From Kate Sweeney
I've found more research on 20mph zones, and specifically how they affect pollution/emissions. It's a recent Belgian study - the whole thing is rather long and, dare I say it, overtechnical, but I'd be happy to supply the link to anyone who wants it.
The results were inconclusive, but to the authors' surprise there was evidence that exhaust emissions, specifically, are less harmful at 20mph. They also suggest (without proving!) that light goods vehicles have higher emissions at 20, but ordinary cars have slightly lower ones.
The authors say what we all know - it's highly complex - but so far I've found nothing to prove that 20mph zones increase emissions. However I'm continuing to burrow. Anyone know of further evidence?
See also: School Crossing at Mytholm for recent discussion on the 20 mph