Consider two ways of commuting in a crowded city: taking public transportation, such as subway and buses, or driving your own car.

A person who chooses to take public transportation in a crowded city imposes a NEGATIVE OR POSITIVE externality on drivers. A policy implication of this result is a SUBSIDY FOR OR TAX ON those who take public transportation.

Persons who choose to drive their own cars to get around in a crowded city impose a NEGATIVE OR POSITIVE externality on other drivers. A policy implication of this result is a TAX ON OR SUBSIDY FOR those who drive their own cars.

A positive externality occurs when the activities of an economic agent is of benefit to another third party that is not part of the activity or transaction while negative externality is the cost an individual bears due to the market activities of another individual or firm. It should be noted that the third parties didn't plan to incur the costs or benefits mentioned above.

Therefore, an individual who chooses to take public transportation in a crowded city imposes a positive externality on drivers. We are told that the city is overcrowded, therefore using public transportation means there will be lesser vehicles on the road and this will be beneficial to the drivers and there may be subsidy for those who take public transportation.

The people who choose to drive their own cars to get around in a crowded city impose a negative externality because there will be traffic congestion and health hazards and a tax can be imposed on them.