Cutting speed limits for cleaner air?

[WelshChris]

The Welsh government have announced plans to cut speed limits from 70 to 50 on certain stretches of busy roads to ‘improve air quality’...

http://www.bbc.co.uk/news/uk-wales-43881650

Is this snake oil? - my initial thought is that it won’t make a blind bit of difference. Another excuse to cut speed limits based upon flawed research. Surely air quality is determined by the volume of traffic rather than the speed it passes through a given zone? - or am I wrong?