Cars use freeways for fast driving, regular roads for slower driving, parking lots and parking garages. In a typical city, every car has an average of eight parking spaces. The next time you walk to school, look around your neighborhood at all the space cars use. One reason kids in some cities don't ride bikes very much is that there are so many cars. Parents worry that the cars will hit their kids.
Cars also get in traffic jams.
A traffic jam is what happens when lots of drivers want to use the same road at the same time. There isn't enough room for them, so all the cars have to go very slowly. Some people don't think that this is the cars' fault. Whenever a new freeway opens or a popular road is widened, people usually take more trips and do more driving. In a lot of places there just isn't any more room for new roads, even if the government wanted to build them.