If I was a coach passsenger, I'd really sooner the driver didn't go in for overtaking other coaches which were travellig at 69 mph.

I think any limiters ought in fact to be set a few miles faster than the speed limit, to allow for emergencies. But I'd like them to be supplemented by flashing lights or sirens to indicate to the world at large - and to the driver and passengers - that the limit is being exceeded, which shouldn't ever be for more than a few seconds.