intersects marginal social cost (N*) and represents the level of driving that occurs when drivers consider the costs they
are imposing on society. The road price that causes socially optimal road usage is r* and is equal to the marginal external
congestion costs (i.e. the hidden costs of driving imposed on society but not felt by the driver, which is equal to MSC -
MPC). The welfare gained from such a charge is given by the shaded area.