Honestly I dont know; but i do know this does not prevail in America nor Canada, and Im sure a few other nations (at least) around the world as well.
In America, of course the majority hates Bush now and would probably kill him, so in DC he has his own underground network (a bus/metro) which doesnt interfere with the public. And in the rest of the country, helipads are everywhere he needs to go and he travels via helicopter.
Over here, why dont the PM/President set up their own flyovers/underpasses/underground network for their travelling. That is safer for them and more convenient for the public.