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?