I've always argued this with the speeding point - speed does not necessarily cause accidents, poor driving skills does. Holding my hands up, the last accident I had, I was only doing 20mph when I hit a motorcyclist how was in my blind spot filtering through heavy traffic (no injuries luckily enough) - the one time I DON'T check over my shoulder.
Whilst not condoning it, I rarely stick to the speed limit (except where the conditions clearly do not allow it, nor should anyone speed).
There was a speed camera on the M4 near me that was positioned just after a junction where it goes from 50 to 70 (in the 50 zone). From day one all that occurred was increased congestion and accidents. It's since been removed, traffic flows freely, and I've not seen an accident since. People are more concerned with checking their speedo's than the road ahead. Equally, I fail to understand why that particular stretch is 50 compared to the remainder of the M4 in that area - it's no different.
Personally, I'd be all for speed camera's outside schools, and in actual accident blackspots, not just where the police feel people break the limit and can get more revenue out of it. However, all speed cameras do is cause you to slow down, and then speed up once you're past it - they do little in the real world