FIGURE 4 ROAD CONGESTION
Assume that this road is fully used and is right at the tipping point before becoming congested. Demand for driving on this road is D0, and the marginal cost of using the road—gas, time, and auto expenses—is MC0. Equilibrium is at point e, with Q0 miles a day being driven. Consumer surplus is area P0ae for the typical driver. When a new driver begins using the road, this increases the marginal cost of driving to MC1 for everyone, because the tipping point has been passed, and the road is now congested. Consumer surplus shrinks because of overuse of this common good.