Portland, Oregon is one of the most bicycle friendly cities,
and should be an inspiration to other cities that are trapped in the
addiction of antiquated petroleum based transportation methods.
The bicycle is the most energy efficient transportation (barring walking)
on the planet.
0 (that's _zero_ )emissions come from using a bicycle.
There are some places where it is not good to ride bicycles, where nature trails
create erosion in natural settings, and disrupt the environment-- it is good
to be 'concious' of even environmentally friendly technologies.
But for a city dweller, and many people who have short commutes, bicycles should be used if possible.