I live in Madrid, where like in most big cities there is a high level of air pollution, especially due to the traffic. The levels of pollution reached the maximum allowable level a couple of months ago and some measures were taken to decrease them. The measures were to decrease the maximum speed in the main roads to 70 km/h and to ban parking in the city centre for non-residents.
The measures made effect and also the rain has helped to maintain a lower level of air pollution during the last month but the situations seems likely to happen again. These measures taken seemed to me that would only alleviate the situation temporarily.
So I was wondering, what measures can be taken in such a big city to permanently decrease the levels of air pollution?