well the road tax is for the maintenance of the roads, charge the people that wear them out. If I only use the car on the weekend and drive a few miles, why should I pay as much as someone that drives hundreds of miles a week. I am doing far less damage to the roads and putting out far less pollution.
Currently the UK motorists put in about 40billion a year into the treasury. Last year the government spent just over 4billion on the road network. I don't see the road charging being any different.