WESTMINSTER, Colo. (CBS4) – Prosecutors around Colorado say they are concerned after more and more arrests for driving under the influence of marijuana are not leading to convictions when they go to trial.

When Westminster police stopped Melanie Brinegar last June for an expired plate they eventually charged her with driving under the influence of marijuana. The case seemed like a slam dunk since she agreed she had smoked pot that morning, just before driving.

“They asked, ‘were you high?’ I said ‘no,’ I was medicating,” explained Brinegar .

A blood test showed she was almost four times over the state’s 5 ng/ml THC limit. Officers said she performed poorly on roadside tests. But even with all that, a jury acquitted the medical marijuana patient who claims she “drives better” and is “able to focus” after smoking marijuana.

Brinegar’s case is one of a growing number statewide in which juries have acquitted marijuana users of driving under the influence of marijuana, even when scientific testing shows they are over the 5 ng/ml limit.

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Tom Raynes, a former prosecutor who now heads the Colorado District Attorneys’ Council, says Colorado prosecutors are becoming increasingly frustrated juries are letting marijuana users off the hook.

“You are putting lives in danger,” said Raynes. “I want the message to be understood. It’s about driving while under the influence of drugs — it’s not about recreational or medical, it’s about being impaired when you drive.”

Now both prosecutors and defense attorneys are conducting “Green Lab” exercises to help educate the public about marijuana and driving. The demonstration tests people after they’ve ingested marijuana to see how they perform on roadside sobriety tests.

The Green Lab is modeled after the “Wet Lab” which helps police identify drunk drivers.

Attorneys say it is a given that if you are over the legal limit for alcohol, you’ll be found guilty. But it’s now becoming apparent that being over the limit for marijuana is not being viewed the same way by juries. The 5 ng/ml limit is only one factor juries are considering.

Brad Wood served as foreman on Brinegar’s jury,

“The law allows you to infer that the person was impaired if they have over 5 ng/ml. But you may also feel free not to infer that and in any case use all the evidence to make your judgement,” Wood

He says Brinegar’s defense attorney, Colin McCallin, highlighted this difference from alcohol on a poster board that hung in the courtroom.

Wood thinks the law was poorly written.

“If the law says we strongly encourage you to weigh this as the biggest factor, I think it would have been a whole different story.”

Wood said the jury believed Brinegar when she testified, “When I smoke I don’t get high.”

So while the THC level in her blood was over the state limit, she says that doesn’t mean her driving was affected and police did not witness her driving erratically.

“If the officer said, ‘We saw her weave,’ it probably would have been a different story,” said Wood.

Wood said jurors tried doing roadside maneuvers on their own and some completely sober jurors failed. So they concluded Brinegar may have been legally high, but wasn’t really impaired.

“Are we sending a message it’s okay to smoke and drive?” asked Wood, “I don’t like that message. In her case maybe its fine.”

That’s a message driving prosecutors to seek more information on how to convince juries to convict drivers who have smoked marijuana.

“I don’t believe anyone can drive better under the influence of any substance,” emphasized Raynes.

Brinegar’s lawyer warns this defense won’t work for everyone. He says a different jury may come to a different conclusion and marijuana users run the risk of arrest every time they choose to smoke and drive.

Mark Ackerman is a Special Projects Producer at CBS4. Follow him on Twitter @ackermanmark

CBS4 Investigator Brian Maass has been with the station more than 30 years uncovering waste, fraud and corruption. Follow him on Twitter @Briancbs4.