Driving Under the Influence

Impaired driving—driving under the influence of alcohol or drugs—is an illegal activity with potentially fatal consequences.  In 2009, over 10,000 people in the U.S. died in alcohol-related crashes; this accounts for approximately one-third of all traffic-related deaths.  Other drugs (often used in combination with alcohol) were involved in 18% of all driver deaths.  Many children die every year while riding in a car with an impaired driver.  Clearly, driving while intoxicated is like playing Russian roulette with your own life and the lives of others.  Yet many, many people continue to drive under the influence of drugs and alcohol either because they do not understand how seriously drugs and alcohol affect their ability to drive or because they are too intoxicated to realize that they should not drive and there is no one nearby to prevent them from driving.

How does Blood Alcohol Concentration (BAC) affect your ability to drive?

In all states, it is illegal to drive with a BAC of .08% or above, although some states have stricter BAC laws.  Many states have zero tolerance laws for drivers under 21; this means that it is illegal for them to drive with any amount of alcohol in their bodies.  BAC is measured by testing the breath, urine, or blood.  It can be estimated according to the amount of alcohol consumed, person’s body weight and sex, and the amount of time that has passed since drinking.  For instance, if a 160 lb. man consumes three drinks one right after the other, his BAC will be approximately .07%, and it will gradually decrease by about .01% every 40 minutes, as his body metabolizes the alcohol.  Women generally reach a higher BAC level more quickly than men do because they tend to be smaller and to metabolize alcohol differently.

Even when a driver’s BAC is only .02%, his or her abilities to visually track a rapidly moving target and to pay attention to more than one thing at a time are diminished.  At .05% BAC, a driver’s coordination, steering ability, and response time are reduced.  At .08% BAC, the legal limit, a driver’s concentration, speed control, and perception are significantly impaired.  By the time a person’s BAC has reached .10 –.15%, their speech is generally slurred, they show a loss of coordination, balance, and thinking skills, and they may even be vomiting; their driving is obviously impaired.  They will have difficulty staying in their lane and controlling their vehicle. 

How do other drugs affect one’s ability to drive?

            Marijuana.  Marijuana is the second most common psychoactive substance found in drivers (after alcohol).  We know that marijuana targets the parts of the brain that govern movement, reaction time, balance, coordination, judgment, and memory.  Studies have shown that the greater the amount of THC (the active chemical in marijuana) in the blood, the more impaired a driver’s cognitive and behavioral skills will be.  Marijuana affects a driver’s performance for about 3 hours.  Other studies have shown that when alcohol and marijuana are combined, the effects on driving are much more serious than when either substance is used alone.

            Cocaine.  In laboratory studies, low doses of cocaine enhance attention and reduce fatigue.  At higher doses, after chronic use, and during withdrawal or crash, cocaine users suffer from agitation, inability to concentrate, confusion, poor coordination, and visual distortions, among other problems.  Drivers who have used cocaine and are arrested while under the influence have typically been driving recklessly and at high speeds.

            Methamphetamine and amphetamine.  These drugs can be prescribed by a doctor to treat attention deficit disorder, narcolepsy, and obesity, but methamphetamine is also abused for its ability to increase alertness, decrease fatigue, control weight, and produce a high.  At high doses, methamphetamine can lead to increased risk-taking and aggressive driving.  During withdrawal, great impairment can also be expected.  Drivers who have tested positive for methamphetamine have been observed speeding, driving erratically, and crashing.  They have demonstrated unintelligible speech, violent behavior, disorientation, and unconsciousness. 

            Ecstasy.  Experts conclude that Ecstasy exerts a relatively mild influence over basic driving skills, but does increase risk-taking.

            Heroin and morphine.  When morphine is taken regularly as prescribed for chronic pain, individuals experience only mild to moderate driving impairment.  People who have not developed a tolerance for morphine and people who have taken large doses of morphine or heroin recreationally experience severe driving impairment.

            Other prescription drugs.  Many prescription drugs—including muscle relaxants, cough suppressants, sedatives, sleep aids, and pain medications—include warnings from the manufacturer regarding their potential to impair driving.  It is generally prudent to heed these warnings and to consult your physician if you must take these medications regularly.  The National Highway Traffic and Safety Administration’s website includes an extensive set of fact sheets enumerating the effects on driving of many prescription drugs and illicit drugs, which you may consult here.

Bottom Line: Driving a vehicle is a complex, potentially dangerous task.  It is incompatible with consuming excessive amounts of alcohol or doing drugs recreationally.  Never drink or do drugs and drive, don’t ride with drivers who do, and do what you can to prevent your family and friends from driving under the influence.