I-502's DUI Provision: A Look At The Facts And FAQs

One of the most controversial provisions of New Approach Washington’s I-502 is its per se DUI limit of 5 nanograms of active THC metabolite per milliliter of blood. It's a limit that some critics have dubbed “unscientific” and “draconian.” Others claim that it is not a measure of impairment and would threaten the driving rights of every medical cannabis patient in Washington State.

These are serious criticisms. So how does New Approach Washington defend its 5 nanogram provision?

When 502 was filed with the Secretary of State's office in June 2011, the New Approach Washington website listed one study to justify the 5 nanogram limit. It was a meta-study (or survey) of many dozens of currently existing studies and was authored by Grotenhermen, Leson and others. Here's what NAW said of the study at the time:

A meta-analysis of 90 experimental studies of the impact of smoked and oral marijuana on driving, and comparison with the results of a meta-analysis on alcohol and driving, suggest that a THC concentration of 5 ng/mL whole blood corresponds approximately to 0.08 BAC.

What did the study itself say? It's a lengthy study and it made many interesting observations, including this telling one:

The crash risk apparently begins to exceed that of sober drivers as THC concentrations in whole blood reach 5–10 ng/mL (corresponding to about 10–20 ng/mL in blood serum or plasma). Because recent studies involved only a few drivers with THC concentrations in that critical range, a reliable assessment of the associated crash risk is still lacking.

Translation: scientists think there's some statistical noise between active THC concentrations of 5 to 10 nanograms, but there's not enough evidence to reach firm conclusions. It's telling that NAW no longer includes the claim that the above study asserts that 5 nanograms “corresponds approximately to 0.08 BAC” on its DUI FAQ sheet.

Instead, NAW now uses the same Grotenhermen study to back its assertion on its FAQ sheet that “THC impairs driving skills, and the impairment can last for a few hours after smoking or consuming marijuana.” This, despite the fact that the same exact study states, as I quoted above, “a reliable assessment of the associated crash risk is still lacking.”

NAW's DUI FAQ sheet now leans heavily upon yet another meta-study from which the 502 group plucks a graph showing that automobile crash risk begins to double at 5 nanograms of active THC metabolite. But in using this graph, NAW is being deceptive.

The 5 ngs doubles crash risk figure touted by NAW is taken from crash data gathered in Australia. A graph of the data is reproduced on NAW's DUI FAQ sheet and the group claims “Studies of marijuana use and driving suggest that THC levels exceeding 5 ng/mL are associated with increased risk of accidents.”

Why would data from Australian drivers be used to impose DUI limits on American drivers? I don't even have a guess, but it does strike me as highly unusual to impose driving standards on an American state based upon crash results from an entirely different country with different driving standards—in Oz, they drive on the other side of the road, after all—and what is likely to be a different driving culture.

Besides, this same meta-study—which examines a dizzying range of studies—also points to many studies that show no link (or weak links) between cannabis use and accidents. NAW simply chose to pick one single study from one single section of the meta-study to buttress its case for 5 nanograms. Picking evidence in this fashion is not a good way to set public policy, especially when one of the final points of the study is this:

Overall, though, case-control and culpability studies have been inconclusive, a determination reached by several other recent reviewers. Similar disagreement has never existed in the literature on alcohol use and crash risk.

(BTW, an interesting fact that I ran across recently is that a Canadian study finds a 70 percent increase--so almost a doubling of crash risk--in the risk of traffic accidents when it is raining. Do we automatically hand drivers a DUI for driving in the rain? Do we call them impaired because they are driving in the rain? Of course not.)

NAW also uses one other study to support its 5 nanogram limit. It's a study by a National Institute on Drug Abuse researcher named Erin Karschner. In it, 25 chronic daily cannabis users were put into a locked facility and abstained from cannabis use for seven days. Their blood levels were taken at regular intervals to measure for active THC metabolite among other things. NAW uses this study to assert that “Even heavy marijuana users like medical marijuana patients should have their THC levels drop below

5 ng/mL if they wait a few hours before driving.”

But that's not an accurate representation of what the study concluded. Here's what Karschner wrote:

Substantial whole blood THC concentrations persist multiple days after drug discontinuation in heavy chronic cannabis users. It is currently unknown whether neurocognitive impairment occurs with low blood THC concentrations, and whether return to normal performance, as previously documented following extended cannabis abstinence, is accompanied by removal of residual THC in brain.

And here's another finding from the study that NAW fails to mention and it's something that should make most frequent cannabis users feel real concern:

For the first time to our knowledge, negative whole blood specimens were found interspersed between positive samples.

In other words, some of the study subjects would test positive for THC metabolite on admission to the study and then test negative a bit later—only to test positive again after that. Regardless of the reason for such fluctuations or at what nanogram level they occur, how can NAW claim measuring active THC metabolite is an appropriate measure of anything, much less impairment, when its presence varies so widely? Would jurors believe a prosecutor who said a set of fingerprints matched a suspect one day, but then didn't the next day, until they were tested a third time when they again matched? Probably not.

And, in case you were wondering, that positive-negative fluctuation has been confirmed by a second study, one I'll soon discuss.

It is clear from a hard look at NAW's defense of its 5 nanogram limit that it is playing loose with the facts and cherry-picking studies in order to justify this provision. Is that any way to appropriately set public policy?