Ahh, a speed limit debate.
Yeah, sometimes they're too low, sometimes they're too high but all in all they're right a lot of the time. Yes, you can drive down a motorway at 140mph at 3am but not a 5pm rush-hour. The speed limit is designed to cater for all conditions.
The physical roads themselves may be safe at higher than legal speeds, but the environment they are in which is full of people crossing the road, overtaking, parking, pulling out of junctions etc is not, which is why the speed limit down a straight piece of road is sometimes 40mph.
That's just my 2p. Generally, I stick to speed limits (never more than 5mph above as that's where the speed cameras are set) and always use my digital dash speedo which is more reliable than the analog one above it.