Marijuana affects motor ability: reaction times are slower so people can’t respond quickly enough to a dangerous situation.

Ask an expert: how will legalizing marijuana affect road safety?

Q: If marijuana is legalized, does that mean it’s safe to drive after using it?

A: No, just because it’s legal doesn’t mean you can drive after using. It is illegal to drive if you are impaired by any drug. That includes cannabis and also other prescription medications that can cause impairment. Even if someone is using marijuana for medical reasons, they may be considered legally impaired, just as they would be if they took prescription drugs like opiates or sleeping pills or even some over-the-counter antihistamines.

Q: How long should a person wait to drive after using marijuana?

A: I think a safe approach is that if you feel any effects at all you shouldn’t be driving. After you’ve smoked a joint, you should probably wait about four hours. It should have passed out of your system by that point.  However if you’re eating it—taking an edible—the effects are delayed, and they last longer. In that case, you should wait at least eight hours before you drive. Even if you feel okay to drive sooner, your ability to assess your own impairment level is itself impaired whether you are high or drunk. We regularly see people involved in accidents who felt they were fine to drive.

Q: How will police know if a driver is too high to drive?

A: The federal government has just announced laws that will give police new powers to administer a saliva test for THC at the roadside if drug impairment is suspected. If the saliva test indicates that the driver was using cannabis, police may call in a drug recognition expert or obtain a blood test. There will also be three new impaired-driving offences, with significant fines and penalties, for drivers with elevated levels of THC, alone or in combination with alcohol, in their blood. THC, of course is the active ingredient in cannabis.

Q: Should I be worried that more people will be driving impaired once marijuana is legalized?

A: That will depend on whether enforcement is increased and how effective public education on the risks of drug driving is. But chances are good that the rate of cannabis use will increase. That has been the experience of states that legalized marijuana. On the other hand, there is the possibility that the rate will decrease if the new drug driving laws are effective.

About the writer

Dr. Jeff Brubacher is an emergency physician at Vancouver General Hospital where he sees the impact of impaired driving every day. He is also associate professor in the University of British Columbia, Department of Emergency Medicine. Dr. Brubacher is completing a large scale study in BC trauma centres, looking at the association of cannabis with car crashes.

Dr. Jeff Brubacher 

One comment

  1. Concerned says:

    Just wondering what are the repercussions to those that DUI with any drug in their system and would they be considered enough of a deterrant. It seems that cell phone use and DUI with alcohol is still happening and the punishment does not fit the crime any more!!!!

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