In order for the government to fairly collect based on the utility of the roads, they would have to go out and pay all the transaction costs to determine the correct amounts -- how much should someone without a car be paying for the roads for the benefit of allowing mail and packages to be delivered, or of having goods they can buy delivered to local stores, or of having a timely emergency response? Shouldn't people more likely to need an ambulance be paying more for roads ambulances can use? Shouldn't trucks that deliver necessities be paying a higher toll since the recipients clearly benefit from them more than trucks delivering frivolous things? How do we determine what proportion of the value of some delivery of goods or other use of the road is attributable to the existence of the road, and what do we use as a reference to calculate the alternative in each case?
Apportioning the value of the roads to the people who find them useful is utterly hopeless. And if you're not going to do it right, and doing it wrong causes both unfairness and economic inefficiency, you ought not to do it at all and just fund it through general taxation.