If you’re a regular marijuana user, how do you know when you’ve ingested too much to drive? You probably judge it by how you feel, since everyone’s tolerance is different, and there are no hard-and-fast rules about how long you need to wait between smoking a joint and getting behind the wheel.

That’s a big problem. There’s simply no clear way to determine — even with chemical tests — when someone has enough tetrahydrocannabinol (THC) in their system to have their senses and reaction times impaired.

That won’t stop the police from trying, however. If you’re accused of driving under the influence of marijuana, the police may decide to test you for THC. The results of those tests, however, can be wildly inaccurate. Lag times between when you’re taken into custody and when you’re tested can give you a false negative when you’re a light user — or the levels of THC in your system could read artificially high if you’re a regular user.

Much of the time, DUI cases involving marijuana rely heavily on an officer’s testimony about how you were driving, how you behaved during the traffic stop and what you said after your arrest. That’s why it’s very important for marijuana users to understand their right to remain silent during a traffic stop.

It’s also generally wise to decline to participate in roadside sobriety tests (such as the “walk-and-turn” test or the horizontal gaze nystagmus test). These can be difficult to do correctly when you’re under stress in the first place. Talking to the police or participating in voluntary testing can only end up giving the prosecutor on-camera evidence that could be used against you.

If you’re facing criminal charges related to drug use and driving, make no mistake: The situation is serious. That’s why it’s wise to talk with an experienced attorney.