Old English rad "riding, hostile incursion," from Proto-Germanic *ridanan, source of Old English ridan (see ride). Also related to raid. In Middle English, "a riding, a journey;" sense of "open way for traveling between two places" is first recorded 1590s. Modern spelling established 18c.
Road test is from 1906. Road hog is attested from 1886; road rage is from 1988. Road map is from 1786; road trip is by 1950, originally of baseball teams.