With pot becoming legal for recreational use in Colorado and Washington, the new move poses a challenge in drafting the rules behind toking-and-driving.
Nationally, 23 percent of those killed in auto accidents have some drug other than alcohol in their system. The fear is that legalizing pot will only make things worse.
Now, pot advocates say that's not true that smokers know when they're too high to drive. Well, Washington state is taking no chances imposing a strict limit on the amount of pot drivers can have in their system.
Driving while high. While illegal, research shows more drivers are toking up. Yet studies, like this one in Europe from Maastricht University, show too much marijuana affects coordination and judgment.
California Highway Patrol Sgt. Tyler Eccles says, "One of the first and most important is an inability or reduced ability to divide ones attention."
Marijuana advocates argue pot is far less debilitating than alcohol. Yet a new Canadian study says those who drive within three hours of smoking post are twice as likely to cause a crash.
But how much is too much?
"I don't feel we are in unchartered territory, I know we are in unchartered territory," said Seattle Deputy City Attorney Craig Sims.
12 states have a zero tolerance level for pot. In Washington state, where marijuana is now legal, the limit is 5 nanograms per blood sample -- an impairment level some argue is roughly equal to alcohol.
Sims says, "Because we are early on in the research phase it is very difficult to tell if that 5 nanogram level will change, similar to how DUI standards have evolved over the years."
The problem is, heavy users, though not impaired, can test positive weeks after smoking. Also, what you smoke and how you smoke it affects people differently, so even experts don't know how much pot causes impairment.
"For anyone to say 2 hits or dosages would get me to 5 nanograms -- it is nearly impossible to make that determination," said Sims.
Now THC is the compound in pot that makes you high. THC is stored in fat, so heavy users can test positive weeks after smoking, which is why Colorado defeated the 5 nanogram standard. Bottom line, this is going to be argued and litigated. In alcohol, we started at 1-5 and came down with pot. Advocates argue it should be about 10 nanograms to reach impairment.