Such a checkpoint would be unlawful. But what if the sign was just a ruse to get drivers to exit the interstate?
Marion County Sheriff Jerry Devore said the sign warning motorists of a drug interdiction checkpoint ahead just before southbound traffic reached the the [sic] Kinmundy-Patoka exit — or milepost 127 — was only up for about 90 minutes before being taken down.
Devore admitted the sign was a ruse to get drug-possessing suspects off the interstate at an isolated exit with no services.
“Anyone from out of state would have no reason to pull off,” Devore said. Once off the interstate, deputies looked for Illinois Vehicle Code violations as a reason to initiate a traffic stop. Once the vehicles were stopped, deputies were able to look for drugs or other illegal items.
As described above, this ruse is probably OK. (But, much is going to depend what happens after the stop.)
Why do I say this? Consider the Eighth Circuit case of United States v. Carpenter.
In that case, the police set up a ruse checkpoint on the interstate and then looked for a reason to stop non-local vehicles that exited the interstate. The defendant, who had out-of-state plates, exited the interstate. When the defendant saw an officer following him, the defendant maneuvered to the side of the road in order to make a U-turn. While on the side of the road, an officer stopped the defendant. The defendant reportedly took the exit looking for a gas station, but there were not any gas stations at the exit. The defendant appeared nervous, had a quarter tank of gas, and gave a travel route inconsistent with his car’s rental agreement. The police brought a canine to the scene and the canine alerted to the presence of drugs in the defendant’s vehicle. The police found and seized a quantity of cocaine. Under the totality of these circumstances, the Court said, the police had reasonable suspicion for an investigative detention to let the canine sniff the vehicle and no Fourth Amendment violation existed.
With all that said, the police still can violate the Fourth Amendment while executing these ruses. For example, if a car exits the interstate and travels in a normal manner without violating any traffic laws (and no other unusual circumstances exist), any stop of the car will be problematic.