It's largely a training issue, although approved screening devices are not administered in Canada, at least to my knowledge, in an individual's own vehicle. The individual is escorted back to the police vehicle. They are put in safe conditions, away from traffic, in the back of the vehicle, and they are tested in that environment, which the police have control over. The contemplated use of the ambient detection would be in the environment of that individual's car, which there's less control over.
On October 20th, 2016. See this statement in context.