I originally wrote a really long post on this, parts of which I may still use in other polemics, but I think the main points are best expressed succinctly as follows:

1. “Road tax” is an erroneous colloquialism for Vehicle Exise Duty, a charge that is levied on certain vehicles based on the emissions they produce. It’s a levy on pollution, not a charge to use the road.
2. Roads are paid for out of the general government taxation pot or local taxes, depending on who is responsible for maintaining the road. “Road tax” revenues are not ringfenced for road maintenance.
3. If you ride a bike on the road, you don’t need to pay anything because bicycles are exempt. That doesn’t just apply to “cyclists”, it applies to everyone. We can all make that choice.
4. If you drive a zero-rated vehicle (e.g. any of these: http://www.nextgreencar.com/car-tax/band/band-a-list/) you don’t need to pay anything because those vehicles are exempt.
5. If you drive any other type of vehicle, the charge you pay is graded according to the amount of cack it pumps into the air we breathe.

And that’s really it. By not paying “road tax”, “cyclists” are not doing anything underhand, illegal or immoral. They are simply people exercising a right that is open to anyone. If you don’t like paying “road tax”, reassess your own choices and ditch the pollutionmobile in favour of something cleaner and cheaper. And that’s the long and short of it.

For more information see: http://ipayroadtax.com/.