Devil's Advocate Time Again:
If you use a resource more than the next person, shouldn't you pay more for it?
Why should someone who only uses the roads once a fortnight to get the shopping in pay as much as the commuter who drives 15,000 per year?
Sure, some people will pay considerably more, but if road tax is indeed abolished, there are some people who will pay substantially less.
(By the way, I see horrible implications in terms of inflation - road haulage companies will have to pass on increased costs to the consumer which will lead to inflation - putting it simplistically...that's how I see it working).