CDL: Alcohol, Drugs, and Driving
Alcohol, Other Drugs, and Driving
Few topics on the CDL exam carry higher real-world stakes than this one. Alcohol and other drugs impair exactly the skills a commercial driver depends on most — judgment, coordination, reaction time, and vision. This article covers how alcohol affects the body and driving performance, common myths about drinking, and the rules surrounding other drugs.
Alcohol and Driving
Drinking and then driving is both dangerous and a widespread problem. Crashes involving drivers who have been drinking result in more than 20,000 deaths every year. Alcohol interferes with muscle coordination, reaction time, depth perception, and night vision, and it also affects the parts of the brain responsible for judgment and self-control. For some people, it only takes a single drink to show signs of impairment.
What Counts as "a Drink"?
What matters isn't the type of beverage — it's the amount of alcohol it contains. A 12-ounce glass of 5% beer, a 5-ounce glass of 12% wine, and a 1.5-ounce shot of 80-proof liquor all contain roughly the same amount of alcohol, even though they look and taste completely different.
How Alcohol Moves Through the Body
Once consumed, alcohol enters the bloodstream directly and travels to the brain. A small portion is eliminated through urine, sweat, and breathing, while the majority is processed by the liver. The liver, however, can only break down about one-third of an ounce of alcohol per hour — a fixed rate that's considerably slower than the amount of alcohol in a typical drink. This means that if you drink faster than your body can process it, alcohol builds up in your system and your driving becomes more impaired. Because the liver's processing speed can't be sped up, nothing — not black coffee, not a cold shower — will make you sober faster. Only time works.
Blood Alcohol Concentration, or BAC, is the standard measurement of how much alcohol is in a person's system. Three factors determine BAC: how much alcohol you drink (more alcohol raises BAC), how quickly you drink it (faster drinking raises BAC), and your body weight (a smaller person reaches a given BAC with less alcohol than a larger person would need).
How Alcohol Affects the Brain and Driving
As BAC rises, alcohol impacts more and more of the brain. The first area affected governs judgment and self-control — which creates a troubling problem, since impaired judgment can prevent a drinker from even realizing how intoxicated they've become. Good judgment and self-control are essential for safe driving, so this early effect is especially dangerous.
As BAC continues to climb, muscle control, vision, and coordination are increasingly affected. This can show up behind the wheel as straddling lanes, jerky starts, failing to signal or use lights, running stop signs and red lights, and unsafe passing. These effects raise both the chances of a crash and the risk of losing a driver's license.
Alcohol use is also linked to a range of other driving errors, including slower reaction time to hazards, driving too fast or too slow, drifting into the wrong lane, running over curbs, and weaving.
Common Myths About Alcohol
Believing inaccurate ideas about alcohol makes a driver more likely to get into trouble. Here are some widely held myths, along with the reality:
| The Myth | The Truth |
|---|---|
| Drinking makes you a better driver. | Alcohol is a drug that reduces alertness and impairs your ability to drive safely. |
| Some people can drink heavily without being affected. | Everyone who drinks alcohol is affected by it. |
| Eating a large meal beforehand will prevent intoxication. | Food does not stop you from getting drunk. |
| Coffee and fresh air will sober someone up. | Only time can sober a person up — nothing else actually works. |
| Beer is weaker, so it's safer than liquor or wine. | A few beers contain the same amount of alcohol as a few shots of liquor or glasses of wine. |
Other Drugs
Alcohol isn't the only substance drivers need to be cautious about — use of both legal and illegal drugs has become increasingly common. Regulations prohibit possessing or using many drugs while on duty, and they specifically prohibit operating under the influence of any controlled substance, amphetamines (including so-called "pep pills," "uppers," or "bennies"), narcotics, or any other substance that could make a driver unsafe. This can include a wide variety of prescription and over-the-counter medications — even something as ordinary as a cold pill — if they cause drowsiness or otherwise interfere with safe driving.
There is one important exception: possessing and using a drug prescribed by a doctor is permitted, as long as the doctor has confirmed it won't affect the driver's ability to drive safely.
Drivers should always read warning labels on medications and follow any guidance from their doctor regarding potential side effects, and should stay away from illegal drugs altogether. It's also important never to use a drug simply to mask fatigue — rest is the only real cure for being tired. Alcohol can make the effects of other drugs significantly worse, so the safest approach is to avoid mixing any drugs with driving whatsoever.
The consequences of drug use behind the wheel extend well beyond the immediate safety risk. Traffic crashes tied to drug use can cause death, injury, and property damage, and violations can lead to arrest, fines, jail time, and even the end of a driving career.
Alcohol and other impairing substances attack the exact abilities a commercial driver relies on most: clear judgment, quick reactions, and steady coordination. Understanding how these substances work — and rejecting the common myths surrounding them — is one of the most important parts of driving safely.