Congestion vs environment
According to The Observer, MPs are surprised that "while a national road-charging scheme to charge motorists by the mile is being piloted, its aim is to cut congestion, not to discriminate between a higher-emissions Land Rover and an environmentally less damaging Toyota Prius".
You see, that's because road usage is not the best way of determining how much environmental damage a vehicle does. The total pollutant output is, over the lifetime of the vehicle, pretty much proportional to the amount of fuel put in to it. Consequently, while road charging may be a great way to ease congestion (as has been proven in London), if you want to reduce environmental damage, you need to encourage the use of more fuel-efficient vehicles instead. The way to do that is to charge more for fuel and perhaps to subsidise the purchase of efficient vehicles such as the Prius. It is an entirely different problem from congestion, and so is best solved in entirely different ways.