I think another reason is to pulse the traffic stream for the 7-9 am slot as its a major bottleneck for people travelling into the city. But then for the rest of the day it can seem excessive.
As a bike braking to come down to 40mph on a dual carriageway this has always been a big safety concern (especially at night) that some cager behind you who hasn't been paying attention, wipes you out over what is just a civil offence. There are a couple of these on the A12 around the corner from the new 2012 Olympic site.
