In May, anticipating an increase in marijuana consumption now that the plant is legal for general use in Colorado, the state legislature officially defined "too stoned to drive." The new law allows juries to convict someone of driving under the influence of a drug (DUID) based on nothing more than a test indicating a THC level of at least five nanograms per milliliter of blood.

Addy Norton is a walking (and driving) refutation of that standard. Norton, a 27-year-old who smokes medical marijuana every day, recently participated in an experiment conducted by KIRO, the CBS station in Seattle, where a five-nanogram rule also prevails as a result of I-502, Washington's marijuana legalization initiative. Norton arrived with a THC level of 16 nanograms, more than three times Colorado's new DUID cutoff, but nevertheless completed a driving course satisfactorily, according to the instructor who accompanied her with his foot hovering over a second brake and his hand ready to take the wheel.

After Norton smoked three-tenths of a gram, she tested at 36.7 nanograms, more than seven times the legal limit, but still drove OK. Even after she consumed nine-tenths of a gram, a drug recognition expert from the Thurston County Sheriff's Office said her driving was merely "borderline." Only after consuming a total of 1.4 grams of pot and achieving a THC level of 58.8 nanograms, almost 12 times the legal limit, was Norton clearly too stoned to drive.

The two other participants in KIRO's study, a weekend smoker and an occasional smoker, also drove competently at THC levels far above five nanograms. So did "Justin," a medical marijuana user who volunteered for a study sponsored by KDVR, the Fox affiliate in Denver. Justin's THC level was already 21 nanograms when he arrived, even though he had not consumed any marijuana that day, and it rose to 47 after he smoked some pot. He performed fine on a driving simulator at both levels. "Justin is doing pretty well," a drug recognition expert from the Thornton, Colorado, police department said as he watched the second test. "He's being a safe driver. It's doubtful that I would have pulled him over. He hasn't shown any degree of impairment."

While some people may be noticeably impaired at five nanograms, it is clear that some people are not. The way people react to marijuana is partly a function of pre-existing characteristics, but it also depends on experience. Regular users develop tolerance and learn to adapt to marijuana's effects, so they may be able to drive safely at THC levels that would incapacitate novices.

Because THC is absorbed by fatty tissue, it can linger in the blood of regular users long after the drug's effects have worn off, meaning that daily smokers may never fall below the five-nanogram threshold. "Patients like me," Teri Robnett of the Cannabis Patient Action Network told Colorado legislators in April, "will continually maintain a blood level far above five nanograms and without any impairment." A five-nanogram standard exposes people like Robnett to the risk of being treated as a public menace whenever they get behind the wheel.

Recognizing that a five-nanogram THC level is not a good indicator of impairment, the Colorado General Assembly rejected that standard five times before finally approving it in May, the same week it passed a law regulating pot stores. "It's an arbitrary limit by uninformed people," complains Kayvan Khalatbari, co-owner of a medical marijuana center in Denver. "They just don't know what they're talking about."

The inclusion of a five-nanogram standard in I-502, the Washington initiative, was so controversial that it turned some legalization advocates against the measure. Alison Holcomb, who oversaw the I-502 campaign, points out that police still need "reasonable suspicion" that a driver is impaired to pull him over and seek a blood test. Holcomb says pro-legalization critics of I-502 were never able to cite cases where drivers who tested above five nanograms were acquitted under the old legal standard, which as in Colorado required evidence of impairment, including but not limited to blood tests. She also argues that a five-nanogram standard could help defendants with lower levels who otherwise might have been convicted. She says jurors "might be skeptical that the person was actually impaired if the results were not at that threshold level."

In a concession to critics of the five-nanogram cutoff, Colorado's new law, unlike Washington's, does not make people automatically guilty of DUID if they test above that level. Instead it creates a presumption that defendants can try to rebut by presenting evidence that they were not in fact impaired. But Denver attorney Rob Corry, who frequently represents DUID defendants, thinks that opportunity will not make much difference in practice. With a "permissible inference" of DUID at five nanograms, he says, "A person coming into court is guilty until proven innocent. If you put a number on it, juries are going to latch onto that five-nanogram number, whether it's a permissible inference or a per se [standard], and the effect will be that innocent people are convicted."