ps, I believe in prevention, also. I just think there should be probable cause before you start questioning people and subjecting them to roadside tests, breathalyzers, and so on. You don't have to wait until they have an accident. And as to the deterrent effect of roadblocks, back in the day when I did, admittedly, occasionally drive drunk, I knew perfectly well what routes to take to avoid any possible sobriety checkpoints.
Well of course you knew the routes to take, that is a drawback of the system. Scotus ruled that roadblocks are ok provided they are done out in the open by accepted standards. one of those is to notify the public of there locations prior to holding them.
Think how effective they would be if that was not the case.
I would argue that the 'probable cause' is found in the numbers of people that die and are injured day after day year after year.
Just like we have to act to stop the terrorism we have to take action to stop the dying on our nations roads.