With pot smoking now legal in Colorado, the state is closer than ever to defining just how stoned is too stoned to drive, say legislative leaders who’ve seen similar proposals die again and again at the state Capitol.

Law enforcers want to set the legal limit at 5 nanograms of active THC per milliliter of blood and recently made a critical compromise to get that standard into the state’s DUI statute.

Under the latest draft, people caught over the THC limit could argue in court that they weren’t impaired, an opportunity not afforded booze-drinking drivers caught over the legal blood-alcohol limit.

Disagreements over whether one size truly fits all marijuana smokers when it comes to the amount of active THC needed to impair driving abilities have confounded three attempts at passing driving-under-the-influence-of-drugs bills in two years.

But newly elected House Speaker Mark Ferrandino said that allowing DUI defendants to argue their side seems to be swaying people who have opposed similar bills in the past.

And pro-pot activists are giving tentative nods to the idea.

“With Amendment 64, it’s going to be important that we clarify a lot of laws, and that is definitely one of them,” Ferrandino, D-Denver, said of the drugged-driving question. “Some of the people in my caucus who have not been supportive of the bill before do support (this proposal). It gives me some hope that we’ll have something that can be supported in a bipartisan way.”

Gov. John Hickenlooper on Monday signed a proclamation officially adding Amendment 64 to the state constitution, legalizing private pot use by most adults and allowing people to grow a small number of plants.

Buying and selling anything but medical marijuana remains illegal until the state legislature approves a regulatory framework next year.

Unlike Washington state, where voters also legalized pot in November, Colorado’s Amendment 64 included no provisions setting a cap on the amount of psychoactive THC that a driver could legally have in his or her bloodstream.

Washington set its limit at 5 nanograms per milliliter of blood. Two other states have set legal limits at 2 nanograms in criminal statutes, and several more have zero-tolerance policies.

But the science used to arrive at those limits is relatively young — and highly debated — compared with what is known about the effect of alcohol on drivers.

This proposal could give jurors and the pot-consuming public a better measuring stick of what’s safe and will also make it easier for prosecutors to win drugged-driving convictions, says House Minority Leader Mark Waller.

“It’s hugely important. If you ask any person on the street, ‘What does it mean to be driving under the influence of alcohol, they’ll talk about (blood-alcohol content). Society has come to know that standard,” said Waller, R-Colorado Springs. “There’s no standard associated with marijuana, which is becoming as prevalent as alcohol in this state.”

A person deciding when he or she has ingested too much pot to drive will likely be trickier than counting empty cocktail glasses to gauge a blood-alcohol level. Pot potencies vary depending on the strain, how the marijuana is consumed and other factors.

Waller is part of the group of police officials, prosecutors, lawmakers, defense attorneys and advocacy groups called the Commission on Criminal and Juvenile Justice that crafted the draft legislation that Waller will carry this year along with Rep. Rhonda Fields, D-Aurora, and state Sen. Steve King, R-Grand Junction.

Waller backed the decision to cut from draft legislation the so-called “per se” provision — which said anyone caught at 5 nanograms or higher was too stoned to drive, period — and instead allow pot users to present evidence that they weren’t impaired by the THC in their system.

Even those who have opposed DUI limits for marijuana in the past, such as defense attorney Sean McAllister, acknowledge that the compromise on the drugged-driving front will likely lead to the passage of a legal standard.

Still, McAllister points out that an amount of THC that would impair an infrequent smoker may have little effect on chronic users, such as medical- marijuana patients, who typically have much higher base levels of the chemical in their systems.

He would like to see that population exempted from the DUI-drugs bill, which he thinks will still lead to more DUI convictions of pot users.

“Unless you have some really good facts and experts on your side, you’re likely to be convicted anyway,” McAllister said. “There is an unfairness to convicting medical-marijuana patients who aren’t taking the drug for recreational uses.”