They're only done on motorways (~freeway) in general, and there tend to be at the very least cameras before/after each junction. Any directly on-carriageway parking places tend to be closed off, hard shoulders are for emergency use only, and service areas are considered junctions for this.

I don't think I've ever seen anywhere with them with less than, say, half a mile separation between junctions. At that point, you just get normal speed cameras.