I wasn't going to get involved in this one...
Speed in excess of the limit is a contributary factor in 5%
of deaths and injuries (KSIs) on our roads. Thus speed cameras can logically only 'prevent'* 5% of injuries and deaths.
Excessive speed for the conditions WITHIN the limit is a contributary factor in 26% of KSIs.
That leaves 69% of all deaths and injuries that have NOTHING to do with speed at all, in fact most of these are a result of driver error / inattention (from memory only 5% of KSIs are attributed to mechanical failure)
The ONLY way to improve our roads is education. Teach people to drive safely and judge for themselves what an appropriate speed is at ALL times. At the moment we're expected to be able to judge when the speed limit is to high for the conditions and slow accordingly, but we're not allowed the same privilege if we consider the speed limit too low.
WRT CO2, the best way to reduce CO2 emissions is to allow the traffic to flow freely and teach people how to drive efficiently. Stop / start, slow moving traffic produces MORE CO2 than free-flowing traffic. And don't even start on electric cars as the electricity still has to be generated somewhere.
Enforcing speed limiters on cars (this is what will happen in reality, manufacturers aren't going to design a range of cars specifically for the UK market) won't have any effect on accident rates OR emissions because you'll still have however many horses under the bonnet, using exactly the same amount of fuel as before. The percentage of the time when it's actually possible to exceed 100mph is realisticly so low that the CO2 'saving' will be negligable.
Now FFS no-one start on this topic at the weekend!!
*Cameras in fact don't prevent jack. A letter in the post 14 days later does not reduce the risk of an accident at the time of the offence. SPECS (average speed cameras) do nothing except create bunching, tailgating and dangerous levels of concentration on the speedo and nothing else.
Some interesting statistics relating to injury accidents in motorway roadworks (referred to as PIA or 'Personal injury accidents') where different types of speed enforcement were deployed. The percentage figures quoted show the comparison between the number of PIAs with and without enforcement.
Amalogue cameras 55% INCREASE
Digital cameras (SPECS) 4.5% INCREASE
Police patrols 27% DECREASE.
The same comparison on an open motorway with no roadworks or speed restrictions:
Analogue cameras 31% INCREASE
Digital cameras 6.7% INCREASE
Police patrols 10% DECREASE
All the figures quoted are from DfT's OWN statistics.