Transparency

As I've written a million times, a pure mileage charge system is just an overly complicated way to avoid raising the gas tax by adding an exciting new incredibly intrusive tax which would be much more popular. Or something. A system which also includes congestion charging and tolling is not entirely without merit, but the problem with these types of systems is that they'll probably like the necessary price transparency. People only respond to prices appropriately when they're fairly simple and they're understood. If you have continuously varying tolls based on route choice and congestion, there's no way that people can really process that information in real time in order to make informed choices. Pricing systems just can't be that complicated.