The police are able to pull people over, at random, to test the sobriety level of a driver. Courts have ruled that although this is a violation of one's Charter rights, that violation, according to the Supreme Court of Canada, is "saved" by s. 1 of the Charter (which is known as the reasonable limits clause). In other words, the police are allowed to stop individuals at random to test sobriety levels.
Moreover, a traffic investigation (such as one for speeding) can turn into a criminal investigation (such as a DUI), provided that the police have grounds to change the course of the investigation. If the police, for example, pull someone over for an improper lane change, and then detect an odour of alcohol coming from the driver's breath, they would be allowed to pursue a criminal investigation for Over 80 / Impaired Driving.
The key to defending most drinking and driving cases generally turns on the observations (or lackthereof) that the police make after pulling a driver over, rather than the reason for the stop. For example, most roadside investigations involve the use of a "screening test" (through an Approved Screening Device). If the driver fails that test, they would then be brought back to the police station for actual breathalyzer tests (of which there are normally 2). For the screening test, the law says that the police mus have a "reasonable suspicion" that the driver has alcohol in her/his body in order to make the Approved Screening Device Demand. If if the police do not have those grounds, then there would be a Charter violation resulting from the taking of the roadside test.
There are also several "timing" requirements that follow the roadside investigation. For example, the police must take both samples back at the station "as soon as practicable". Failing that, there would be another potential defence to raise to an Over 80 charge.
You are best off having a lawyer sit down with you and review your disclosure to determine what, if any, defences you might have to the charge.