Why is London imposing lower speed limits and installing speed humps when they contribute to pollution?

There has been a lot in the news recently about pollution, with London's air quality being forefront. Boris Johnson launched a discounted van deal to promote the Low Emission Zone requirement on ordinary vans from Jan 2012 and the DfT is trying to negotiate a £300m fine down to £5m for not meeting EU standards on air quality. Am I alone in realising that unnecessarily low 20 and 30 mph speed limits and "calming" humps are contributors to pollution?

Both diesel and petrol engines use least fuel per mile travelled when running at constant speed in top gear and that means somewhere between 45 and 60 mph for most vehicles. The closer we can get to this ideal the less pollution will be emitted. So raising 20 to 30 and 30 to 40 mph and probably 40 to 50, makes a lot of sense. We could also save the economy over £300m per year by banning speed cushions.

Asked on 3 April 2011 by PO, Claygate

Answered by Honest John
Quite right. Do you know what the urban speed limit is in Bangkok? Not 20mph. Not 30mph. Not 40mph. It's 50mph, and that's with cars, buses and trucks intermingling with costermongers barrows and motorcycle sidecar street kitchens. But in Bangkok, all the Tuk Tuks, most of the taxis and a lot of the trucks now run on virtually pollution-free Compressed Natural Gas, which is an answer to urban pollution that the EC prefers not only to ignore but to discriminate against.
Similar questions
Having recently visited Australia, driven from Melbourne to Sydney and driven around Perth and within rural areas, it was a welcome return to pleasurable driving. There was no aggressive driving and everyone...
Last week your correspondent (SD, London) complained about a pinch point and speed humps and the effect they were having on his property. However, he did say they slowed the traffic. I'm sure you will...
You often publicise issues that seem unfair to road users: speed humps, road repairs, inappropriate speed enforcement and unfair parking charges, for example. Motorists are a huge proportion of the (voting)...