Wouldn't be hard to do.
Best way would be to have the 'lower' limit settable so that the 30mph is not fixed.
Heh, Some folks might also want it cut out above a certain speed e.g. 90+ mph.
Once you get to those sorts of speeds, any distractions (like a Radar alarm) is the last thing you want to be bothered with!
I'd build such a device using a small micro-processor so that the features you want could be built in easily without resorting to soldering irons.
Hmm, Do I sense an add-on board for Patricks Tuner?