As more states push to legalize marijuana, safety on the road could be affected.
50 percent more drivers were high on pot in 2014 than they were in 2007 according to the National Highway Traffic Safety Administration.
But unlike alcohol, marijuana is hard to measure.
The drug is a little more complex and our bodies metabolize cannabis differently, which makes difficult for lawmakers to set a legal limit.
"It's not very cut and dry. People want a black and white answer - they want to say, 'You're automatically impaired at this level' or 'you're not at this level.' And it just doesn't quite work that way with marijuana," said Mason Tvert, communications director for the Marijuana Policy Project.
THC, which stands for Tetrahydrocannabinol, is the chemical found in pot that affects the brain.
The National Institute on Drug Abuse describes THC as a mind-altering chemical, altering the user's senses, body movement, and difficulty with thinking and problem-solving.
THC levels are measured in Nanograms. States like Washington and Colorado, where pot is legal, have set 5 Nanograms as the legal limit.
Lawmakers in Illinois are pushing for a 15 Nanogram limit in their state.
"One of the big challenges is that there is no .08 equivalent for something like marijuana. We don't exactly know where that level of impairment is," said JT Griffin, chief government affairs officer with Mothers Against Drunk Driving.
The current way to test THC levels in someone's body is by taking a blood, urine or saliva test, which makes it difficult for cops to positively identify a marijuana DUI.
But now there's these new marijuana breathalyzers being developed for law enforcement use - which could help solve the problem.