[sustran] Do 20mph zones have a negative impact on the environment?

[Knight, L V]

The UK Road Safety Strategy recently launched by the government stated that
an urban 20 mph speed limit would increase CO2 emissions and have a negative
effect on the environment.

In the next paragraph they gave support to self-enforcing 20mph zones in
urban areas especially around schools.  If anything I would think
self-enforcing 20mph zones where vehicles are forced to slow down and speed
up would not improve the quality of life for the immediate population, due
to noise and vehicle emissions.

However, it made me wonder if a balnket speed of 20mph would be detrimental
to the environment, does anybody know the answer or have any commnets about
20mph zones?