An interesting adjunct to the news that diesel car sales are on the slide in the UK and the effect that this has had on CO2 emissions: the German courts have ruled that two cities will be able to ban some of the most polluting vehicles from city centres in the hope of reducing the negative externalities of pollution. 2 of Germany's most polluted cities, Stuttgart and Düsseldorf have been granted the legal right to ban diesel vehicles in effort to cut nitrogen dioxide emissions.

Is this necessarily going to improve the situation? And can you evaluate the likely effects of such a policy? 

Think in terms of opportunity cost, a potential for government failure and distributional issues.