When a long-running legislative serial opens its third season at the state Capitol on Tuesday, it will start with a twist.

Two relatively new studies promise to shed more light — though not necessarily more consensus — on the debate around driving while stoned. The first, published in BMJ, the journal of the British Medical Association, shows why marijuana-impaired driving is dangerous. The second, from a University of Colorado Denver professor, questions what laws intended to crack down on stoned driving actually accomplish.

They both will become part of the plot of one of the most extended legislative soap operas going under the dome: The battle to set a limit for how high is too high to drive.

This year’s bill on the subject, House Bill 1114, gets its first committee hearing Tuesday afternoon.

“I am very hopeful that it is going to get through this year,” said state Rep. Mark Waller, a Colorado Springs Republican taking his third turn as a sponsor of the marijuana driving bill. “With the passage of Amendment 64, we need a presumptive limit on the books.”

The premise here is fairly simple: Supporters of the bill say that, to combat stoned driving, there should be a limit to how much marijuana people can have in their systems before they are presumed impaired.The idea is similar to the .08 blood-alcohol level for booze and would make it easier for prosecutors to win convictions in stoned-driving cases.

It’s currently illegal to drive stoned, but prosecutors must prove impairment in each case.

Bill sponsors this year will try again to peg the marijuana limit at 5 nanograms of THC — the psychoactive stuff in marijuana — per milliliter of blood.

But the simple premise belies the complicated debate beneath, as the two studies show.

Published last year by researchers from Canada, the report in BMJ analyzes the results of nine different studies worldwide on the impacts of heavy marijuana use and driving. And out of the disparate studies, the analysis pulls a concise conclusion.

“Acute cannabis consumption nearly doubles the risk of a collision resulting in serious injury or death,” the study states.

But a new study by CU Denver professor Daniel Rees and Montana State University professor D. Mark Anderson suggests stoned-driving limits don’t impact traffic fatalities. Rees said he and Anderson looked at fatality data from 16 states that adopted marijuana-limit laws between 1990 and 2010 and found no statistically significant difference between states that did and didn’t have such laws.

“We cut the data a bunch of different ways and the estimate just came back zero zero, zero, zero, zero,” Rees said. “… All we can conclude is that, as currently implemented, they are not effective.”

The new studies add to the ongoing debate about marijuana, impairment and whether the 5-nanogram limit will allow stoned drivers to slip free or cause sober drivers to be convicted. That debate alone tripped up three previous versions of the bill during the past two years.

One tweak to the bill this year, though, could smooth its path. Instead of making the 5-nanogram measure a “per se” limit — which would lead to virtually automatic convictions for those who test above it — this year’s bill would allow defendants greater leeway to argue that they weren’t impaired, even if they hit the 5-nanogram limit.

“We would like per se,” said Thomas Raynes, the executive director of the Colorado District Attorneys’ Council. “But it’s a step in the right direction. At this point in time, we’ll take a step in the right direction.”

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold