Random an discrimination are key regarding 'checkpoints'.
Random: They can't set one up outside a club and be effective at curbing DUIs.The only result of that would be the closing of a business. Folk ain't going there if they know what's waiting just outside.
Discrimination: There can't be any charge of discrimination at a checkpoint because EVERYONE that comes along is asked for the same information. It doesn't matter where you came from, what color or sex you are everyone is equal.