The answer may be a bit more complicated than you think.
In Washington state, a driver who's suspected of using pot receives a blood test that measures the level of Tetrahydrocannabinol (THC): the active ingredient in marijuana.
The test results are accurate, according to Dr. Brianna Peterson, toxicology laboratory manager with the Washington State Patrol.
"Yes, the method is fully validated just as we do for alcohol," she said.
But does that THC level mean anything?
Researchers said depending on the individual, "drivers with high levels of marijuana in their system might not be impaired, while others with low levels may be unsafe behind the wheel."
Because marijuana can affect people differently, it's difficult to come up with THC blood level guidelines.
Another challenge? Peterson says THC is only detectable in the blood for a few hours after an infrequent user consumes marijuana.
"A blood draw often occurs up to two hours after the initial incident, meaning the THC concentration has significantly decreased," she said.
Peterson said field sobriety tests are still an important way to measure impairment.
AAA recommends additional training to help law enforcement officers detect drivers who may be impaired by pot.
"THC is capable of causing impairment in all people," Peterson said.