Chapter 9 of 12

Alcohol and drugs

Learn the legal tiers, then reject the dangerous idea that a threshold can tell you when it is safe to drive.

canvas Updated 2026-08-17 Road glossary

Impairment often steals the ability needed to measure impairment. A driver can feel confident at the exact moment judgment, vision, and reaction are getting worse. That is why the safe plan must be made before the first drink or dose.

The threshold is a legal line, not a safe target

California has different per se blood-alcohol concentration tiers for different drivers and driving roles. The general tier is 0.08% source . A driver under legal drinking age is subject to the lower 0.01% source tier. A driver on DUI probation is also subject to 0.01% source . Commercial driving and driving a passenger for hire with a passenger aboard use the 0.04% source tier.

Memorize those categories because the knowledge test can ask them. Then remember the more important fact: a person can be too impaired to drive below any per se tier. California law also prohibits driving while under the influence. A number does not override observed impairment, unsafe driving, or the effects of a drug.

The California Driver Handbook alcohol and drugs chapter explains both the tiers and the broader impairment rule.

Teaching timeline

Watch an illustrative BAC curve

Change the model inputs and move through time. The threshold lines come from California state data.

Teaching illustration only. Never use this model to decide whether to drive. It cannot measure your BAC or tell you that driving is safe. Choose a sober ride.

Illustrative model point 0.000% Time marker 2.0 hours Inputs 2 drinks, 160 lb

Move the time marker to inspect the teaching curve. No point on this chart is a go-ahead to drive.

Standard drinks in model 2 Body mass in model 160 lb Hours since first drink 2.0 h

Illustrative model curve California threshold lines

General adult threshold 0.08%
Under age 21 0.01%
Commercial vehicle 0.04%
DUI probation 0.01%
Passenger for hire 0.04%

The curve uses fixed absorption, distribution, and elimination assumptions. Real absorption and impairment vary with timing, food, medications, health, drink size, and other factors. California can treat impaired driving as illegal below a numeric BAC threshold.

Official source: BAC limits

Check your understanding

A driver believes being below the applicable per se BAC tier proves it is safe and legal to drive. What is wrong with that reasoning?

Keyboard: press 1 through 4 to answer.

Alcohol changes the tasks driving depends on

Driving is a continuous chain of perception, prediction, choice, and control. Alcohol can weaken each link. Eyes may track less effectively. Attention narrows. A driver may miss information at the edges of the scene. Reactions slow. Steering and braking become less precise. Risk judgment can deteriorate before the person notices a dramatic physical effect.

That last change is especially dangerous. Confidence can rise while ability falls. A driver may accept a smaller turn gap, follow more closely, or drive faster because the internal alarm is quieter. The absence of worry is not evidence of fitness.

Food, body size, biological differences, medications, drinking pattern, and elapsed time can all change a measured result. None creates a reliable personal formula for safe driving. The body removes alcohol slowly. A cold shower, exercise, coffee, or fresh air may change how awake a person feels without rapidly clearing alcohol from the blood.

Drugs do not need a BAC to impair driving

Prescription medicine, over-the-counter medicine, cannabis, illegal drugs, and combinations can impair attention, reaction, vision, coordination, or wakefulness. A legal prescription does not make an impaired drive legal. Read the label, follow medical advice, and learn the effect before you are responsible for a vehicle.

Mixing substances can create effects that are stronger or less predictable than either substance alone. Alcohol plus a sedating medicine is an obvious danger, but even a medicine marketed for a common symptom can reduce alertness. If a label warns against driving or operating machinery, take that warning literally.

Check your understanding

A prescribed medicine makes a driver drowsy. Does the prescription make driving while impaired acceptable?

Keyboard: press 1 through 4 to answer.

Chemical testing is a duty tied to driving

California's implied-consent law connects the privilege of driving with chemical testing after a lawful DUI arrest. The required test and consequences depend on the situation. Refusal or failure to complete a required test can trigger administrative action separate from the criminal case.

At the roadside, an officer may use observations and preliminary tests during an investigation. After a lawful arrest, the statutory chemical-test duties apply. Do not collapse every roadside interaction into one rule. The implied-consent statute sets out the legal framework.

Administrative and criminal consequences are separate

A licensing agency can take administrative action based on a test result or refusal under rules that are separate from a criminal prosecution. A court case and an administrative process can therefore move on different tracks. For a real incident, use qualified legal advice rather than a study guide.

Check your understanding

Which statement best describes California's chemical-test framework after a lawful DUI arrest?

Keyboard: press 1 through 4 to answer.

Build the transport plan while judgment is clear

The safest technique is logistical, not chemical. Decide who will drive before an event. Give the keys to that person. Arrange a ride that does not depend on a last-minute self-assessment. Plan for the vehicle to remain overnight. Protect a friend without debating whether that person “looks fine.”

If an impaired person is about to drive, interrupt the plan when you can do so safely. Offer a specific alternative. Call for help if the person creates an immediate danger. A minor social discomfort is a small cost next to a preventable injury.

Source trail

Know the tiers because the law distinguishes driver categories. Never use them as a green light. The dependable decision is made while judgment is intact: separate impairment from driving. Next, protect the attention that sober driving still requires.

Your progress

Close the loop.

Complete the inline checks to build an objective-level mastery signal.