As more states legalize medicinal and recreational marijuana use, the conversation about how to manage Driving Under the Influence (DUI) laws has become increasingly prevalent– in legal, law enforcement, prosecution, and government circles, amongst others.

There is no presumptive legal limit for THC levels in connection with driving under the influence in most states at this time. That may change quickly in the wake of a fatal collision in Albemarle County, Virginia, in which the truck driver who collided with a train, killing his passenger, was just acquitted.

The County’s attorney, Robert Tracci, is pushing for a presumptive legal limit to address both the safety issue posed by impaired drivers and the evidentiary roadblock to getting convictions in serious cases like this one.

The truck driver prosecuted for the collision, in which one passenger was killed and the other sustained permanent injuries, tested positive for THC, but the judge would not allow the jury to hear testimony from a toxicologist about whether he was impaired.

The challenge for lawmakers, judges, law enforcement, and prosecutors is the questionable reliability of determining impairment based on THC blood levels. The difficulty in connecting THC levels to degree of impairment is recognized by some surprising groups, including the National Highway Traffic Safety Administration, which has found that the level of THC in the blood and the degree of impairment do not appear to be closely related, and the American Automobile Association, which has taken a position against legal limits for THC, citing lack of scientific evidence.

Still 18 states have a zero tolerance for driving with any drugs in the blood, three states have a zero tolerance for THC, seven states have per se limits for THC, and one state, Colorado, has a “reasonable inference” law applicable to driving with THC. Many more states are considering some more definitive legislation on the issue.

If the unsuccessful prosecutor in the Virginia case has his way, Virginia will join those states with a presumptive limit. A similar effort failed in 2006, but many believe it’s time to try again. While it’s not possible to determine with absolute scientific precision whether a THC blood level is going to result in impairment in any given individual, “that’s true of all drugs, including alcohol,” says prosecutor Tracci.