A proposal at the state Capitol to set a limit for how stoned is too stoned to drive died this evening in the Senate.

In a crucial vote, lawmakers rejected a hard cap on the amount of THC — the psychoactive chemical in marijuana — drivers could have in their systems above which they would be presumed too high to drive. Instead, a divided Senate sided with medical-marijuana advocates, who urged more study of the proposal.

“We are being asked to make policy by anecdote,” Sen. Shawn Mitchell, R-Broomfield, said in arguing for extra research. “… Policy should be well-considered.”

With the teeth of the proposal removed, the Senate later voted to kill the bill, a decision that withstood a subsequent procedural challenge 20-15.

Sen. Steve King, a Grand Junction Republican who was one of House Bill 1261’s sponsors, said failing to set a THC limit would have real consequences. He cited instances of fatal accidents in which the at-fault drivers tested positive for THC.

“Lives are at risk here,” he said.

But Mitchell noted that some of those drivers had THC levels below the proposed limit — 5 nanograms per milliliter of blood. Echoing the concerns of a number of lawmakers, Aurora Democratic Sen. Morgan Carroll said she believes the research is inconclusive about how much THC definitively causes impairment, meaning a 5-nanogram limit might snare sober drivers while allowing stoned ones to go free.

It will still be illegal to drive while impaired by marijuana in Colorado, Carroll noted. The bill would have made it easier for prosecutors to prove a driver’s guilt.

“If you’re going to have a shortcut to presuming somebody is impaired, let’s make sure the science is established,” Carroll said.

Other lawmakers attacked the bill from another direction, arguing that any amount of THC in drivers is too much.

King responded that the 5-nanogram limit is supported by a number of studies and was vetted by multiple groups. He also called out the medical-marijuana industry for lobbying against the bill while, he said, not working as hard to discourage stoned driving.

“Why have we never seen them run an ad that says, ‘Friends don’t let friends drive high’?” King asked.

Before running aground in the Senate, the THC limit had a relatively easy time in the House, passing 51-14.

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