Walking barefoot is a lot cheaper than cars (and not as stupid as it sounds - your feet are made to walk barefoot), but it is so much slower nobody considers it reasonable anymore.
Cars (and by extension the policies that enable car dependence), have an extremely high externalities. They require vast amounts of public spending, lower land productivity, make areas more flood prone, damage the health of those living near car infrastructure, contribute to climate change, are traffic, drive suburban sprawl, and kill more Americans than guns do (crashes alone are just 0.3/100k fewer). The reason 'nobody considers it reasonable anymore' to walk are those policies that build for cars. In places with less car-oriented policies, people walk places. It has nothing to do with maximizing the speed of the vehicle you're using.