PC shows how a driver's costs increase as speeds drop below the limit and travel times increase. SC rises at a faster rate than PC because the additional drivers increase the gasoline, oil, time and other costs to all other drivers. Supply curves are another title for these cost curves, for they show how costs change as trips per hour increase. From the demand curve's viewpoint, the costs per vehicle mile, C , indicate the prices motorists would pay for a given number of trips on a highway. D is downward sloping, indicating the higher the price the fewer trips taken.
Prices also reflect the benefits a driver receives from a trip.
If usage of the highway is free to drivers, then N number of trips will be taken, the point where D bisects PC . Up to that point, drivers' willingness to pay for trips exceeds their private costs, and beyond it they do not. Thus, too many trips are taken. This is where D intersects SC . Society is now better off with congestion pricing.