There are three ways security can be implemented; prepare for every eventuality, based upon intelligence and experience prepare for the most likely eventualities, or don't prepare at all. Before 9/11, security within the United States was based on the second and third options. Obviously, it didn't work. Now it's based on the first option, secure from every threat imaginable and hope that any attack gets deterred or caught before any harm is done. A bomb on or in a bicycle isn't an unimaginable danger. They've been used in Ireland, Russia, Sri Lanka, India, Pakistan, Iraq and Afghanistan, the last time I'm aware of was in August in Iraq. In March 2008, a bicycle bomb exploded in front of the U.S. Army Recruiting Station in Times Square in New York City. To the best of my knowledge, no arrests in that case have occurred. It may, no pun intended, seem like over kill to worry about a bomb on a bicycle, but it really isn't.