I think people will continue to drive in pretty much all the same situations in which they currently drive. People with a few drinks in them will continue to display the same overconfidence and bravado as ever ~ some with good-enough reason. Different people with different body-weights and different psychologies have vastly different tolerances for alcohol and other mood-altering substances, and there are many who are perfectly capable of safe driving, even with a blood-alcohol level slightly above the legal limit (and especially if that legal limit is low).

Although there may well be a brief period of extra-enthusiastic enforcement for a brief period once such a new law is enacted, in the long run, and in general, drivers will be pulled over and breathilized only when they have drawn attention to themselves by erratic behavior on the road.