If safety was the real goal the police themselves would announce checkpoints and speed traps. This gives people a chance to not drink too much or speed in the first place. I've lived in places where DUI checkpoints were all announced ahead of time, and I think for many it was a serious reminder to not drink and drive.
But for many DUI checkpoints safety is not the goal. It's simply a pretext to check everyone's papers.
That only works if you actually have a DUI checkpoints all the time everywhere. It is a random check because then people need to be careful all the time. If there is a DUI checkpoint 2 times per year in your area you can just avoid driving drunk at those two days per year.
They do. DUI checkpoints are heavily advertised here in California for exactly that reason — to deter drunk drivers. The only thing they don't do is tell the exact intersection so drunks don't just drink and drive the other direction.
But for many DUI checkpoints safety is not the goal. It's simply a pretext to check everyone's papers.