"Road tax" is a _tax_ you have to pay in order to use the _road_. It's the most natural name for it, which is why people call it that.

Despite the words they use, it's not a vehicle tax: you can own a vehicle without paying it as long as you declare that you will not use the vehicle _on_the_road_ (a.k.a., make a Statutory Off Road Notification). It's a tax for using the roads. A road tax.

That the government don't ring-fence it to use on road maintenance is irrelevant.