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.
Note, this solution does not eliminate all congestion. Congestion still occurs, but only up to the point that the incremental value of a trip is equal to the incremental social cost, or until drivers feel that the incremental costs imposed by congestion start exceeding the incremental value of using the highway. For about the last three decades economists have advocated congestion pricing to control highway usage, but toll collection was an impediment. Stopping traffic to collect tolls is self-defeating. Recent electronic toll collection devices, however, make congestion pricing feasible.