Yes, but for the police, what constitutes a legal reason for taking someone into custody who is suspected of having too much ganja? You can detain anyone who might be a danger to themselves and others, but new laws mean modified ways of how they are to do their jobs.
The law will have to be amended (if it hasn't already) that effective 1 Jul, anyone suspected of being under the influence of cannabis will be dealt with as follows: insert amendment(s) here.
You can bet that defense lawyers are already researching the law. How do they get their clients off? It's that back and forth we see that plays out with the courts and the police.
Legal reason is the Criminal Code that says you can't drive impaired by alcohol or drugs.
253 (1) Every one commits an offence who operates a motor vehicle or vessel or operates or assists in the operation of an aircraft or of railway equipment or has the care or control of a motor vehicle, vessel, aircraft or railway equipment, whether it is in motion or not,
(a) while the person’s ability to operate the vehicle, vessel, aircraft or railway equipment is impaired by alcohol or a drug; or
(b) having consumed alcohol in such a quantity that the concentration in the person’s blood exceeds eighty milligrams of alcohol in one hundred millilitres of blood.
Marginal note:For greater certainty
(2) For greater certainty, the reference to impairment by alcohol or a drug in paragraph (1)(a) includes impairment by a combination of alcohol and a drug.
R.S., 1985, c. C-46, s. 253; R.S., 1985, c. 27 (1st Supp.), s. 36, c. 32 (4th Supp.), s. 59; 2008, c. 6, s.