Last year Kali Su Schram was sentenced to six months in jail because of a fatal traffic accident she did not cause, thanks to Michigan's unjust and unscientific definition of drugged driving. Schram had the right of way when a bicyclist suddenly appeared in front of her at an intersection, but she was blamed anyway because she had a detectable amount of THC in her blood.
California, where state-licensed marijuana stores began serving recreational consumers on Monday, takes a more rational approach to driving under the influence of cannabis, requiring evidence of impairment. But three of the eight states where marijuana is legal for nonmedical use have adopted versions of the Michigan model, falsely equating impairment with arbitrary levels of THC in the blood.
In Michigan any amount of THC suffices for a DUI conviction. The cutoff in Nevada, where legal recreational sales began last year, is two nanograms per milliliter, which is not quite as strict but still criminalizes driving by many marijuana users who pose no threat to the public.
Because THC is absorbed by fatty tissue, it can be detected in the blood of frequent cannabis consumers as long as a month after last use. In a 2009 study, a frequent pot smoker had a THC level of seven nanograms on the first day of abstinence, then between two and four nanograms during the next six days.
A 2015 study found that a cannabis consumer tested at or above five nanograms as long as five days after last use. Five nanograms is the legal cutoff in Washington, where any driver who hits that level is automatically guilty of DUI.
Colorado allows juries to infer impairment at a THC level of five nanograms or more but lets defendants rebut that inference. In 2015 Melanie Brinegar, a medical marijuana patient who tested at 19 nanograms after she was pulled over for driving with an expired tag, used the latter provision to win an acquittal.
Even though Brinegar's THC concentration was nearly four times the level that Colorado treats as presumptively equivalent to driving under the influence, she persuaded a jury that she wasn't. In 2013 KIRO-TV, the CBS station in Seattle, found that regular cannabis consumers could pass driving tests at even higher THC levels.
Nor does a five-nanogram rule make sense for less frequent users. According to experiments at the National Advanced Driving Simulator in Iowa City, occasional cannabis consumers with THC levels exceeding 13 nanograms, more than twice Washington's cutoff, show lane weaving similar to drinkers with a blood alcohol concentration of 0.08 percent, the usual DUI threshold.
"Per se" rules that define stoned driving based on THC in the blood appeal to politicians and the general public because they look similar to state laws that define drunk driving based on alcohol in the blood. But the latter policy, while problematic because of individual variability, has a much stronger scientific basis.
"Whereas the impairment effects for various concentration levels of alcohol in the blood or breath are well understood," the National Highway Traffic Safety Administration notes, "there is little evidence available to link concentrations of other drugs to driver performance." Hence "specific drug concentration levels cannot be reliably equated with a specific degree of driver impairment."
A 2016 report from the AAA Foundation for Traffic Safety noted that THC blood levels do not predict performance on roadside sobriety tests. It said "there is no evidence from the data collected...that any objective threshold exists that established impairment."
As Staci Hoff, research director at the Washington Traffic Safety Commission, noted in a TV interview last summer, "More and more research is coming out debunking this mythical link between THC level in the blood and level of impairment." Instead of adopting pseudoscientific DUI standards that unfairly and irrationally treat sober drivers as if they were stoned, states that legalize marijuana should follow California's example by requiring proof that people accused of endangering public safety actually did.
Jacob Sullum is a senior editor at Reason magazine. Follow him on Twitter: @jacobsullum. To find out more about Jacob Sullum and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate Web page at www.creators.com.