By Aaron Crowe
Voters in both states passed measures in November making recreational use of the drug legal for those 21 and older, though it can only be smoked in private and not in public or while driving. Driving under the influence of marijuana — just like alcohol or drugs — is still illegal.
Each state, however, has different methods to measure if a driver has smoked enough pot to be impaired, leading to some potential problems for smokers unsure when they can drive after using the drug.
“As marijuana becomes more and more available, it’s logical to assume more and more people will drive under the influence,” says Larry Mertes, a criminal defense lawyer in Boulder, Colo., who has also been a deputy district attorney there.
Each state is taking a different view on how to measure how much THC, the active ingredient in marijuana, can be in a driver’s blood before they’re illegally driving under the influence. It’s sort of like the blood-alcohol level required to be legally too drunk to drive.
Washington has a legal limit of 5 nanograms of THC per milliliter of blood, while Colorado hasn’t set a legal limit for THC, although Colorado lawmakers are studying such a limit.
Critics question the limits, the effects of driving while high, how well trained police are to test for driving under the influence of marijuana, and other legal problems such as how to legally buy, sell, grow or transport the drug that’s now legal to smoke in private.
A THC limit is meaningless because it isn’t dispersed in the body the same way alcohol is, and in fact THC levels go down after inhalation while alcohol levels in the blood increase after use, says Paul Armentano, deputy director of the National Organization for the Reform of Marijuana Laws, or NORML.
Peak levels of THC appear at inhalation, and the impairing effects of using marijuana appear 20 to 40 minutes after use, Armentano says. In other words, the maximum effects from using the drug lag the blood content levels of THC.
Because THC levels aren’t always directly linked with levels of impairment the way blood-alcohol levels are, it’s difficult for states to come up with effective and responsible standards of how much pot makes driving under the influence of marijuana dangerous, says Jeffrey Reynolds, executive director of the Long Island Council on Alcoholism and Drug Dependence.
Still, no one should drive under the influence of pot, alcohol or other drugs, Reynolds says.
“Operating a vehicle is the most complex task most of us do all day and being impaired by anything — alcohol, marijuana, prescription drugs and even cell phones — puts us and those around us at incredible risk,” he wrote in an email.
“It’s a shame that as we make progress in reducing drunk driving, we might be going backwards with the legalization of marijuana,” he said.
Effects of marijuana on driving
The impairment effects dissipate rapidly an hour after use, and in three to four hours the THC level in the blood is back to baseline, says Armentano, who has written an analysis on the effects of marijuana on driving.
But as drinking a beer won’t affect a heavier person who drinks often as much as it would an infrequent drinker, someone who smokes marijuana often won’t feel the effects the same way a first-time user will, he says. Someone who smokes a lot of pot could have stopped smoking five hours before they’re pulled over for driving under the influence and still have high THC levels, Armentano says.
While NORML’s stance is that pot smokers shouldn’t be driving while under the drug’s influence, it points out that the effects on psychomotor skills are modest and don’t last long. The effects on driving include slow reaction times, including braking later, weaving, reduced speed, and leaving more distance between their car and the car in front of them.
While alcohol drinkers may think they’re sober to drive when they’re not, marijuana smokers see themselves as being more impaired than they actually are and are more likely to drive slower, make fewer lane changes and overestimate time, Armentano says.
One in 10 adults use marijuana, he says, and they immediately know after smoking that they’re impaired and shouldn’t be driving.
“Who is going to smoke marijuana and then decide while smoking that they’re going to go for a drive?” he asks.
In Colorado, the new law has created more questions as state legislators try to determine what level of THC is needed for impairment while driving. The Colorado Department of Transportation has an “R-U-Buzzed” app to determine blood-alcohol concentration and will call a cab for the driver. It doesn’t have one for pot smokers.
Legalization also raises the question of where to buy it, Mertes says. Cities are tasked with implementing the new law, and some are starting to ban the sale of marijuana for legal use, just as cities have done in banning medicinal marijuana dispensaries in states where it’s legal.
Police tests for marijuana
If an officer pulls someone over for weaving or another sign of being under the influence, one of the first things they may ask a driver is if he’s been smoking marijuana, Mertes says. The police want the driver to admit to using the drug within a certain amount of time so they can determine he’s still impaired by it, he says.
“Almost never does it help you to talk to police,” Mertes says “Almost never does it help you to go into details.”
A roadside exam used for alcohol impairment won’t necessarily work for pot use, and a drug recognition expert may have to be called in to check for symptoms of smoking marijuana, he says.
If a blood or urine sample is taken later, any THC could have been there from smoking weeks ago, Mertes says. THC is fat soluble and a regular pot smoker can have it in their urine for up to six weeks, and can have inert THC in their blood cells.
“Everybody who’s a regular use of marijuana isn’t going to be impaired by having one joint,” he says.
The maximum penalty for driving under the influence of the drug is six months in jail and a $500 fine, along with up to two years on probation, and mandated therapy and drug counseling sessions, Mertes says.