Get 'em off the street. The need to do so outweighs the minimal intrusion into the privacy rights that the roadblocks are.
Even if you believe MADD's and NHTSA's numbers (which thinking people can't), the vast majority of people killed in auto accidents are killed by sober drivers.
What do we do to get those people off the street?
Necessity is always the cry of the tyrant.
Banks are robbed by people with guns. Do you want to take my gun so that I don't rob a bank?
People are beaten with baseball bats, stabbed with knives. Do we confiscate them?
Sober people have car accidents and people die. Perhaps we need to just ban driving altogether.
Are you a reformed drunk?