MARIJUANA HAS 'MODERATE' EFFECT ON DRIVING, ALCOHOL BIGGEST DANGER ON THE ROAD

Drug Policy Letter ** Fall 1994

The National Highway Traffic Safety Administration (NHTSA)
released two studies this year (1994) on drug use and driving.

"The Incidence and Role of Drugs in Fatally Injured Drivers" is
dated October 1992, but was released February 1994. The study found that,
out of a sample of 1,882 drivers killed in car, truck and motorcycle
accidents from 1990 to 1991, alcohol was found in just over half the cases.
By comparison, only 17.8 percent of the fatally injured drivers showed
traces of other drugs, the majority of whom also had intoxicating levels of
alcohol in their system. (A person with a blood / alcohol content (BAC)
greater than 0.1 percent is considered intoxicated in most states.) Of the
drugs that showed up in the fatally injured drivers, the most frequent was
cannabis, in 6.7 percent of the drivers. Other drugs that showed some
prevalence were cocaine (5.3 percent), benzodiazepines (2.9 percent),
amphetamines (1.9 percent) and barbiturates (1.5 percent).

In addition to measuring for drug presence, the study also
attempted to measure causal contributions to the accidents by the various
drugs. This was accomplished by conducting a "responsibility analysis"
of each accident, and comparing the responsibility rates of those with a
drug or drugs in their system with the "drug-free" control group. The
responsibility rates were as follows:
* Alcohol with a BAC greater than 0.1 percent, 93.9 percent;
* Alcohol with a BAC under 0.1 percent, 75.8 percent;
* Amphetamines, 83.3 percent;
* Drug-free, 67.7 percent;
* Benzodiazepines, 66.7 percent;
* THC, the active ingredient in marijuana, 57.9 percent; and
* Cocaine, 57.1 percent.

Even though the responsibility rates in the THC and cocaine-only
groups were lower than the drug-free group, the differences were rated as
statistically insignificant because of the low number of persons in these
categories.

The report concluded: "Althoough drugs other than alcohol were the
basic focus of this study, the dominance of alcohol in fatal crashes was
inescapable....No single drug, nor all the drugs combined, approached the
prevalence of alcohol."

The second report, "Marijuana and Actual Driving Performance,"
reached similar conclusions. Just as the previous study found that there
was no indication that marijuana by itself was a cause of fatal accidents,"
this study found that "THC's adverse effects on driving performance
appear relatively small."

The study, conducted in the Netherlands and sponsored by NHTSA,
measured the effects of marijuana on drivers on a closed road, but with
traffic, and compared the effects of a modest dose of THC to alcohol (BAC
of 0.04 percent). The report concluded that:

1) "Marijuana, when taken alone, produces a moderate degree of
driving impairment, ....manifesting itself mainly in the ability to
maintain a steady lateral position on the road."

2) "The magnitude of impairment is not exceptional in comparison
with changes produced by many medicinal drugs and alcohol,"

3) "It is not possible to conclude anything about a driver's
impairment on the basis of his / her plasma concentrations of THC," and

4) When comparing the effects of alcohol with marijuana,
marijuana's effects on driving, even at the highest dosage tested (300
micrograms / kg), never exceeded alcohol's effects at BAC's of 0.08
percent. The marijuana users also tended to overestimate the adverse
effects of the drug on their driving quality and compensated for this by
increasing effort and / or slowing down. Alcohol had the opposite effect:
Drivers under its effect tended to underestimate the adverse effects of the
drug and did not "invest compensatory effort."

As the researchers stated: "THC's effects differ qualitatively
from many other drugs, especially alcohol. For example, subjects drive
faster after drinking alcohol and slower after smoking marijuana.... This
evidence stronly suggests that alcohol encourages risky driving whereas THC
encourages greater caution, at least in experiments. Another way THC seems
to differ qualitatively from many other drugs is that the former's users
seem better able to compensate for its adverse effects while driving under
the influence."

It would be a mistake to infer from either report that it is safe
to drive under the influence of any drug.

Scott Ehlers


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