Doh!

"No it isn't. That's a tax on the use of fuel. This isn't the same thing as road usage. A gas guzzler will need a lot more fuel to use as much road as a supermini, all other things being equal."

...So a bigger car (occupying more road space) will cost more?

Still sounds like a road use tax to me.

...And travelling a larger, faster, less congested road at higher average speed will cost less?

Yep. Still sounds more efficient way to charge for road usage than a huge network of cameras and ridiculous administrative / bureaucratic overhead.