Q: Am I allowed to turn around before approaching a sobriety checkpoint?
A: The answer to this question depends on what you mean by “turning around.” If you're still a couple of miles away from the checkpoint, and a friend calls you on your cell phone to warn you about what's ahead, then you're perfectly within your rights to turn around and head back home or take a different route to your destination. If you're that concerned, however, the ethical thing to do would be to get off the road entirely until you sober up! Even if the checkpoint is within view, there's nothing wrong with turning off onto the nearest side street or highway exit, provided you can execute this maneuver smoothly and legally; after all, the police will never know if you intended to go in that direction in the first place.
As you may already have guessed, though, it's not a good idea to turn around when you're already being funneled into a sobriety checkpoint. Generally, police can only inspect random cars at sobriety checkpoints according to a mathematical formula (every fourth car, every tenth car, etc.) If you start behaving suspiciously, though—say, by squeezing into the right lane, climbing up onto the shoulder, switching to reverse, and backing up to the nearest exit—the police may take notice. At that point, they are legally entitled to infer probable cause from your behavior, and an officer will be dispatched to examine you for evidence of DUI, either at the checkpoint itself or along the side of the road.