Three years after Colorado voted to legalize recreational marijuana, little is known about whether the state's roads are less safe - and law enforcement efforts remain largely focused on alcohol, not pot.
One glaring example: A Coloradan who pleads guilty to driving under the influence of marijuana is required to install a device on his car's ignition to measure the alcohol in his breath.
From a lack of statistics to laws that haven't been updated to disagreements over how to measure whether a person is stoned, there is a lot to be discussed and learned about marijuana and driving.
"There's a lot of gray area," said Colin McCallin, a Denver defense attorney who specializes in DUI laws.
In Colorado, DUI laws don't distinguish between drivers who are drunk on booze, high on pot or even reeling from oxycodone.
Because Colorado was the first state to legalize recreational marijuana, it has been expected to be a leader in highway enforcement. But there is work to do here and around the country, multiple experts on impaired driving said.
The entire system is based on catching drunken drivers. It has been developed and tweaked for decades by law enforcement, researchers, lawmakers and attorneys.
Colorado police officers spend 24 hours during their basic training academy learning how to detect and test for drunken drivers. And the state keeps extensive data on alcohol-related arrests, collisions and fatalities.
But the legalization of recreational marijuana in 2014 introduced a whole new set of questions. Among those are how an edible affects driving compared to smoking weed and whether a regular user can smoke more than a novice and still be in more control of a car.
It's too early to tell how legalization has affected road safety.
The Colorado State Patrol started measuring marijuana-related traffic citations in 2014, said Sgt. Rob Madden, a state patrol spokesman. That year will serve as the baseline for years to come.
"Statistically speaking, you need more than two years of data, and we don't even have two years yet," Madden said.
In Washington state - where recreational pot sales have been legal since July 2014 - state patrol officers separate DUI charges into alcohol and drug categories. From there, the office's crime lab will categorize the drug
While Colorado law enforcement does test for different drug categories, the charge falls under a general DUI category. Madden said this is because there are many labs in Colorado that do toxicology testing depending on the law enforcement agency, whereas Washington uses a central lab.
In marijuana-specific cases, the Washington department can also test whether the THC in the driver's system was active or whether it had been metabolized.
Because of the way Washington tests, the state is able to keep more in-depth statistics on impaired driving.
The state's report shows an increase in the number of drivers involved in fatal crashes who tested positive for THC between 2010 and 2014. But it also notes that more information is needed to draw conclusions about the impact of legalized recreational marijuana in the state.
The Colorado Legislature has determined that the legal limit for impairment by marijuana is 5 nanograms of THC in the blood. But that limit is a presumption only, unlike the blood-alcohol limit that affirms a driver is drunk if their blood-alcohol concentration is more than .08 percent.
That pot standard already has been rejected by a jury in at least one case involving a medical marijuana patient.