The following diagram shows the effects of increasing traffic along a particular stretch of road. At first, additional cars have no effect on the traffic speed. But, after a point, additional cars slow down the traffic and thus impose a 'congestion cost' on other car users. Beyond thus point, therefore, the marginal social cost of using the road is greater than the marginal private cost.

Assuming that there are no externalities on the demand-side, what will be the actual level of road use?