Well yes, I mostly agree with you, S. I completely agree that engine size is a blunt instrument, although to be fair the tax disc is a relatively minor part of the cost of running a car. And if we abolished the tax disc, we'd have to find another way to police MOTs and insurance. Insurance discs have been proposed, though, I quite like that. Fuel tax is also slightly blunt, but it is simple and cheap to apply and does most of the things we need to achieve - with the noticeable exception of congestion charging. However I would be prepared to accept that congestion is somewhat self-limiting: people affected by congestion will try to find an alternative route or method of transport.