How do alcohol and other drugs affect driving?
When you drive, your hands, eyes and feet control the vehicle, and your brain controls your hands, eyes and feet. To drive safely, you need to be alert, aware and able to make quick decisions in response to a rapidly changing environment.
Alcohol and other drugs alter the normal function of the brain and body, and interfere with even the most skilled and experienced driver’s ability to drive safely. While different drugs can have different effects on driving, any drug that slows you down, speeds you up or changes the way you see things can affect your driving—too often with tragic consequences.
Alcohol and other depressant drugs
Alcohol blunts alertness and reduces motor co-ordination. People who drive after using alcohol can’t react as quickly when they need to. Their vision is affected, and may be blurred or doubled. Alcohol alters depth perception, making it hard to tell whether other vehicles, pedestrians or objects are close or far away. And because alcohol affects judgment, people who drive after drinking may feel overconfident and not recognize that their driving skills are reduced. Their driving is more likely to be careless or reckless—weaving, speeding, driving off the road and, too often, crashing.
Alcohol is a depressant drug, which means it slows down your brain and body. Other depressant drugs, including some prescription drugs such as sedatives and painkillers, affect a person’s ability to drive safely, in a way similar to alcohol. Any drug that causes drowsiness, including some cough, cold or allergy medications, can also affect a person’s ability to drive safely. When alcohol and another depressant drug are combined, the effect is more intense and dangerous than the effect of either drug on its own. When taking prescription or over-the-counter medications, it is wise to consult with your doctor or pharmacist before driving.
Stimulant drugs, such as caffeine, amphetamines and cocaine, may increase alertness, but this does not mean they improve driving skills. The tired driver who drinks coffee to stay awake on the road should be aware that the stimulant effect can wear off suddenly, and that the only remedy for fatigue is to pull off the road and sleep. Amphetamines do not seem to affect driving skills when taken at medical doses, but they do make some people overconfident, which can lead to risky driving. Higher doses of amphetamines often make people hostile and aggressive. People who use cocaine are also likely to feel confident about their driving ability. But cocaine use affects vision, causing blurring, glare and hallucinations. “Snow lights”—weak flashes or movements of light in the peripheral field of vision—tend to make drivers swerve toward or away from the lights. People who use cocaine may also hear sounds that aren’t there, such as bells ringing, or smell scents that aren’t there, such as smoke or gas, which distract them from their driving.
Cannabis and other hallucinogens
Cannabis impairs depth perception, attention span and concentration, slows reaction time, and decreases muscle strength and hand steadiness—all of which can affect a person’s ability to drive safely. The effects of hallucinogenic drugs, such as LSD, ecstasy, mescaline and psilocybin, distort perception and mood. Driving while under the influence of any of these drugs is extremely dangerous.