WATCH | You know how much alcohol you can have in your system and still be legally allowed to drive. Marijuana doesn't work that way -- a lot of DUI laws don't reflect that.
Melanie Brinegar, a resident of Westminster, Colorado, suffers from degenerative disk disease that gives her chronic pain. She uses medical marijuana to cope and also smokes pot recreationally.
In 2014, she was pulled over for an expired license plate. Then officers smelled weed. She took roadside tests, including standing on one leg. She said she had muscle spasms, which officers interpreted as a sign she was impaired. She was arrested and charged with driving under the influence, even though police reportedly said she wasn't driving erratically.
Yet despite being almost four times over Colorado's legal limit of tetrahydrocannabinol or THC-- the main mind-altering ingredient found in the cannabis plant -- she was acquitted after proving in court she wasn't actually impaired.
Redefining DUIs gets hazy
States that have legalized or decriminalized marijuana have all handled DUIs differently. Colorado treats marijuana use like alcohol use, based on the premise that the more you drink, the more dangerous you are on the road. And similar to a blood-alcohol concentration of 0.08, which has been scientifically proven to impair drivers, the state sets the limit at 5 nanograms per milliliter of THC in the blood for pot.
Marijuana isn't so clear-cut
If you have a high enough concentration of THC in your blood in a state with a per se law, you could get charged with DUI. These laws set a numerical limit for marijuana use similar to alcohol use.
But there's not a solid scientific link between THC concentration and impairment, according to the National Highway Traffic Safety Administration, and critics of these laws have used research like this to argue against the use of numerical thresholds.
"Case-control studies are inconsistent, but suggest ... concentrations of THC higher than 5 ng/mL are associated with an increased risk of accidents."
There's some support for per se laws based on a 2009 Yale study published in the American Journal on Addictions, but the study authors admit their conclusion is "inconsistent."
By the states
On Nov. 8, California, Maine, Massachusetts and Nevada approved ballot measures to legalize recreational cannabis. Here's how some other states handle impaired driving laws.
Time for some chemistry
The main reason for the complication is that marijuana is fat-soluble, while alcohol is water-soluble, former White House drug czar Robert DuPont said. This means alcohol spreads pretty evenly throughout your body in your blood. But THC is fat-soluble, so THC concentrations spike quickly after someone takes a hit (up to 18 nanograms per milliliter of blood) and then linger for weeks afterward.
"It is inadvisable to try and predict effects based on blood THC concentrations alone."
If you smoke marijuana regularly, it'll take longer for THC to exit your system. Marijuana significantly affects driving performance for a few hours, but chronic smokers could have a THC concentration of 45 ng/mL 12 hours after smoking, according to the National Highway Traffic Safety Administration.
Here's a breakdown of how long marijuana impairs your driving, according to NHTSA, NORML and the journal Therapeutic Drug Monitoring.
Back to Brinegar
Colorado law lets juries consider other factors besides THC levels when determining impairment, which worked in Brinegar's favor. They can infer impairment if a driver's over the THC limit, but they may decide the driver was not impaired. Brinegar's THC concentration was 19 ng/mL, nearly four times Colorado's limit (which, as previously stated, is 5 ng/mL). Jurors in Brinegar's case told CBS Denver they failed the roadside tests while sober. Police also reportedly did not see Brinegar driving erratically.
Other states aren't so forgiving. Arizona drivers were not allowed to use medical marijuana cards as a defense, The Arizona Republic reports. The Arizona Supreme Court later allowed using the card as a legal defense.
There's no perfect test yet
Companies like Hound Labs have started to work on a marijuana "breathalyzer" to try to address scientific concerns.
"We need to get away from this notion that we're going to get a magic bullet detection test on the side of the road."
—Paul Armentano, deputy director of NORML
But Paul Armentano, deputy director of the National Organization for Reform of Marijuana Laws, said even a marijuana breathalyzer might not be as useful as it seems.
The science gets more complicated
As if the science wasn't complicated enough, when your body processes THC, it produces two chemicals: hydroxy-THC, which affects your brain, and carboxy-THC, which does not, but can linger in your body for months, according to the NHTSA.
However, in Arizona, Delaware, Georgia, Indiana, Oklahoma, Rhode Island, South Dakota and Utah, the presence of carboxy-THC in one's blood can lead to DUI charges.
So what can the police do?
"There are tools already at law enforcement's disposal," Armentano said. "There should be a greater focus on expanding those existing tools."
Lt. Robert Sharpe, from Washington State Patrol's impaired driving unit, said officers are told to look for impairment, whatever form it takes, in a DUI stop. In tricky cases, officers with special drug training arrive.
"I'm going to make an arrest because they're unsafe to drive. And in Washington state law, that's all I have to have."
—Lt. Robert Sharpe
Sharpe said if someone blows a .04 on a Breathalyzer, but still seems drunk, he still might have grounds for arrest, depending on what he observes from the driver.
The roadside tests for driving while high are mostly similar to alcohol DUI investigations, Sharpe said. Suspected drivers will stand on one foot, try to walk in a straight line, and take a Romberg test - trying to guess how long it takes 30 seconds to pass while balancing on one foot.
Law enforcement's strategy hasn't changed since Washington made recreational marijuana legal in 2014.
"Legalization just put a different name on it," Sharpe said. "Impairment by marijuana is impairment by marijuana."
Twelve other states have zero-tolerance laws for THC while driving, meaning any THC in a driver's blood is evidence of impairment.
- Rhode Island
- South Dakota
"The reason it's there is to make most marijuana smokers stay under it, not because it has to do with impairment."
That zero-tolerance standard is supported by DuPont, who served as White House drug czar in the 1970s and was the first director of the National Institute on Drug Abuse. He says airline pilots and commercial truck drivers have a similar standard, and they're less likely to cause crashes than non-professionals.
The court of public opinion
DuPont said per se laws set a legal threshold that can also be used in the court of public opinion.
"It gives the appearance of....having a program to deal with marijuana-impaired driving...We do that for political reasons, and I'm all in favor of that," he said.
In the meantime, Brinegar says she'll continue to use marijuana and drive. She says it saved her life.
"I was in a state where my depression took over, and I didn't want to live," she said. "I'm at a place where I'm probably the best in my life. It's all because of cannabis."
Video editing by Adam Fleishman. Fernando Hurtado contributed to this report.
CORRECTION | An earlier version of this article listed Illinois as state that listed the presence of carboxy-THC as a cause for arrest, per GHSA data. Illinois now uses a 5-nanogram per se standard to determine impairment.