So why do we pay Road tax?

Rhetorical question I know, I suspect we all know it's just a means to fill the governments coffers, after all if they spent all they took in on the actual roads we'd have the best transport system in the world bar none...
As it is, they keep looking for ways to dip their hands in our pockets again and again to pay for transport infrastructure, rather than taking the money from the fund that's actually supposed to pay for these things...
BBC.

A new Thames crossing could be built without spending public money, Kent County Council (KCC) says.
Paul Carter, KCC leader, said the council had been in discussions with companies in Canada over plans to build the crossing using private funding.
He said many of Canada's road were funded by income from tolls.
In 2011, the government said a new crossing would alleviate congestion at the Dartford Crossing tolls caused by "the success" of the M25.
"They do a 'fund design build and operate' scheme in Canada which has been an enormous success," Mr Carter said.

All well and good, but as I've pointed out above we already do pay 'public' money into the road fund license and that's the money which should (all) be spent on new infrastructure, rather than be pissed up the wall in MP's expenses and green lunacy with the rest of our taxes. They can't even keep their promises on the tolls they grab from other projects such as the Dartford crossing which initially was supposed to suspend tolls after the damned thing was paid for but several years after it was paid for we're still paying. Plus no-one believed the local councils claims that somehow or other they had to keep the barriers in place as a traffic calming method, nor the governments statement that the toll money would be spent in the county on other roads...
I rather suspect that the Canadian companies assuming they want to get involved with UK politics and taxation will do a reasonable job, but then as in all these schemes milk us for the rest of our lives maxing out their profits, not that I have a problem with companies making profits, I just have a problem with government both local and national pissing down my back whilst telling me that it's raining Which is exactly what this is, just another tax grab on top of the road fund license...