I was wondering what is the reason for having a "Radar" sign on the highways? Is it mandated by the law, or what? Surely, the mobile radar stations must not have pre-warning.
Having said that, I have seen a portable "radar" sign just next to the border crossing; and the mobile radar station located sneakily on the border crossing (where limit is, presumably, 10km/h). From that I guess that there needs to be some kind of notification (on the Swiss territory) that there will be radar measurements.
With a very few exceptions, they're used to try and slow traffic down even though there's no radar trap nearby. In theory just a warning that there _may_ be radar, so best slow down (unless you know where they really are and are not )
But in practice they are not normally a pre-warning of a real one, unlike those in France, which very much are, as I believed was a condition of acceptance of their use. So in France the sign means you should really slow down and/or keep a sharp eye out, in CH it means you're clear to go for the next stretch of road.
Mobile radars are also normally placed in places where there's no warning signs.
Exceptions include the sign as you're approaching Luzern Southbound on the A2, where there's a big sign showing how many drivers have been fined and banned as a result of the five (I think) cameras between Rotsee and Hergiswil.