The federal government has officially passed Bill C-46 to crack down on drug-impaired driving, but a Winnipeg defence lawyer says the new rules miss the mark.

The bill gives police the power to do roadside screening tests and creates new criminal offences for driving with a blood-drug concentration equal to or higher than the permitted levels.

Defence lawyer Michael Dyck said it's possible someone may be over the THC limit but may not be displaying or showing signs of impairment.

"There seems to be consensus about impairment levels with alcohol consumption but there isn't that same consensus with consumption of marijuana and the amount of THC that you have in your blood stream and how that correlates necessarily to impairment and it might not be dangerous for them to be operating a motor vehicle,” said Dyck. “Yet, if they’re over the limit it becomes criminalized.”

“If people are charged with impaired driving for marijuana, that’s the debate, it’s whether it’s just consumption or if they’re actually feeling the effects and if it’s unsafe for them to be driving because they’re impaired. If a person is over the limit but there isn’t a sign that it’s connected to public safety in some way and they can drive and operate a vehicle normally that’s the type of law that may be challenged in court.”

Bill C-46 gives the Governor in Council the power to establish blood-drug concentrations.

The federal government has previously proposed driving with more 2 nanograms of THC per 1 millilitre of blood would result in a fine of up to $1000, while the penalty for driving with more than 5 nanograms of THC in your system would be the same as alcohol-impaired driving.

Bill Blair, Parliamentary Secretary to the Minister of Justice told CTV News it’s no surprise lawyers will challenge the law.

“Of course they are that’s their job but at the same time I’m very, very confident in the constitutionality and the effectiveness of the legislation that we brought forward,” said Blair. “The passage of Bill C-46 and its royal assent this morning now creates the opportunity for the attorney general to approve devices and we’re confident in the not very distant future we’ll be able to get those devices out to the police.”

Denise Elias, the president of the Winnipeg chapter of Mothers Against Drunk Driving said the organization supports the new measures in Bill C-46.