaluation and Classification Program
Implied Consent and Refusing the Test
Nearly every state has implied consent laws. By driving on public roads, you’ve already agreed to submit to chemical testing if an officer has probable cause to believe you’re impaired. Refusing the test rarely helps. States impose administrative penalties for refusal that are separate from the criminal charge, and the most common consequence is an automatic license suspension that often lasts longer than the suspension you’d face for failing the test.8National Highway Traffic Safety Administration. BAC Test Refusal Penalties In at least a dozen states, refusal is itself a criminal offense. Prosecutors can still bring the underlying charge using officer observations, field sobriety results, dashcam footage, and the refusal itself, which a jury can treat as consciousness of guilt. For commercial drivers, refusing an alcohol test triggers the same one-year disqualification as a failed test.
Enhanced Penalties at Higher BAC
Most states treat very high BAC readings as an aggravating factor. The common trigger points are 0.15% and 0.20%, and a majority of states impose enhanced consequences at one or both.9National Conference of State Legislatures. Increased Penalties for High Blood Alcohol Content Enhancements typically mean mandatory minimum jail time even for a first offense, higher fines, longer license suspensions, and mandatory ignition interlock installation. In a few states, a first offense at 0.20% or above can be charged as a felony.
What a Conviction Actually Costs
A first offense is a misdemeanor in most states, but the practical burden stacks up fast.
Jail and Fines
Jail time for a first conviction is typically capped at six months to one year, though many first-time offenders receive probation. Fines generally run $500 to $2,000, before court costs, surcharges, and program fees. Repeat offenses bring longer sentences, higher fines, and in many states a felony charge.
License Suspension
A conviction almost always triggers a license suspension. The length depends on your state, your BAC, and whether it’s a first offense, with 90 days to one year common for first-timers. Getting your license back requires a reinstatement fee, and some states make you retake written or driving exams, particularly if the license was revoked rather than suspended.
Ignition Interlock Device
An ignition interlock is a breathalyzer wired into your vehicle. You blow into it before starting the car, and it prevents the engine from starting if it detects alcohol. More than 30 states and the District of Columbia require interlock devices for all offenders, including first-timers.10National Conference of State Legislatures. State Ignition Interlock Laws You pay installation plus a monthly monitoring fee for the duration of the requirement, which commonly runs six months to a year for a first offense and several years for repeat offenses.
SR-22 Insurance
Most states require you to file an SR-22 certificate after conviction, which is proof that you carry at least the state minimum auto insurance. The requirement typically lasts three years. If your policy lapses, your insurer notifies the state and your license is suspended again. The filing fee is small, but the real cost shows up in premiums: a DUI conviction is treated as a major risk factor, and rate increases of 50% or more are common for several years.
The Total
Add the fine, court costs, mandatory education, interlock fees, insurance increases, reinstatement fees, and lost wages from court or jail, and a first-offense conviction routinely costs $10,000 or more before any legal representation. Repeat offenses, felony charges, and convictions involving injury or death multiply those numbers and can bring years of imprisonment.