Rest Area Stops

I’ve heard of these roadside fishing expeditions before, but this was the first time I’ve encountered one.
I was heading out of town to visit relatives for Thanksgiving on Tuesday morning and discovered that the sheriff had set up at a local rest area (I-55 Funk’s Grove Rest Area at mile marker 149, 10 miles south of Bloomington, Illinois). There were two signs on the roadside before the rest area exit. The first one apparently indicated that there was a roadblock ahead (I quite frankly didn’t pay attention to it), and the next one said “All vehicles subject to search.” The second sign was just 20 yards or so before the rest area exit.
The idea is, of course, that they don’t actually have a roadblock ahead, but they search the cars that exit into the rest area, figuring that anyone carrying drugs will want to take that opportunity to get off and destroy the evidence before hitting the roadblock. The Supreme Court has ruled against drug interdiction roadblocks, but I can’t remember if they’ve heard one of these fake-roadblock/go-after-those-who-avoid-it situations (anyone have the answer?)
As I passed the rest area, I could see cars being stopped as an officer with a dog went around the car sniffing for drugs — perhaps using the horrible Supreme Court ruling in Caballes v. Illinois for guidance. (I’m wondering if the Caballes ruling will result in an increase in these fishing expeditions.) Of course, Caballes was based on a valid traffic stop. Would this kind of stop also be considered valid for a dog sniff that would, by itself, justify a full search?
Now, I can’t report this without pointing out the “stupid” factor. Even assuming you don’t know about this fake-out technique used by the cops, still — how stupid do you have to be to assume that the cops are stupid enough to warn you about car searches while giving you an easy way to get off the road before the roadblock? And yet, I suppose that’s the reason the sign is so close to the exit — not enough time to think it through. All you can do is react.
Unfortunately, relatives and a meal were waiting for me so I couldn’t take the time to stop and check it out — I would have loved to have had a chat with the officers about constitutional rights.
Here’s another question for you: In this situation, could I pull into the rest area and stand on the sidewalk with a sign that read “You are not required to consent to a search’? Since the officers have no evidence that a crime has been committed but are simply fishing, would my free speech rights trump laws against interfering with an officer?
What do you think?
Update: Radley Balko provides excellent background on the status of the Supreme Court thinking regarding these stops.