What will your sign say? If was going to be out there, depending on what mood I was in, I'd either have a sign that said "Warning - police state ahead" to ridicule them, or I'd have a sign that said "don't waive your rights, use the u-turn!" to encourage people to go a different way instead of waiving their rights.
I watched an episode of, I think Dog the Bounty Hunter, where they "helped" (??) at a "fake" DUI checkpoint. As you pointed out, even though the turn around "opportunity" is supposed to be their "escape" from being in violation of people's right against unwarranted searches/seizures (right?) they often end up using that, too, against the people and try to target those who do turn around as opposed to going through the checkpoint. This tactic was highlighted, praised really, in the Dog the Bounty Hunter episode, where they actually didn't even have a DUI checkpoint at all, just a sign that said DUI checkpoint ahead. They staked out near the sign, and watched and waited for people to turn off right after seeing the sign.