Coloradans, recently empowered to smoke weed legally, may soon face charges of driving under the influence while high — and while there remains a loophole as to whether their driving was actually impaired, newfound smokers may be more at risk on the road and with the law.

On Tuesday night, the state's House Judiciary Committee sent the measure, House Bill 13-1114, to the full legislature for consideration. In addition to revoking a standard which allowed for "habitual users" of controlled substances to be charged with DUI (whether or not they were under the influence), the bill adds a legal limit of 5 nanograms of THC, the intoxicating component of marijuana, per milliliter of blood.

As Mother Jones notes, that level of THC was found by the National Institutes of Health to cause crashes at several times the rate of non-high drivers. Moreover:

Cannabis smoking increases lane weaving and impaired cognitive function. Critical-tracking tests, reaction times, divided-attention tasks, and lane-position variability all show cannabis-induced impairment. … Evidence suggests recent smoking and/or blood THC concentrations 2-5 ng/mL are associated with substantial driving impairment, particularly in occasional smokers.

As that caveat at the end suggests, impairment based on THC blood-level is less clear-cut with pot than with alcohol. The Huffington Post notes a test conducted by the Denver alternative paper Westword, in which a reporter smoked pot, got a night of sleep, and 15 hours later had blood-level THC higher than the legal limit. To work around situations like this — particularly given the state's new crop of legally licensed medicinal users — the new Colorado bill provides a loophole for marijuana smokers charged with DUI: they can argue that their driving wasn't impaired.

[Bill sponsor] Waller said his bill is different from previous versions because it would allow someone to rebut that they are impaired at the 5 nanogram level.

"For example, if you did not exhibit poor driving, you can put that on as evidence to say, "Look my driving was not poor...I'm not unsafe to operate a motor vehicle," Waller said.

A recent (not terribly rigorous) test conducted by a television station in Seattle suggested that even very high drivers might be hard for police to identify. In the test, three drivers were given marijuana to smoke and then let loose on a closed course. One hit a cone. All three drove more slowly than normal.

Granted, this is a sample size of three. And the one who fared worst, hitting the cone, was not what the NIH would term an "occasional smoker"; she was already high when she arrived to take the test.

The distinction, then, between Colorado outlawing illegal blood levels of THC and its existing laws banning reckless driving are subtle. "Intoxicated" drivers may not be impaired; impaired drivers, not intoxicated. Just like any driver on the road.