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.