DENVER (CBS4) – A plan is in the works to set a limit for people driving while under the influence of marijuana, and this time lawmakers say they’ll get it done.

There’s a lot of pressure on lawmakers after legalizing pot. As the number of users grows, there is growing concern the number of people driving under the influence will as well. In 2011, the most recent data available, 13 percent of deadly crashes in Colorado involved pot.

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This is the third year lawmakers have tried to pass the bill, and they watered it down this time to make sure it gets through.

When it comes to alcohol, the law is clear. At .08 a person is too drunk to drive. But when it comes to marijuana, proving a person is too high to drive may be tougher.

“So we’re saying you’re presumed to be under influence of marijuana at five nanograms,” said Rep. Mark Waller, R-Colorado Springs.

Under a bill by Waller the DUI limit will be five nanograms of THC, the psychoactive ingredient in pot. But even if a driver reaches that limit, he or she could get off.

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“You can put on evidence that says you’re not under the influence when over the five nanograms,” Waller said.

Waller, a former prosecutor, admits the burden of proof will be higher for police and prosecutors. They can’t just point to the test as proof of impairment, but will need evidence like dangerous driving or slurred speech.

The bill is meant to address the concerns of medical marijuana users — people who are chronically above five nanograms, but function as sober.

“I think that’s what’s going to help it pass this year,” Waller said.

Democrats and Republicans, defense attorneys and prosecutors are on board. While marijuana advocates insist five nanograms is an arbitrary limit, Waller says it could be lower.

“It is time to act. We need to do something to protect the citizens of Colorado, and that’s exactly what this driving under influence of marijuana does,” Waller said.

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Critics say it will also lead to a lot of drivers who simply smell like pot being hauled in and given blood tests unnecessarily. Unlike alcohol, there’s no breath test for THC. Waller says police will still need probable cause, and like DUI laws involving alcohol, this one too will evolve over time.