With the push to legalize marijuana surging in popularity, states want to assure the public that roads will be safe, reports McClatchy. But they face a perplexing question: How stoned is too stoned to drive?

"The answer is: Pretty damned stoned is not as dangerous as drunk," said Mark Kleiman, professor of public policy at the University of California, Los Angeles, who served as Washington state's top pot consultant.

He said Washington state has a law that's far too strict and could lead to convictions of sober drivers, with many not even knowing whether they're abiding by the law.

With no conclusive research, states are all over the map as they try to assess intoxication by measuring blood levels of THC, the main ingredient in marijuana.

"The problem is that science is lagging really far behind with drugs versus alcohol," said Diane Goldstein, a former police lieutenant and a member of Law Enforcement Against Prohibition, a pro-legalization group that opposes zero-tolerance laws.