Is it reasonable to expect that you have alcohol in your body? That is all that it is necessary an officer to believe in order to demand that you supply a suitable sample of breath for an approved screening device. What happens to you after the analysis depends on the reading.
I recall stopping a driver shortly before lunchtime one day. He smelled strongly of stale liquor from a previous night's party. I asked him when he had last had a drink, anticipating using a screening device to test him at the roadside.
Whoosh! Here comes a car overtaking my cruiser at 144 km/h in the posted 110 km/h zone. It's dark at 11:30 pm and at that speed, any animal or object on the road won't be identified in time and a collision is almost sure to occur. This is only the beginning of the story.
Using alcohol helps drivers make poor decisions. Of course, the decision of importance to us all is "do I drive, or have I had too much?" This is a decision that should be simple, if you drink don't drive, and the law should reinforce that.