As Andy T says it's not a road tax - although many people, myself included, call it that. However words matter. They convey meaning and the wrong words may convey the wrong meaning. No matter how many of us call it a 'road' tax, the fact remains that it isn't.
In order to drive a motor vehicle anywhere except on private land, a driver needs two licenses. One is to prove physical and mental ability to drive the vehicle safely and is, logically, called a driving license. The second license is to to be allowed to actually drive it somewhere and is a vehicle license. It's not for the road, it's for the vehicle.
This second license is a bit like a fishing license which allows the holder to fish in a river, pond, lake etc. It does not convey 'ownership' of the river and nor does it give the fisherman any extra rights over the river. Other river users eg. canoeists, houseboats or small boys throwing stones, can continue to use the river - often to the annoyance of the fishermen - even though they have not paid any license fee.
In a similar way pedestrians and cyclists have an absolute right to use the roads without paying a fee for a license, although as John Brown points out, many cyclists are, in fact, also motorists and so will have paid for a motor vehicle license which remains unused while they are on their bicycles.
Motorways have the distinction of being the only British Roads where pedestrians and cyclists are not allowed, and in this respect I think they are unique.
Bob