Old English rad "riding expedition, journey, hostile incursion," from Proto-Germanic *raido (cf. Old Frisian red "ride," Old Saxon reda, Middle Dutch rede, Old High German reita "foray, raid"), from PIE *reidh- "to ride" (see ride (v.)). Also related to raid (n.). In Middle English, "a riding, a journey;" sense of "open way for traveling between two places" is first recorded 1590s. Meaning "narrow stretch of sheltered water" is from early 14c. (e.g. Hampton Roads in Virginia).
Modern spelling established 18c. In 19c. U.S. use, often meaning "railroad." On the road "travelling" is from 1640s. Road test (n.) is from 1906; as a verb from 1937. 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.