What is Impairment? Drivers Need to Know for Safe Driving

It’s never safe to drive when ability is impaired. But what is impairment?

Whatever it is, people are opposed to it. Surveys find that the most people consider impaired driving to be a major problem. It can result from any of number of things.


what is impairmentPeople know that alcohol can cause impairment. And impairment rises gradually at lower levels of blood alcohol concentration (BAC). But it rises greatly at higher levels of BAC .

About half of all alcohol related traffic deaths involve drivers with BACs of .16 or higher. Impairment occurs at lower levels. Yet it’s especially important to prevent driving with high BACs.

The best advice is don’t drink and drive.


what is impairmentThe number of crashes caused by alcohol is unknown. Alcohol related crashes include accidents that are clearly not caused by alcohol. For example, a driver who has been drinking is stopped at a red light. They’re then rear-ended by a sober but inattentive driver. That’s counted as an alcohol related crash.

Most crashes do not involve any alcohol. And many that do would have happened anyway. But when alcohol is involved, reports tend to say they’re “caused by alcohol.” The implication is that none of these alcohol related crashes would have occurred if alcohol had not been consumed.

In fact, alcohol often contributes to the crash. The Insurance Institute for Highway Safety (IIHS) points this out. It says “caused by alcohol” is “incorrect and misleading. That’s because alcohol is only one of several factors that contribute to crashes involving drinking drivers.”

Good News

The proportion of alcohol related traffic deaths continues its drop. The rate has gone down greatly over time. Yet we must continue our efforts to reduce it even more.

A single death caused by drinking is one too many. Each such death is a needless tragedy. And it that traumatizes many others.


what is impairmentDrugs and driving may be a bigger problem than generally thought. A New England Journal of Medicine published a report on drivers without alcohol in their systems. But police stopped them for reckless driving. It found that 45% had marijuana and 25% had cocaine in their systems.

The IIHS studied interstate tractor trailer drivers. It found that 15% of all drivers had pot and 12% had non-prescription stimulants in their systems. Also, 5% had prescription stimulants, 2% had cocaine, and fewer than 1% had alcohol in their systems.

The National Transportation Safety Administration (NTSA) studied fatal truck crashes. It found that stimulants were the most frequently unidentified (15%) drug class among fatally injured truck drivers.

Stimulants may be widely used to combat another under-reported killer. That’s driving while sleep-deprived or fatigued.

Sleep Deprivation

what is impairmentDrowsy driving is greatly under reported because there is no test for it. And many states don’t even have a place for it on their vehicle crash reporting forms.

NTSA estimates that about 100,000 police-reported crashes annually involve drowsiness or fatigue as the principle cause. Driver inattention and lapses probably contribute to about 1,000,000 crashes each year. That’s one-sixth of all crashes. Sleep deprivation and fatigue make such lapses of attention more likely.

The National Science Foundation found that 62% of all adults surveyed in the US reported driving while were drowsy. And that was just during the previous year. Twenty-seven percent reported that they had, at some time, fallen asleep while driving. New York State Police estimate that 30% of all fatal crashes on the Thruway occur because drivers fall asleep. Studies suggest that truck driver fatigue may contribute to at least 30 to 40% of all heavy truck accidents.


In addition to preventing impairment, it’s important not to become distracted. Such things as using a cell phone, eating, combing hair, or applying make-up while driving easily cause distraction.

What is Impairment?

    • All references are from the Ins Inst Highway Safe web site.