Roadblocks are "seizures" under the Fourth Amendment, but they are not subject to the normal Fourth Amendment rules.
When Can Police Set Up Roadblocks?
When police stop only "suspicious" cars at roadblocks, the stops may not be legal.
Can the police ticket me for something other than the purpose of the roadblock?
If the roadblock is legal, what the officer sees in plain view is fair game.
The police ran a roadblock to ask drivers about an accident. Was the roadblock legal?
Informational roadblocks are permissible.