Earlier this month, the Vermont legislature became the first in the nation to pass a marijuana legalization measure. Today, Gov. Phil Scott became the first state executive to veto such a bill.

According to Vermont alternative weekly Seven Days, Gov. Phil Scott "said he does not consider marijuana legalization a priority and has concerns about the lack of a roadside test to detect driver impairment."

The absence of a reliable roadside sobriety test for THC is a misleading excuse for striking down legislation that would do more to remove criminal penalties for adult possession than it would to create a marijuana market that might conceivably increase drug use. Similar to D.C.'s legalization measure, Vermont's S.22 decriminalized possession of an ounce (or less) and allowed residents to possess two mature plants; in both cases, only for adults older than 21. The bill would not have established a tax-and-regulate system, or legalized person-to-person sales. In essence, it simply removed the civil penalties the state adopted when it decriminalized possession in 2013.

As for the road safety issue: The link between THC content in the blood and impairment is very murky, and no one has spent more time exploring that issue than Reason's Jacob Sullum. In 2016, he reviewed several reports from AAA that looked at THC-related traffic incidents. One of those reports was based on THC-related traffic incident data in Washington state between 2005 and 2014 (Washington, along with Colorado, legalized marijuana in 2012). AAA reported a 30 percent increase in THC-related traffic incidents events over that period; yet 66 percent of drivers who tested positive for THC in fatal accidents also tested positive for other substances, which was also true for 73 percent of THC-positive drivers involved in non-fatal accidents. It is not possible, in such circumstances, to isolate marijuana as the cause of a traffic incident.

Even the Rocky Mountain High Intensity Drug Traffic Area concedes that a positive test for THC–which remains in the body long after the drug's impairing effects have subsidided–does "not necessarily prove that marijuana was the cause of the incident."

Another study from AAA that sought to identify a THC blood level cutoff for impairment found "no clear relationship between THC blood levels in DUI arrestees and performance on roadside sobriety tests." While people with THC in their systems performed less well than the sober control group, Sullum noted that the failure rate in the latter ranged from 33 percent to 51 percent. That "makes you wonder how accurate these tests are as measures of impairment."

The National Highway Traffic Safety Administration, meanwhile, reported in 2015 that while "the impairment effects for various concentration levels of alcohol in the blood or breath are well understood, there is little evidence available to link concentrations of other drugs to driver performance."

Scott's apparent insistence on a black-and-white measure for determing THC-related driver impairment puts Vermont legalization advocates in a tough spot: The best science can tell us if a person has THC in their body, but not whether (or how) it affected their driving. Scott is reportedly open to reconsidering the bill during a special session dedicated to vetoed legislation. If the Vermont legislature is able to negotiate on the impaired driving issue, they should be mindful of the consequences of accepting a zero-tolerance policy. Michigan has such a law, and it recently resulted in a woman who tested positive for THC being sentenced to six months in jail for an accident she did not cause.