I agree; it doesn't add up. We need detailed stats on accidents before and after the speed camera installation, plus stats about accidents during the same times of day and year.
It's far from clear whether fining people actually reduces road accidents. From a Guardian report in 2013:
"Professor Richard Allsop’s RAC Foundation report found that the number of fatal and serious collisions near 551 fixed speed cameras the number of crashes either went down or did not increase by a significant amount; at 21 sites the number of crashes went up by a significant amount."
The entire issue of speed cameras and statistical benefits is fraught with imponderables and unjustified confidence from 'experts'. People will often say because they've seen the results of serious accidents they 'know' that speed is the problem. And as DVT points out, if all vehicles remained parked all the time, then there'd be almost no accidents on the roads.
But we live in a society where personal transport and goods transport are critical aspects of that society. The question, therefore, has to be not 'does speed kill' but 'how can we ensure that inevitable accidents result in the fewest possible injuries?'.
The statistics for roads throughout the world don't help that much, either; so much information relevant to statistic outcomes isn't used or incorporated - road maintenance, weather, changes in temperature, catastrophic failure, improvements in vehicular design and much more. The only fact we can be completely sure of is that as speed increases so does the probability of an accident. In other words we're back to DVT's point: if the vehicle stays parked and in the garage, there would be far fewer accidents.
Given that the society in which we live has chosen to allow us to drive on roads, how are speed limits set? Interestingly, there's no scientific studies of any merit that established the 70mph limit. The 70mph and 60mph limits were introduced as a result of the oil crisis in 1974 and kept for road safety purposes afterwards. 40 and 50 mph limits are required in places where the highway design means that it is not safe to travel any faster (usually defined by the safe stopping sight distances to a stationary vehicle in the carriageway). (DoT)
So the basis of their introduction was nothing to do with safety but to save the country money. There's a substantial body of evidence that suggests reducing speed limits on all motorways to 45mph would allow traffic to move more easily (RTL) but the bottom line is that the main cause of road accidents is driver error.
And you see this every day, on every road. Inability to use roundabouts - particularly those at Black Cat - are a prime example, lack of observation, following too closely behind other vehicles, lack of anticipation, failure to know what signals are for, talking to and looking at the passenger (!!!) while driving, use of the mobile 'phone in the hand - all these add up to not only sloppy and thoughtless driving but dangerous habits and perhaps if these drivers were targeted instead of those who infringe an entirely arbitrary speed limit we might have safer roads.
The UK focuses entirely on speed - because it's the easiest way to collect cash - but perhaps a re-think of targeting is needed to eliminate those drivers who, even at low speed, present a clear and unmistakable danger to the rest of us.