As detailed in SF Weekly, you can be arrested "for a marijuana DUI in California if law enforcement makes the determination that you are high while driving and that [it's] impairing your driving ability."
But hundreds of drivers each year are wrongfully charged. But why?
Because there is no effective way to gauge marijuana impairment at the time a driver is pulled over.
Authorities determine drunkenness by measuring the amount of alcohol in a driver's bloodstream when the driver is pulled over.
Lawmakers have tried desperately to replicate such tests to determine if drivers are under the influence of pot while driving-but to no avail.
This is because a blood test is not the appropriate method to determine impairment by marijuana.
Unlike alcohol, marijuana stays in the body long after its effects have worn off. So it's difficult to determine if a person consumed pot 15 minutes before getting behind the wheel or 15 days.
One effort sought to establish the limit at two nanograms of THC (marijuana's active ingredient) per milliliter of blood. But researchers soon discovered that even this level could be found in individuals who hadn't used marijuana for almost a week.
The AAA Foundation for Traffic Safety has denounced such limits as "arbitrary and unsupported by science."
Another attempt, another failed method
Technological entities, like Hounds Labs Inc, have developed breathalyzers that could be used roadside to detect a driver's marijuana consumption.
But this method has become suspect as well. THC in breath only lasts for a few hours. Hound Labs even admitted that their device wouldn't be able to detect the timeframe a driver had consumed marijuana before getting behind the wheel.
It remains to be seen what technological advances will come down the pipeline.
Despite public acceptance of the use of pot-even for recreational purposes-authorities everywhere are still adamant about cracking down on such a "dangerous" substance.
There will no doubt be some device available in the years to come.
Until then, drivers remain subject to the whims of an unreliable blood test. Those who have been subject to such a test should seek legal counsel ASAP.