A rational consumer is one who always seeks to maximize his utility but if he does so at the cost of the depletion of resources, then such behavior is described by the term ‘tragedy of the commons’.
It arises when over-consumption of a product or service occurs neglecting the well-being of the society. For example, if a land is open to all, then this land could lose its productivity, if villagers bring their cows to make them graze on the land and to achieve maximum benefit out of it, they could make the land overgrazed.
In the short run, people can maximize their utility level by over exploiting a resource but as soon as the resource is depleted, it makes everyone worse off in the long run.
Congestion on roads happens to be the result of the tragedy of the commons. As the roads are public owned and government does not charge any tolls for using the roads, so congestion are most common to see there. But if the roads are privately owned, it would not have been so.