By that same argument, why do taxes collected nationally have to be spent on roads? (Presumably you bike on roads, right?) Because transportation is vital to the health of the economy. Bicycles are just another transportation option in a menu of options that include buses, subways, light rail, trains, airplanes, trucks, and cars. Oh, and your own two legs. If we don’t have an efficient way to move workers and goods around through a variety of methods, then our economy comes to a halt. Your tax dollars, and mine, go in part toward ensuring that we can move efficiently around the country and obtain the foods, goods, and services we need to maintain our privileged lifestyles.

And one important fact that often gets lost in the shuffle — no project is funded 100% through federal tax dollars. There is always a local match required which comes from taxes collected locally, not to mention private sector funds from foundations and corporations.