Is riding a bike good for the environment?

If you normally drive, and then switch to riding a bike, you have just reduced the emissions produced by cars, decrease congestion and the need for a parking space. It seems like a clear win.
But what if you switch from riding the bus to biking? The bus will still run whether you are on it or not. The change in a single rider will probably not cause the addition or subtraction of bus trips. If the bus is already stopping at your stop, your boarding will only add a minuscule amount of time. The extra energy needed to add you to the bus is extremely small. (Would the marginal emissions be smaller than your additional emissions from biking? Almost surely if it is an electric bus powered by hydro. Possibly even for diesel buses.) If it is a school bus or you already have a bus pass then there is not even cost savings. A single day switching from bus to bike may help you get exercise, but not do much to help the environment. Biking may also be a little more costly, but allow for greater flexibility, and be more enjoyable. (And this all assumes you are switching from taking the bus... If the alternative is driving then biking wins hands down.)