The Denver Post reports that Colorado legislators are moving toward a new standard for driving under the influence of marijuana now that Amendment 64 has made the plant legal for adults 21 and older to grow and consume. Under current law, DUI is defined as "driving a motor vehicle or vehicle when a person has consumed alcohol or one or more drugs, or a combination of alcohol and one or more drugs, that affects the person to a degree that the person is substantially incapable, either mentally or physically, or both mentally and physically, to exercise clear judgment, sufficient physical control, or due care in the safe operation of a vehicle." While a blood alcohol concentration of 0.08 percent or more is treated as equivalent to DUI, there is no such per se standard for marijuana. Previous efforts to set a limit at five nanograms of THC per milliliter of blood—the same as the standard established by Washington's marijuana legalization initiative, effective last Thursday—have failed due to objections from critics who said it would unfairly penalize regular consumers, especially people using marijuana for medical purposes, who might exceed the ceiling even when they are not impaired. Now the Post reports that supporters of a five-nanogram rule are offering a compromise that has won over some critics: Instead of being automatically guilty of DUI, drivers who test above the limit could present evidence that they were in fact OK to drive. Newly elected House Speaker Mark Ferrandino (D-Denver) told the paper:

With Amendment 64, it's going to be important that we clarify a lot of laws, and that is definitely one of them. Some of the people in my caucus who have not been supportive of the bill before do support [this proposal]. It gives me some hope that we'll have something that can be supported in a bipartisan way.

The Post says "even those who have opposed DUI limits for marijuana in the past...acknowledge that the compromise on the drugged-driving front will likely lead to the passage of a legal standard." But one such critic, defense attorney Sean McAllister, argues that there should be an exemption for patients because a DUI presumption at five nanograms will be hard to rebut:

Unless you have some really good facts and experts on your side, you're likely to be convicted anyway. There is an unfairness to convicting medical-marijuana patients who aren't taking the drug for recreational uses.

Critics of Washington's Initiative 502 raised similar concerns. They also objected to I-502's "zero tolerance" standard for drivers younger than 21. Given individual variation in how people respond to alcohol and other drugs, per se standards are inherently problematic. Assuming an alcohol-like per se standard for marijuana is politically necessary (as I-502's sponsors did), it is unclear whether five nanograms is a reasonable cutoff and how common it is for unimpaired drivers to exceed it. The Post notes that "the science used to arrive at those [THC] limits is relatively young—and highly debated—compared with what is known about the effect of alcohol on drivers." Alison Holcomb, director of the Yes on I-502 campaign, tells me it is certainly true that regular users can test positive for pot when they are no longer impaired, but she has never seen a study finding that "someone is safe at five nanograms."

In any case, a cutoff of zero is clearly nonsensical, regardless of the driver's age. Although the National Organization for the Reform of Marijuana Laws supported I-502, its executive director, Allen St. Pierre, said, "We fully recognize the per se DUI marijuana provisions in I-502 are arbitrary, unnecessary, and unscientific, and we argued strongly with the sponsors for provisions that would require proof of actual impairment to be shown before one could be charged with a traffic safety offense."

[Thanks to CK for the tip.]