What is Impairment?

It's never safe to drive a vehicle or operate machinery when ability is impaired. Surveys reveal that the majority of people in the U. S. consider impaired driving to be one of the nation's most important social issues, more important than healthcare, poverty/hunger, racism, and education. Nearly 97% of Americans see impaired driving by others as a major threat to themselves and their families. 1

Impairment can result from a number of things.


Well known is the fact that the consumption of alcohol can cause impairment. Perhaps less recognized is that fact that impairment rises gradually at lower levels but dramatically at higher levels of blood alcohol concentration (BAC). About half of auto fatalities involve drivers with BACs of .16 or higher. Graph: Relative risk of fatal crash as a function of BACImpairment occurs at lower levels, but it's especially important to prevent driving with high BACs.

The Best advice is don't drink and drive.

The proportion of auto accidents caused by alcohol is actually unknown. Alcohol-related crashes include accidents that are clearly not caused by alcohol; as, for example, when a driver who has been drinking is stopped at a red light and rear-ended by a sober but inattentive driver.

A more accurate estimate comes from fatal crashes. In 1999, 28% of fatally injured drivers had BACs of at least .10. Of course, alcohol was not a cause of some unknown number of these crashes. 2

In 1999, the National Highway Traffic Safety Administration estimated that 30 percent of all traffic deaths occurred in accidents in which at least one driver or nonoccupant has a BAC of .10 or higher and at least some alcohol was present in 38% of all fatal crashes. 3

This statistic is the basis of assertions that a third or more of all fatal crashes are "caused by alcohol" and the implication that none of these alcohol-related crashes would have occurred if alcohol had not been consumed. However, the Insurance Institute for Highway Safety points out that "this is incorrect and misleading because alcohol is only one of several factors that contribute to crashes involving drinking drivers. Furthermore, some fatally injured people in alcohol-related crashes are pedestrians with positive BACs, and these fatalities would occur even if every driver were sober." 4

A single death caused by alcohol consumption is one too many. Each such death is a needless tragedy that permanently traumatizes many others.


Drugs and driving may be a bigger problem than generally recognized. A New England Journal of Medicine report on drivers without alcohol in their systems who were stopped by police for reckless driving found that 45% had marijuana and 25% had cocaine in their systems. 5

A study by the he Insurance Institute for Traffic Safety of interstate tractor-trailer drivers found that 15% of all drivers had marijuana, 12% had non-prescription stimulants, 5% had prescription stimulants, 2% had cocaine, and fewer than 1% had alcohol in their systems. 6

A National Transportation Safety Board investigation of fatal truck accidents found that stimulants were the most frequently unidentified (15%) drug class among fatally injured truck drivers. 7

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

Sleep Deprivation

Drowsy driving is greatly under reported because there is no test for it, as there is for intoxication, no clear way to identify it, and many states don't even have a code for it on their vehicle accident reporting forms.

The National Highway Traffic Safety Administration (NHTSA) estimates that about 100,000 police-reported crashes annually involve drowsiness/fatigue as a principle cause. About 1,000,000 crashes annually -- one-sixth of all crashes -- are thought to be caused by driver inattention and lapses. Sleep deprivation and fatigue make such lapses of attention more likely to occur.

The National Science Foundation found that 62% of all adults surveyed in the US reported driving a car or other vehicle while they were drowsy during the previous year. Twenty-seven percent reported that they had, at some time, fallen asleep while driving. The New York State Police estimate that 30% of all fatal accidents on the New York State Thruway occur because drivers fall asleep at the wheel. Studies suggest that truck driver fatigue may contribute to at least 30 to 40% of all heavy truck accidents. 8


In addition to preventing impairment, it's important not to become distracted. Distraction is easily caused by such things as using a cell phone, eating, combing hair, or applying make-up while driving.


  • 1. America's impaired driving prevention campaign. The Peer Education, 2001, 24 (4), 4-5.
  • 2. Insurance Institute for Highway Safety. Q&A: Alcohol: General. www.highwayfafety.org/safety_facts/quanda/alcohol_general.htm
  • 3. Insurance Institute for Highway Safety. Q&A: Alcohol: General www.highwayfafety.org/safety_facts/quanda/alcohol_general.htm
  • 4. Insurance Institute for Highway Safety. Q&A: Alcohol: General. www.highwaysafety.org/safety_facts/qanda/alcohol_general.htm
  • 5. Insurance Institute for Highway Safety. Q&A: Alcohol: General. www.highwaysafety.org/safety_facts/qanda/alcohol_general.htm
  • 6. Insurance Institute for Highway Safety. Q&A:Drugs (Other than Alcohol) www.highwaysafety.org/safety_facts/quanda/drugs.htm
  • 7. Insurance Institute for Highway Safety. Q&A:Drugs (Other than Alcohol) www.highwaysafety.org/safety_facts/quanda/drugs.htm
  • 8. Facts about drowsy driving. The Peer Educator, 2000, 23(4), 9 & 14. To learn more, visit www.dui.com/whatsnew/slep.html.

filed under: Drinking and Driving