If you want usage fees to be proportionate, I think you'd need to tax according to the maintenance costs caused by the vehicle per mile (eighteen wheelers cause

If you want usage fees to be proportionate, I think you'd need to tax according to the maintenance costs caused by the vehicle per mile (eighteen wheelers cause vastly more road wear than cars). That's really all you need, except perhaps for some fixed cost per mile for the size of the vehicle, to tax congestion.