Massachusetts is one of several states that have made marijuana use legal for all adults 21 years of age and older. However, there are some restrictions on when and where the drug can be used. In particular, it’s still illegal to drive when you’re high — and doing so can easily lead to a charge of operating under the influence (OUI).

There’s just one problem with this rule: Unlike alcohol, marijuana can stay in someone’s system long after the psychoactive effects of the drug have worn off. Plus, individual reactions to the drug vary greatly. A frequent user, for example, may have a lot of the drug in their system and not seem impaired, while an infrequent user could become impaired after smoking or ingesting a small amount.

How, then, can a police officer tell if you’re “too high” to be behind the wheel? It isn’t easy. Here are some of the methods:

  • They may look for signs of smoke in your car or a heavy odor of the drug. Marijuana has a fairly distinctive smell. If smoke rolls out of your car when you roll down your window at a traffic stop or your clothes smell like you’ve been smoking a joint, the officer will likely suspect that you’re impaired.
  • They may look for physical signs of drug intoxication. While the drug is active, marijuana users may experience dry, reddened eyes, a lack of focus and some impairment of their motor skills. An officer might decide you’re impaired based on your actions at a traffic stop and your appearance.
  • They may try to get an admission. It’s always dangerous to answer an officer’s leading questions at a traffic stop. You may find yourself admitting — against your interests — that you smoked a joint or a bowl “hours” before, hoping that the officer will decide you’re not impaired. Instead, that may be the only information they need to arrest you.

If you’re facing OUI charges over marijuana use, find out more about your options for a defense. There are many ways to challenge what are — at best — the imperfect methods used to determine impaired driving.