The above are examples of a typical case.  Other opportunities arise when it’s not a random spot check case.  A person may be found sleeping in a car, requiring consideration of the vast jurisprudence of “care or control”.  Or police might stop a vehicle fitting a description given by a civilian, which implicates another line of jurisprudence.  It also creates issues of proof that the right vehicle or driver was stopped.  Effective cross-examination of civilian witnesses often undercuts the prosecution’s case.  Other times, the arrest is at a hospital, where a detainee has been taken after a collision, and police arrive after the fact.  Yet the condition of people under treatment after an accident arguably does not reflect their condition at the time of driving, so observations made by police at the hospital are often unreliable bases for arrest or conviction.  The same goes for assistance given to police by medical personnel, themselves governed by rules of patient privacy that typically require a search warrant or explicit informed consent by the patient.  Without either, medical personnel assisting police could poison the Crown’s case.

It can go on and on like that.  Plugging one hole can spring another leak.  While governments occasionally attempt to legislate some of these difficulties away, those efforts can themselves invite new problems.  Parliament and the Provinces remain bound by the Constitution and can only go so far.