I was thinking about this the other day, and I don't like the idea of road charging schemes, as they are just an extra tax on top of everything else.
So I thought about credits instead. After all, we are supposed to be trying to reduce emissions and unnecessary journeys. Take the Kyoto Treaty as an example. Every country gets a certain amount of credits, when they're gone, they either buy some from someone with spare or improve their efficiency to fall within their own limits.
Why can't that work with cars ?
Every licence holder gets given a certain amount of credits per year (1 credit = 1 mile) and they are free to use them as fast or slow as they wish. If they think they can get away with less, then they can flog them off to other more "needy" motorists on ebay.
Admittedly the reduction of emissions argument is only a by product here, but with less traffic, total efficiency would be up anyway.
Maybe I'm biased because I only drive less than 2000 miles per year (privately) and could benefit from such a plan, but there surely has to be a way to positively discourage car useage that largely consists of a free-form, accident prone train up the M4 (1,3,6,32, whatever) twice a day (that costs stupid amounts of cash just to have a private room while commuting).