Why Bikes Are a Sustainable Wonder goes over some aspect of bicycling which are very positive. Riding a bicycle uses minuscule amounts of energy compared to any other form of transportation. Riding a bicycle gives exercise and promotes better health. Bicycling uses less land area, leaving more land available for other things such as houses or farms.
In the U.S. cars are so popular that they're basically crowding themselves to a standstill. This standstill is otherwise known as gridlock. It's very hard to achieve gridlock with bicycling.
A lot of talk is made about cars powered by alternative fuels, like ethanol, biodiesel, electricity, hydrogen, etc. But changing the fuel in cars, while it would help the environment in one way, does nothing to solve other transportation problems. Gridlock will remain just as bad as always, and horrible traffic has many followon effects such as promoting road rage, and the rage you experience on the road often causes undesirable effects at home or at work.
A lot of talk is being made of obesity and the dangers of sedentary lifestyles. Riding a bicycle counteracts sedentarism, and promotes better health.