? quoting Loco
Additionally, even if you did have a system where a guy in a Nissan Micra wasn't charged the same amount as a guy in a huge fuel guzzling machine, the system would be so horrendously complicated to enforce.
Hardly, since your car type is linked into your vehicle registration & numberplate. If vehicles were classified in different bands, it's simply a case of saying "this person has driven X miles, with a class Y car, so they need to pay Z amount." It should be fairly easy to manage with computer systems, though the government has proved several times it's incompetence in this area (c.f. NHS computer systems, PFI, air traffic control, etc...) The hard part about enforcing it is installing the tracking equipment in cars in the first place and locating cars without it (though lots of hidden 'checkpoints' should be able to get round this effectively).
Again, I still think charging by the mileage you do is the fairest system. What do you do for taxis and freight and buses? You zone areas (city centres, motorways, rural roads, etc.) and class them in such a way that the people who have to use them pay the least. That way, it's fair!