The Colorado Senate today gave initial approval to a bill making it easier to convict people of driving while stoned, in the toughest test yet for the proposal.

The approval came by the slimmest of margins and only after the Senate had appeared to kill the bill on a voice vote. Using a procedural maneuver, bill sponsor Sen. Steve King, R-Grand Junction, revived the bill for a recorded vote. The bill passed that vote 18-17.

The bill still needs another, recorded vote in the Senate before going over to the House, where it will need at least another four OKs before it can go to the governor.

The measure, Senate Bill 117, would set a limit of THC — the psychoactive chemical in marijuana — in the blood above which it would be illegal to drive. King said numerous studies suggest that the large majority of people with more than 5 nanograms of THC per milliliter of blood are impaired.

King said the bill is needed to stem what appears to be an increase in stoned driving in Colorado. Drivers whose blood tested positive for THC at the state toxicology lab have increased from a couple hundred in 2009 to more than 1,000 last year, King said.

“With this drastic increase, we are well on our way to a doped driving epidemic that will match the DUI epidemic we had 10-15 years ago,” King said.

Opponents say that research isn’t conclusive that everybody is stoned at 5 ng and that the bill would result in sober drivers being convicted. Sen. Morgan Carroll, D-Aurora, called the bill, “a shortcut on burden of proof.” Sen. Pat Steadman, D-Denver, said state law already makes it illegal to drive while stoned — including for those drivers who are impaired at less than 5 ng.

“I would prefer to stick with current law, where the question of impairment is put to a jury and where evidence of someone’s conduct is presented in court,” Steadman said.

Steadman said the bill would hurt medical-marijuana patients who regularly use marijuana and may have higher baseline levels of THC in their blood.

But King said the bill sends an important message that driving high is not OK.

“What I’m saying is, you can’t get high and drive,” King said. “It has an impact on the rest of us. You can smoke and wait. You can smoke and walk. You can smoke and find a ride. But you cannot smoke and drive.”

Fourteen other states have laws creating a THC limit for driving — laws that are known as “per se” laws. Several other states have zero-tolerance driving laws for THC.

Today’s approval in the Senate is significant because a nearly identical proposal failed at the same step last year after first clearing the House easily. While the Colorado Senate is controlled by Democrats and the House is controlled by Republicans, party lines have been unreliable in determining voting patterns on the bill. Today’s vote in the Senate split party caucuses.

John Ingold: 303-954-1068 or jingold@denverpost.com