how much would that cost then? Since countriea have different costs of living and use the money to maintain the roads (with personell) it would have to be paid by the countries standards. if you try tonunify that, and charge i.e. the price of austrian vignete, then other, poorer countries people could not use the roads in their country anymore. Except you exclude them from paying it, but that is against the european law (germany tried that and failed). If you make it the price of a country like croatia, then roads in austria could not be maintained as the personal (that needs to be paid) could not live from that.

the only possible solution would be something like the Eurail pass fr r trains, where you pay a flat amount for a sepecific set of countries.