In other words, commuting by car is a dominant strategy in this game. In a dominant strategy equilibrium, all drive their cars. The result is that they all have negative payoffs, whereas, if all rode busses, all would have positive payoffs. If all commuters choose their mode of transportation with self-interested rationality, all choose the strategy that makes them individually better off, but all are worse off as a result. This is an extension of the Prisoners' Dilemma , in that there is a dominant strategy equilibrium, but the choice of dominant strategies makes everyone worse off.
Some people do ride busses.
Thus, the game no longer has a dominant strategy equilibrium. However, it has a Nash-equilbrium. When a fraction q of commuters drives cars, that is a Nash-equilibrium. On the other hand, starting from q, if one car commuter switches to the bus, that moves into the region to the left of q, where bus commuters are worse off, so, again, the switcher is worse off. No-one can be better off by individually switching from q.