Re using a 3rd gear as an aid to stay below 30 mph. Many if not most modern cars are more efficient (and therefore less polluting) on 4th at 30 mph than on 3rd.
Given that pollution, in big cities at least, is almost certainly more damaging and ultimately kills and maims more people than those who might be killed every year *as a direct result* of going slightly over 30 mph, suggesting travelling in 3rd at 30 mph simply to keep speed in check seems to me an extraordinarily absurd proposition, to be perfectly honest.
Even more so when you think that those tempted to use such technique are cautious and safe drivers in the first place, who are already trying to be as safe as possible. How many serious accidents a year are there going to be that are directly caused by a cautious driver who accidentally goes a few mph over the limit? And any driver with basic skills should be able to tell if he is travelling far too fast (as opposed to a little bit over the limit).