London's new Ultra Low Emissions Zone
Recently we have seen the introduction of the new Ultra Low Emissions Zone in London, designed to charge the owners of the most polluting vehicles an extra £12.50 for entering the area, in addition to the existing congestion charge.
Critics argue that it's a form of regressive taxation, likely to punish people unable to afford newer, cleaner cars, and worry about the effect on small businesses. However, supporters argue that the negative externalities of air pollution fall most heavily on the poorest Londoners and that the scheme is going to reduce toxic pollution by as much as 45%.
Either way - it will be an interesting case study to see whether it has the effects that are anticipated.