The federal government is giving signals that there will be a series of unknowns when cannabis becomes legal October 17th.
Officials say some police forces will not be able to grapple with procedures for laying drug-impaired driving charges.
To prove the offences, the federal government says police will have to take blood samples within a two-hour window.
And, even before that, a specially trained officer will be required to get a suspected high driver out of their vehicle and administer preliminary tests.
Another problem is a lack of clarity on Canadians travelling over the border.
Ahead of legalization, the Liberals are sending information cards to more than 15 million households, running ads and slipping notes into new passports about the rules around cannabis, hoping to avoid problems before they arise.
The government is spending 46 million dollars over five years on the advertising campaign.