It's well known that drunk driving can have fatal consequences, but a new study suggests that alcohol is not the only drug that's a danger on the road.
It might make sense that drugs like marijuana or amphetamines would impair drivers and lead to crashes. But few studies have actually looked specifically at the impact of other drugs on traffic deaths -- even as there is an increasing push to pass "drugged driving" laws nationally. In the new study, reported in the July issue of the Journal of Studies on Alcohol and Drugs, researchers found that of U.S. drivers who died in a crash, about 25% tested positive for drugs. The most common drugs were marijuana and stimulants, including cocaine and amphetamines, which each accounted for almost one quarter of the positive tests.
It's not clear whether the drugs were to blame for the crashes, the researchers say. Some people who use illegal drugs may simply be reckless drivers in general, for instance.
On the other hand, a recent government study found that of U.S. drivers who were randomly pulled over, 14% tested positive for drugs. The fact that drug use was almost twice as high among drivers in fatal crashes suggests that drugs do contribute to road deaths.
"The suspicion is there, because when you look at drivers who've been in fatal crashes, the percentage using drugs is a good deal higher," said study co-author Robert B. Voas, Ph.D., of the Pacific Institute for Research and Evaluation in Calverton, Maryland.
For the general public, the message is simple.
"Don't drink or don't consume drugs when you're going to drive," said Eduardo Romano, Ph.D., the lead author on the study.
The issue is more complicated, though, when it comes to lawmaking, according to the researchers.
With alcohol, drivers' blood levels can be easily tested, and because studies have found that levels above a certain limit -- .08% -- impair driving, that blood alcohol concentration is the legal limit in all U.S. states. With other drugs, however, there are no agreed-upon levels that impair driving, and testing drivers is not straightforward. For one, certain drugs can linger in the body for days or weeks after they are used. Right now, states differ in how they tackle drugged driving. More than a dozen have drugged-driving "per se" laws. In most states, that means "zero tolerance" for any detectable amount of certain drugs in a driver's blood or urine. The specific drugs that are prohibited vary by state.
Last year, the White House announced that it would be encouraging more states to adopt drugged-driving per se laws.
So it is important, Romano and Voas say, for researchers to keep studying how various drugs might impair drivers.
The current findings are based on a government reporting system that collects data on all U.S. traffic deaths. All states report drivers' blood alcohol levels, whereas 20 test for drugs.
Between 1998 and 2009, there were more than 44,000 fatally injured drivers with drug-test information -- one quarter of whom tested positive for drugs. Marijuana and stimulant drugs including cocaine and methamphetamine were the most commonly implicated.
It turned it out that stimulants were linked to all types of crash fatalities -- whether from speeding, failure to obey other traffic laws, inattention, or forgoing seatbelts. Marijuana, on the other hand, was tied only to speeding and seatbelt non-use. That lays out the possibility that stimulants are particularly impairing, but that's not yet clear, the researchers say.
Whatever the effects of different drugs, alcohol still appears to be the biggest roadway hazard.
This study found that, in general, other drugs seemed to be key only when drivers had not been drinking as well. That is, when someone drinks and does drugs, the alcohol is the main reason for impaired driving.
"Alcohol is still the largest contributor to fatal crashes," Romano said.
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