Part of many cities fight against emissions involve areas of the city where they enforce low emissions from cars. This means that only those that pass current emission standards can drive in those zones or incur a fine. London has recently taken steps to institute one of the toughest with the ULEZ, Ultra Low Emissions Zone.
If you have a non-compliant car or vehicle, you have to pay an additional charge to drive in a high congestion area of the city. Those who refuse will be slapped with a fine. The idea is to encourage people to drive less and/or switch to emission free vehicles.
The ULEZ is a tough zone but there are many around the world who have similar standards. The plan is to expand the zone which will capture more people and hopefully encourage them to change cars or rely more heavily on other modes of transportation.