When automobiles crash, they can become damaged and hurt people, and the life of a person is more important than keeping a automobile from damage. When too many automobiles try to go the same way, traffic congestion slows them all. Automobiles can cause air pollution if too many are used in a small area like a city, and the combined pollution of the world's automobiles is partly to blame for climate change. Many places where people live close together have public transportation such as buses, trains, trams and subways. These can help people go more quickly and cheaply than by automobile when traffic jams are a problem. Some of these problems can be made smaller, for example by carpooling, which is putting many people together in one automobile.