The gas tax and vehicle registrations paying for infrastructure is an example of a fair and representative tax. You essentially purchase the services of the government and they cannot spend road tax revenue on anything besides the road.
Apparently, 60% is so ring-fenced. The other 40% can be spread wider, and yet also external revenues 'top up' the road-spending pool, so it's, maybe not like the UK (goes into the genetal pot, and out of it, with merely "raising £Xmillion more by this tax adjustment, we will be able to spend £Xmillion more on that (maybe related, maybe not) expenditure" as a public justification for opening and closing the valves on the money pipelines), but it's messier than you claim.
The government generally attempts to spend according to a relevant revenue source. It would bad governance to say spend revenue collected from a tax on potash mining to fund prisoner rehabilitation or something.
If your country could readily produce, for export, oh... I don't know... butterfly wings, and had no need to fund the butterfly-wing producers because they could just so easily (and willingly) dominate the butterfly-wing market even with taxation of a massive market, but at the same time found itself with the problem of insufficient private manufacture of the few vital and much needed pogo-sticks (that your country seems to need) unless there was a flow of public money into the pogo-stick market, then what would be the problem in collecting revenue from one sector sufficient to publicly support the other sector? Even if you had to use a fiscal-hosepipe to directly tie the two together via a dedicated fund (rather than the shared-pot system, exact incomes and outgoings only nominally associated, perhaps a little extra butterfly cash one year that gets spent on painting the fire stations, in another year the pogo-stick demands require a boost but cream-cake sales have boosted (this may not even be correlated!) and the cream-tax provides, the actual details year on year get absorbed by all the other fiscal instruments but can be fully justified so long as there is no permanent drift nor unforeseen dramatic changes in any of the incoming or outgoing streams...
Yes, ask for an idealised "balancing of books", but the modern world just has too many little tricks and traps with money, much 'working wealth' is virtual and continues to be promisary in nature and you can be your bottom zloty that pork-barrel skimming is part of every bit of legislation, to various degrees and in various directions, which may or may not come out to spending-neutral in some areas if you're lucky, but I think we can be sure that fuel/vehicle taxes don't just go to roads on a 1:1 basis. Even if that was desirable. (With your low fuel taxes, either there's not enough or your're nog spending enough. From what I've heard, it's both!)