See my other comment -- why not just have an added petrol tax at city gas stations? They are trying to address both issues, but the issue of just 'being around'

See my other comment -- why not just have an added petrol tax at city gas stations? They are trying to address both issues, but the issue of just 'being around' (the source of congestion) is a very dubious thing to tax, whereas carbon emissions are not and taxing them produces the same intended incentives.