From Envirowiki

A bicycle is a two-wheeled contraption for transporting humans via pedal-power. It's useful because it allows humans to move around much faster than walking (walking speed is about ~4km/h, but it's pretty easy to go ~30-40km/h on a bike). It can also be modified with racks, baskets, and panniers to allow someone to carry a fairly large load and still move fairly quickly, due to gearing.

Bikes are, outside of construction and maintenance, pretty much greenhouse emission-free. (It would be good if we could get a stat for emissions during construction here^{[reference needed]}). They can potentially last decades if treated well, and are often relatively easy to fix when they break.