The need to provide for highway safety, by getting drunk drivers off the road, trumps the normal need for reasonable suspicion.
Police must follow an articulable process for stopping cars to check for drunk drivers. For example, they could stop every third car, or every other car, and so on. Once they begin stopping cars whose drivers they suspect might be drunk drivers, they must be able to support their stop with facts that would lead a reasonable officer to suspect that the driving was impaired. In other words, the police must have reasonable suspicion to stop that car, just as they must when stopping any car on the road.
Police need not announce to the public that they intend to operate a sobriety checkpoint, though they often do. The reason for such announcements is to alert the public to the presence of police on the streets, which will hopefully have a deterrent effect. Some drivers may choose not to drink and drive when they learn that checkpoints will be stationed at popular intersections and highways.