For years the State of Mo and the DEA have run a drug stop on I44 in a remote unpopulated area. They don't actually set up on the interstate, but put up a sign stating that there is a drug checkpoint ahead. Drug runners see the sign and jump off on the next exit, where the cops are set up. At this point you have an out of state vehicle exiting the interstate for no apparent reason (there is absolutely nothing at the exit except a rural road going to nowhere) which gives them probable cause to ask questions.
Yes - and each state's statutes are handled differently. In the case I was referring to, there were totally different circumstances. The stops were made randomly (every so many vehicles) and the officers would ask permission to search the vehicles. If the person was smart enough to say "no", there was a judge sitting in a parked vehicle down the road who signed warrants to search. It was blatently unconstitutional. Warrants are only valid if based on reasonable probably cause - and a random stop checkpoint did not amount to probable cause.
The courts in some states have ruled checkpoints unconstitutional, while in other states the courts have upheld checkpoints which stop the flow of traffic in both directions for license checks, informational stops, DUI's, etc. However, "general crime" enforcement reasons have been deemed unconstitutional by the Supreme Court.