This is completely bone-headed. A person who drives for 20km in a small, fuel-efficient car not only has a smaller environmental impact and uses up less of the limited and dwindling oil reserves, but also causes vastly less erosion of the roads than a person who drives the same distance in a large SUV. 'Distance driven' is a terrible metric to base a tax on; 'fuel consumed' is much better, since it roughly correlates with the weight of the vehicle multiplied by the distance driven, and thus the damage caused to the road surface, it provides an additional incentive to conserve a limited resource, the consumption of which is damaging to the environment and it even takes into account infrastructure pressure from congestion, since fuel efficiency is lower in congested conditions.