Last Wednesday, ostensibly seeking to illuminate the issue of "how high is too high to drive," the Fox affiliate in Denver, KDVR (a.k.a. Fox 31), aired a report implying that setting the DUI limit for marijuana at five nanograms of THC per milliliter of blood would be too generous. "We wanted to find out if the five-nanogram law would be fair for Colorado drivers," says Fox 31 correspondent Mark Meredith. So the station conducted an experiment involving four medical marijuana users, a cop, and a driving simulator. "The results?" says Meredith. "While some drivers could barely handle our simulator, under the previously proposed law some of those drivers actually would have been considered safe to drive because their blood didn't show any nanograms [of THC]."

That story, especially timely now that the state legislature is mulling how to regulate the driving of newly legal pot smokers, was based on material recycled from reports that aired on May 16 and May 17, describing what an anchorwoman back then called "the shocking results of our one-of-a-kind investigation." One of the medical marijuana users who participated in that investigation, Max Montrose, claims in a YouTube video he posted last week that the simulator tests were rigged, and today Fox 31 posted a response to his criticism. But after you piece together the information from the three reports, it looks like the real problem is that Meredith misrepresents what the station actually found. Rather than showing what a menace pot-smoking drivers are, the experiment suggests (consistent with research by actual scientists) that they pose less of threat to public safety than drunk drivers do and that a per se standard is a poor measure of impairment.

Fox 31 recruited four medical marijuana users who took a driving simulation test under the watchful eye of Officer Mark Ashby, a "drug recognition expert" with the Thornton, Colorado, police department. They drove the simulator before and after consuming marijuana, and their blood was tested in both conditions. At the beginning of the experiment, two of the subjects, Montrose and a guy identified only as Justin, already tested above five nanograms, "even though they hadn't smoked or ingested marijuana all day." Those results reinforce the point that the five-nanogram limit could prohibit regular users from driving even when they have not consumed marijuana recently enough to be impaired. Montrose, whose THC level rose from six nanograms in the first test to 32 in the second, did "significantly worse" the second time around, according to Ashby. But Justin, whose THC level rose from 21 nanograms to 47, did fine in both tests. "Justin is doing pretty well," Ashby says while watching 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."

In his May 16 and December 5 reports, Meredith does not mention that Justin drove competently despite a THC level suggesting he was high as a kite. Instead he describes Justin's performance in the May 17 report, which focuses on a comparison between Justin and Brian, a volunteer drinker who bombs his second test, when his blood alcohol concentration is a bit more than 0.08 percent (the legal limit). "It's clear after just a few minutes that Brian's drinking is taking a heavy toll," Meredith says.

In the May 16 report, a third medical marijuana user, Robert, goes from 1.9 nanograms in the first test to 10 in the second, and his performance worsens. "He's driving about 15 miles, 20 miles under the speed limit," says Ashby. "He's stopping before the stop lines, 10, 15 feet." The fourth subject, Fran, presents the biggest puzzle. Her performance on the second test sounds terrible, even though there is no detectable amount of THC in her blood at that point. Unlike the other subjects, she swallowed two capsules containing cannabis instead of smoking it. She did so about an hour before the second test, so it's possible, depending on what else was in her stomach, that it had not been absorbed yet. But then why did her test performance get worse? "She showed a lot of mental impairment, not only just the physical impairment," says Ashby. "She was stopping for no reason in the roadway." He says he would have pulled her over.

Since one subject showed no impairment at 47 nanograms, while another was somehow impaired at zero, these results cast doubt on the validity of not just the five-nanogram rule but any per se standard. "Somebody who smokes on a regular basis for a year is going to be able to handle more than somebody who doesn't," observes the phlebotomist who drew and tested the subjects' blood in the May 16 report. "I think there should be another way to base it on, not on five nanograms, because [marijuana] affects everybody so differently." Or as Meredith puts it toward the end of the same report, "That question really does remain unanswered tonight: Is there a fair standard that would legally show somebody who is impaired while under the influence of marijuana?"

Meredith's report last week gave quite a different impression. He does not mention that Justin drove fine at a THC level more than nine times the proposed legal limit, and he seems to transform a single anomalous subject, Fran, who drove poorly at zero nanograms, into "some drivers" who "could barely handle our simulator" even though they "would have been considered safe to drive" under a five-nanogram rule. It is hard to see how any per se standard could guard against a terrible driver with no THC in her blood. Maybe the limit should be a negative number?

In its response to Montrose's charges, Fox 31 claims it was not trying to scare people about the danger posed by pot smokers who meet the five-nanogram test. "In fact," it says, "at the 6:30 mark of Montrose's report, our Mark Meredith points out that 'some of the participants passed with flying colors' even though they were under the influence of marijuana, as defined by the previously-proposed state statute." Here is what Meredith actually says at that point in the video:

None of our volunteers put much faith in the simulator, but can you put faith in the delta-9 [THC] law when some of the volunteers who admitted they were under the influence would pass with flying colors?

In other words, drivers who were admittedly impaired during the simulator experiment nevertheless tested below the five-nanogram limit. Again, Meredith seems to have poor Fran in mind, although he speaks in the plural. In any case, he said exactly the opposite of what Fox 31 now claims.

[Thanks to Rob Collister for the tip.]