Among the unanswered questions is: Would drivers who are ‘high’ travel too fast or too slow for safety?

Congressman John Mica, a Florida Republican, convened a hearing Thursday of a Transportation subcommittee. He says he’s worried that growing numbers of drivers are impaired, increasingly with a mix of drugs and alcohol. But he says it’s nearly impossible to gauge the danger. Unlike with alcohol, there there’s no way to quickly test drivers.

Congressman Gerald Connolly, a Virginia Democrat, says it’s just as possible that pot doesn’t pose as much a risk as alcohol. He says drivers who are high on marijuana may even drive slower.