There&#39;s obviously a lot of variables to have to equalize to get to apples to apples, but this is a decent article comparing the impacts of bikes vs cars, and the number of miles it would take to offset one or the other. <br>

<br><a href="http://www.slate.com/id/2300676/">http://www.slate.com/id/2300676/</a><br><br><br>