The Border Patrol habitually sets up roadblocks as far as 90 miles north of the border with Mexico and demand you declare if you are a US citizen. If you refuse to comply they harass you by holding you longer for special attention.
The TSA has mobile units called “Stinger Squads” (or something like that) that participate in roadblocks, often with local LE units, also demanding information from you when stopped. They also occasionally set up and bus and train stations.
Several years ago the local Sheriff’s Dept. set up a roadblock on a four-lane highway here in the county checking permits, inspections, registrations, and looking for possible DUIs. A man driving a truck did’t stop fast enough in the backed up traffic and rear-ended the vehicle in front of him and was killed. Sheriff’s office took a lot of heat over that. Now they set up on side roads, but still set up.