The relationship between speed and CO2 emissions
Access to relatively high speed streets for travel can protect the environment:
Most people would expect that lowering urban speed limits would save petroleum in an era when it is likely to become scarcer and more expensive, and would reduce greenhouse gas emissions (CO2) from vehicles in an era where the problem of toxic emissions from new cars has been progressively reduced to the point where the air is getting cleaner every year and its now only a matter of waiting for older dirtier cars to reach the end of their lives for the problem to be solved altogether. But there remains the real possibility of climate change.
The following graph is from a recently-published official United Kingdom Department of Energy, Transport and the Regions document entitled "New Directions in Speed Management - A Review of Policy". A document of that sort whose purpose is to justify "speed management" is not where you would expect to find evidence that is sypathetic to speed.
It graphs CO2 emissions against speed for petrol-powered car compliant with European emission standards, which are similar to those which apply in this country.
Contrary to what I would think most people would expect, and certainly what I would have expected, this is another U-shaped curve of the sort that keep turning up in road safety - one that shows that while a very high speed is not best, neither is a very low speed. The best fuel economy and lowest CO2 emissions are actually at about 83 km/h. Whatever else it does or doesn't do, reducing speeds on urban and residential roads does not improve fuel economy and greenhouse gas emissions, it increases them.
For a given journey fuel used and greenhouse gas emitted are less if that journey can as much as possible be done on higher speed roads, even expressways at 110 km/h, than if it has to be done on surface streets restricted to 50 km/h as so many now are in urban areas.
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