Why not simply price it into the petrol?
Sounds like an excuse for surveillance.
Why not simply price it into the petrol? You want people to reduce the journeys taken by car whether they are on busy roads or quiet roads. Congestion isn't the only problem, lack of oil (UK is now a net importer of oil) and global warming are 2 others. Fewer journeys and more efficient cars are the way to go.
So you shift as much fixed annual cost onto per-mile cost, to encourage people to leave their car at home and save the money instead by making as many journeys without the car.
1. shift the annual car tax, to a per mile tax.
2. Encourage insurance companies to give deep discounts to low mileage drivers.
3. Avoid fixed one time costs (including expensive GPS/Receiver/Charging equipment!) .
4. Increase the MOT interval to 2-3 years (common in Europe) or per mileage (e.g. every 20,000 miles).
So if a person wants to reduce their costs they can simply drive less, and automatically save, not just the cost of the petrol, but a proportion of the insurance, a proportion of the tax etc..
Conversely, say you pay 700 GBP in fixed charges annually, you'll never save that 2GBP daily charge, regardless of whether you cycle to work or drive to work. Any public transport alternative has to be AT LEAST 2 GBP better value to make it worth switching to public transport.
As public transport usage goes up, presumably the congestion goes down, and presumably the quality of public transport will improve.
It has other advantages too, it's non discriminatory, if I don't have the kit, I can't drive in the charging zone. That discriminates against me. (How do these works for EU license plates too?)
It's non invasive, since when did we give up our right to privacy?
Plus road charging creates a company to monitor the charges, once you've done that that company will lobby for more and more charges and cameras to expand their business. That's counterproductive. The more money spent on road infrastructure, the more locked into using the roads we've spent so much money on.