WASHINGTON — A new roadside survey by the National Highway Traffic Safety Administration confirms a continuing decline in the percentage of legally intoxicated vehicle drivers over the last three decades.

In 1973, 7.5 percent of drivers had a blood alcohol concentration (BAC) of .08 or higher. In the latest survey, that figure had fallen to 2.2 percent.

The survey found 16.3 percent of nighttime weekend drivers were drug positive. The survey focused on weekend nighttime drivers and found that the drugs used most commonly by drivers were: marijuana (8.6 percent); cocaine (3.9 percent); and over-the-counter and prescription drugs (3.9 percent).

Motorcycle riders were more than twice as likely as passenger vehicle drivers to be drunk (5.6 percent compared with 2.3 percent). Pickup truck drivers were the next most likely to have illegal BACs (3.3 percent).

Truck drivers are routinely tested for drug use in the U.S., indicating the likelihood that the majority of drug positive results come from four-wheel drivers.

Transportation Secretary Ray LaHood said he is concerned about the prevalence of drivers who use drugs.

"I’m pleased to see that our battle against drunk driving is succeeding," said Secretary LaHood. "However, alcohol still kills 13,000 people a year on our roads and we must continue to be vigilant in our efforts to prevent drunk driving."

NHTSA is conducting further research to assess how drug traces correspond to driver impairment since some drugs can remain in the body for days or even weeks.

Should further research indicate that drugs pose the same type of traffic safety risk as alcohol, NHTSA is committed to applying lessons learned in fighting the drunk driving problem.

Among the findings of the latest roadside survey are these: The percentage of male drivers with illegal BAC levels was 42 percent higher than the percentage of alcohol-impaired female drivers.

Drivers were more likely to be illegally drunk during late nighttime hours (1 a.m. to 3 a.m.) than during daytime or early evening hours.