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With cannabis legalization just days away, some questions remain unanswered about how legal limits on the use and home growing of marijuana will actually work.

In an interview with the West Block’s Mercedes Stephenson, Justice Minister Jody Wilson-Raybould was asked how much is too much when it comes to marijuana consumption and driving, and how the government plans to deal with potential legal challenges in provinces that restrict home growth of marijuana despite federal rules allowing it.

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She was also asked at what point someone would actually be charged with impaired driving, something Wilson-Raybould said will depend on the situation.

“It depends on a case-by-case basis,” she said.

“Police officers have the opportunity to go through a series of tests. They can conduct standard field sobriety tests. We are training drug recognition experts to be able to determine whether or not they are of the view, or see probable grounds, that someone is impaired by drugs, and can build the probable grounds to take a blood sample to determine whether someone has been impaired by drugs.”

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The regulations set by the federal government create two classes of limits that have raised concerns from impaired driving activists over whether they can adequately be measured, and whether there is a potential for public confusion over exactly how long consumers need to wait to drive after lighting up.

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Under the rules, police can lay a summary conviction charge against a person driving with between two and five nanograms of THC per millilitre of blood.

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Those who have more than five nanograms could be charged either with a summary or indictable offence. The latter is the more serious of the two and carries a required sentence if someone has racked up multiple convictions for impaired driving.

Think of the difference between summary and indictable offences as resembling those between misdemeanors and felonies south of the border.

However, what those limits translate to in terms of joints smoked or time waited between lighting up and getting behind the wheel is not clear.

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And despite the government passing legislation that gives police more powers to stop and evaluate drug-impaired drivers, it can be notoriously tricky to measure the concentration of THC in someone’s blood and determine when it came from.

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Even days after someone smoked, THC can still show up in the blood.

“I believe fundamentally there is no amount of alcohol or drugs one can consume and get behind the wheel,” said Wilson-Raybould, when asked how such limits will work. “For the sake of safety on the roads, we recommend people not get behind the wheels of their car if they’ve consumed any alcohol or drugs.” Tweet This

She also acknowledged differences of opinion between the provinces about the contentious issue of whether citizens should be allowed to grow pot at home.

The federal legislation sets a limit of four plants per dwelling, and the government rejected an amendment by the Senate earlier this year that aimed to respect the decisions of some provinces to ban home growing outright.

Manitoba and Quebec have both said they plan to take a zero-tolerance approach.

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That could result in a lawsuit if a resident in either province tries to challenge the ban in favour of the federal limit.

Wilson-Raybould would not comment on how such a challenge might work or whether the government was preparing for one.

“We believe and we have been working very cooperatively with the provinces and territories, recognizing that provinces and territories are different,” she said.

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“We are going to continue to work with the provinces to ensure this is seamless.”

Marijuana becomes legal for recreational consumption on Oct. 17.