英语词源字典 - 在线词源网站

roundabout (adv.)[roundabout 词源字典]: "by a circuitous route," mid-14c., from round (adv.) + about. As an adjective from c. 1600. Noun sense of "traffic circle" is attested from 1927.[roundabout etymology, roundabout origin, 英语词源]
traffic (n.): c. 1500, "trade, commerce," from Middle French trafique (15c.), from Italian traffico (14c.), from trafficare "carry on trade," of uncertain origin, perhaps from a Vulgar Latin *transfricare "to rub across," from Latin trans- "across" (see trans-) + fricare "to rub" (see friction), with the original sense of the Italian verb being "touch repeatedly, handle."

Or the second element may be an unexplained alteration of Latin facere "to make, do." Klein suggests ultimate derivation of the Italian word from Arabic tafriq "distribution." Meaning "people and vehicles coming and going" first recorded 1825. Traffic jam is 1917, ousting earlier traffic block (1895). Traffic circle is from 1938.