Smoking marijuana in Washington state is legal as of Thursday, but driving while high is not.
It's a fine line that police and prosecutors are hoping to walk.
"I don't feel we are in unchartered territory -- I know we are," said Seattle Deputy City Attorney Craig Sims.
Possessing pot is legal now in Washington and soon in Colorado. In 17 other states, marijuana for medical use is easily obtainable. Yet unlike alcohol, there is no national legal limit. Some states have a zero tolerance policy, while others like Colorado and California leave it up to the discretion of the arresting officer.
Yet multiple studies show marijuana can cause dizziness and slowed reaction time, affecting coordination and judgment. Drivers are more likely to drift and swerve while they're high, according to research in the Netherlands.
"First and most important is an inability or reduced ability to divide one's attention," said California Highway Patrol drug recognition expert Sgt. Tyler Eccles. "Another effect of those who drive under influence of marijuana is a reduced ability to perceive time and distance."
In a field sobriety test, Eccles said it is not uncommon in the finger-to-nose tests for those high on marijuana to touch their eye socket or forehead. Those who support legalization say, however, that pot is less debilitating than alcohol, and that pot smokers are more aware of being impaired than those who drink.
That debate was front and center this summer in Colorado, which considered adopting a standard of five nanograms per milliliter of blood -- above that would have qualified as DUI-D, or driving while under the influence of drugs.
There are different types of tests for marijuana -- most involve THC, the ingredient that makes one high. The National Highway Traffic Safety Administration says peak THC concentrations are reached during the act of smoking, and within three hours they generally fall to less than 5 nanograms.
However, researchers say THC is stored in body fat and in heavy pot smokers remains detectable weeks after use. The pot lobby successfully argued in Colorado that innocent people could be found guilty based on prior use, and yet show no signs of impairment.
The legislature there agreed and defeated the bill. However, in Washington state, advocates knew the ballot measure would only pass with voters -- especially those in conservative eastern Washington -- with the 5-nanogram standard.
"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," said Sims. "There absolutely needs to be some public education campaign to educate John Q citizen on the effects of marijuana."
So how much pot does it take to reach five nanograms? Police say it is impossible to know since THC concentrations vary based on where and how the marijuana is grown, and how much is consumed.
"People always want to know how much does it take to reach five nanograms. That is nearly impossible to determine," said Eccles. "For anyone to say two hits or dosages get me to five nanograms, it is nearly impossible to make that determination."
Practically speaking, in Washington, police will still have to observe signs of impaired driving before pulling someone over.
Drivers will likely undergo some type of field test. If the officer believes the driver is impaired, they would be taken to a hospital where blood would be drawn. Tests above five nanograms would automatically subject the driver to a DUI conviction. With no priors, they may receive a day in jail and a fine.
William La Jeunesse joined FOX News Channel (FNC) in March 1998 and currently serves as a Los Angeles-based correspondent.