With the recent article claiming that (paraphrasing) cyclists are part of a communist plot to turn the country into a third world country, I thought I would list the reasons why it makes sense from a variety of political sides to support cycling and walking. Sure, that article was extreme, but more moderate variants of such are not unheard of.
So here is my personal take on why each supporters of each political philosophy should support cycling and walking.
Conservatives should support biking and walking because first, it is individualistic, where the person takes control of his/her destiny in travel and is not at the mercy of others. Second, the savings in gas are good for the country. It's foolish to be sending billions of dollars abroad to countries that we don't like much, and where they don't like us much either. Being dependent on foreign oil is a national security threat. And finally, having a fit population helps keep the country strong - if needed, a fit populace can be more readily trained to be soldiers than an unfit populace. A good environment is something that the country can showcase to the world.
Libertarians should support biking and walking because they provide freedom. "Freedom" to travel is illusory if one is forced to do so via car; people should have real choices. All modes should get to compete on an equal and fair basis.
Finally, liberals should support biking and walking because such leads to a greater sense of community, as well as being beneficial to the poor. Imposing added transportation costs onto the poor hurts the most vulnerable because they can least afford it. The environment belongs to everyone, so it's only fair that people try to care for it.
In terms of projects, the arguments typically raised against them are generally that they are a waste of money since the usage levels will be too low. Depending on the project in question, such may or may not be the case. Efficient allocation of resources would dictate that projects tie together into a network with few breaks - the network will increase usage because only then does the number of possible trips increase exponentially. Use of a project should be monitored at varying times of day. While sidewalks often seem deserted around noon in the summer, they often host joggers in the mornings, for instance. Road shoulders can be easily converted to bike lanes, and the shoulders themselves are plenty useful for road maintenance and for situations such as breakdowns. But yes, not all roads need sidewalks and/or bike lanes; it depends on the traffic speeds and volume.