Is the stoned driver ‘safer’ than the ‘drunk’ one?

What’s the difference between alcohol and marijuana?

“If you’re drunk, you run red lights. If you are stoned, you stop at green lights,” says Dr. Kevin Hill, an assistant professor of psychiatry and Harvard Medical School.

The question is becoming more relevant as more states consider legalizing marijuana. In Massachusetts, however, a driver apparently slammed into a state police cruiser, an hour after visiting a medical marijuana dispensary. The state trooper was killed, the Boston Globe says.

With medical marijuana legal now in Minnesota, has carnage on the road, presently caused by alcohol, multiplied with stoned drivers? It’s too soon to say.

And funny thing about that; scientists who study the brain can’t seem to agree on whether it impairs drivers the way booze does.

Hill says while too much alcohol seems to make drivers commit “errors of omission,” marijuana users make “errors of commission,” such as driving too slowly.

Washington state, which began recreational-marijuana sales nearly two years ago, tracks THC-involved accidents and found that fatal accidents involving “marijuana-positive” drivers climbed from 58 in 2013, the year before recreational sales were allowed, to 92 last year, according to preliminary data.

Authorities in Washington caution that the state’s blood testing for THC does not distinguish between active levels and what’s known as “carboxy THC,” the chemical that lingers long after the marijuana high has dissipated.

“As such,” a report from the state concluded, “the actual impairment, or contributing crash effect by marijuana, if any, is unknown.”

Marijuana stays in the blood for days; alcohol does not. But while police have a test to determine the impairment from alcohol, no such test exists for determining whether a driver is stoned.

“If I used a lot of marijuana Tuesday night, I may still have a blood level above that threshold on Thursday, but I may not be impaired to drive,” Hill said.

But the impact of use on teenagers might accelerate, another researcher says. “It’s divided attention, for kids who already have less ability to inhibit the things that get their attention, [marijuana] makes it more dangerous for them,” said Marisa Silveri, an associate professor of psychiatry at Harvard Medical School.

The National Institute on Drug Abuse says a large study from the National Highway Traffic Safety Administration (NHTSA) found no significant increased crash risk attributable to cannabis after controlling for drivers’ age, gender, race, and presence of alcohol.

The study from researchers in Iowa focused on three driving habits: weaving within the lane, the number of times the car left the lane, and the speed of the weaving, says.

While alcohol had an effect on the number of times the car left the lane and the speed of the weaving, marijuana did not. Marijuana did show an increase in weaving. Drivers with blood concentrations of 13.1 ug/L THC, the psychoactive ingredient in cannabis, showed increase weaving that was similar to those with a .08 breath alcohol concentration, the legal limit in most states. For reference, 13.1 ug/L THC is more than twice the 5 ug/L numeric limit in Washington and Colorado.

The researchers said many people who get stoned often have a few beers first. That combination is more lethal, they said, because it creates a more intense high and delayed the peak of alcohol impairment.