The problem is that there is no clear "definition" to be given as to what roads cost
There are estimates of teh "real cost of motoring" that include pollution, accidents, policing , injuries and other factors in addition to the simple building and maintenance of roads
Other estimates simply look at maintenance, others include new Roa building as well
This means that if you took a "road tax" equivalent to the VED income, in some cases it would require subsidy of up to £1200 per vehicle per year, through breaking even, to being a profit.
It would be nice though to introduce a "road tax",, hypothecate it entirely, and the when it runs out stop all road building and maintenance because motorists haven't paid enough