If there has been an improvement in air quality then the increased usage of electric vehicles has probably been a bigger contributor.
I wonder if, in years to come when there's very few polluting vehicles on the road, the limit will be put back up to 70? Similar with the new ULEZ zone in London, in a few years time when there won't be many vehicles on the road that pay, the scheme will make a loss - I wonder if that'll be scrapped aswell?