In the case of sidewalk vs rad vs road, you're making the wrong argument.
We do not have "rads" anymore, we have "roads." "Road" is derived from "rad" but they are two different words with different meanings.
A sidewalk is a walking area on the side of a road, and that meaning of the word has not changed. Bikes, for example, or prohibited from sidewalks in most Western jurisdictions (lack of enforcement notwithstanding), and essentially all Western jurisdictions prohibit motorized vehicles from sidewalks. In both cases, it is because the sidewalks are meant for "pedestrians", i.e., people walking.
