Originally Posted by ElJamoquio

Density has nothing to do with stiffness. Look at Mercury and Boron/Carbon. Even if you had looked up the correct numbers, though, it wouldn't matter - 3.3Xinfinity = infinity. The surface doesn't deflect, your shoes and body does.

So concrete deflects .001" and asphalt .002", while your shoe deflect .200". No difference from a stiffness perspective.

From a ease of running... sidewalks are a pain in the ass to run on because of curbs, driveways, expansion joints, etcetera.