DENVER (AP) — A marijuana blood limit for drivers was rejected Tuesday for a third time in Colorado, as lawmakers from both parties argued about how to fairly gauge whether someone is too stoned to get behind the wheel.

The bill would have made Colorado the third state in the nation with a blood-level limit for marijuana, much as the nation has a blood-alcohol limit of .08.

Currently, drugged-driving convictions depend on officer observations.

The Colorado Senate fell a single vote short on the bill setting a drivers’ blood standard for THC, the psychoactive ingredient in marijuana. The measure failed on a 17-17 tie, one vote short of the number needed to advance it.

Earlier Tuesday, the state House signed off again on the bill that would limit drivers to 5 nanograms of THC per milliliter of blood. Sponsors talked about Colorado’s rising arrest rates for people driving under the influence of drugs, as well as data from the National Highway Traffic Safety Administration showing more drivers in fatal accidents test positive for marijuana.

"It is past time to get this done," said Republican Rep. Mark Waller, sponsor of the bill.

But, marijuana activists and some lawmakers from both parties argued that the blood standard is an unfair measure of driver impairment. They pointed out that more than 90 percent of Colorado’s drugged-driving criminal cases already end in convictions, so they questioned whether the 5 nanogram limit would change behavior.

"I don’t think it’ll make our roads any safer," said Democratic Sen. Pat Steadman of Denver.

Some Republicans opposed the bill, arguing that the measure considered Tuesday should have targeted more than just marijuana use.

Opponents tried to amend the bill to exempt state-certified medical marijuana patients from the limit. The amendment failed.

"Impaired is impaired, whether you have a (medical marijuana) card or don’t have a card," argued Republican Sen. Steve King.

After the amendment failed, the entire bill collapsed. Its fate appeared to hinge on the absence Tuesday of a lone senator — Republican Sen. Nancy Spence of the Denver suburb of Centennial.