Kind of like being caught between a rock and a hard place.... at least with a petrol tax, you have to pay it when you fill up your car but, those who can least afford it in cars which aren't that fuel efficient subsidise those who can afford a more modern, fuel efficient car because those with the less efficient cars have to buy more petrol to travel the same distance.
But, you bring in road user charges and unless you're going to expect mechanics when it's WOF time (that is if the car even has a WOF), or an AA clerk when it's time to renew the Rego (if the car is registered) or the petrol station attendant (but not if you use pay at the pump) is expected to check to see if someone has purchased their RUC's, it's going to be a nightmare to enforce.
Buggered if I know what the answer is but it seems paying at the pump is a lot easier to administer even if it isn't exactly a "fair system".
Any talk about tolls is generally meant by calls of privatisation by stealth so that doesn't seem to be the answer either.
Dallas has a system where, on their freeways, at peak times, some of the lanes are tolled while others aren't.... so motorists have a choice, pay a toll and get home quicker or stay in the untolled lanes and get stuck in traffic. The money from the tolled sections is then used to pay for the next new piece of freeway to be built.