The social best possible amount of vehicles on the road must be exceeded if congestion results. The marginal cost (The amount spent on producing one extra unit. The marginal cost is the increase in total cost when one more unit is produced) to the consumer is the only cost really considered when a driver makes the decision to use the car. What is not taken into account are the costs to other road users (Social Cost), the cost to society collectively. The marginal cost to other road users is the added congestion caused by the extra car on the road. The marginal costs to society collectively are the increase in emissions produced by the extra journey made, the follow on effects from this are large, rising asthma levels in the local area, decaying buildings and collapsing roads could be caused because of the high congestion rates. The marginal cost to the individual could be the opportunity cost of the time spent in congestion. If the more space efficient bus made the journey, the traveler would be able to read the newspaper, play on a hand held computer or even do some work, this is not possible if the car is chosen to make the journey. The marginal utility of existing users of the congested roads would decrease with the addition of an extra motorist, an extra 10 or even 100 motorists would lower the marginal utility levels dramatically. But each individual marginal cost wouldn't be affected, which explains why the marginal cost and marginal social cost differ. This can be shown by the graph that shows negative externalities in consumption.
This diagram shows, if there are negative externalities then the marginal social benefit will be less than the marginal private benefit. The best equilibrium for society would be where the marginal social cost is equal to the marginal social benefit (Q!). However, when market failure occurs it would be because a market would produce where the marginal private cost is equal to the marginal private benefit (Q^).