Montpelier — Gov. Phil Scott said he wants lawmakers to pass a marijuana driver impairment law before they consider a legalization bill. But transportation officials said the kind of test that Scott wants to use doesn’t exist at this time.

As one House committee considers a bill to legalize marijuana and another one looks at plans to enact new marijuana driver impairment standards, Scott has sent a clear message to the entire House.

“I believe that we should be determining level of impairment first, before we legalize,” he said.

The House Transportation Committee is drafting a bill that would create a new level of driver impairment.

Currently, a driver is considered to be impaired if their blood alcohol level is .08 percent or higher. This bill would lower the limit to .05 percent if there was any trace of marijuana in the driver’s system.

“If you were to be stopped for erratic driving and you tested at .05 for alcohol with the presence of THC in your system, no specific level, then you would be deemed to be impaired,” Colchester Rep. Pat Brennan, the chairman of the committee, said.

But the governor doesn’t think this House plan goes far enough. He wants a test that measures the specific amount of THC in a driver’s system. THC is the psycho-active ingredient in marijuana.

Scott has other questions as well.

“Just like if you have any alcohol it doesn’t mean you’re impaired, so what is the level of THC that causes impairment?” Scott asked. “I don’t have the answer to that, I don’t think we have a good way to measure that, and that’s what I am seeking, trying to make sure that we have that in place before we move forward.”

Rep. Brennan notes that a blood test and a salvia test can measure a person’s THC level, but he said there’s very little agreement on what constitutes a level of impairment because people metabolize marijuana very differently. That’s why Brennan says it will be very hard to satisfy the governor’s request.

“I think to pin something down as to the exact content in the system doesn’t exist. The test, as I said, tests for presence, but not content,” Brennan said.

The debate over impairment is a reason for the Vermont Legislature to move very cautiously on the legalization bill, Scott said. He also wants to study how other states are dealing with this issue.

“I’m not sure what the other states are doing at this time but I think we have a perfect opportunity to find out,” the governor said.

Colorado and the state of Washington use a blood test to measure driver impairment. They set a level of 5 nanograms of THC as the “presumed” level of impairment, but this approach doesn’t have the same force of law that an alcohol test has.

There are other ways to measure impairment.

There are roughly 40 drug recognition experts in Vermont who work for local, county and state law enforcement agencies. These DREs can be dispatched to evaluate a driver after the person has been pulled over for erratic driving behavior.

They’re trained to identify all types of drug impairment, from prescription medications to marijuana. A determination of probable impairment can lead to a blood test.

Matt Simon, the director of the Marijuana Policy Project, a group that’s working to legalize pot in Vermont, said he thinks this system of expert evaluation is the best way to deal with the impairment question.

“Successful prosecutions are today and should be in the future based on observation of a driver actually being impaired and being able to demonstrate that impairment in court,” Simon said.