ocrat ization ▁of ▁travel ▁occurred ▁during ▁the ▁early ▁tw enti eth ▁century ▁when ▁the ▁autom obile ▁revolution ized ▁travel . ▁Similarly ▁air ▁travel ▁revolution ized ▁travel ▁during ▁ 1 9 4 5 – 1 9 6 9 , ▁contrib uting ▁greatly ▁to ▁tour ism ▁in ▁the ▁United ▁States . ▁P urch ases ▁of ▁travel ▁and ▁tour ism - related ▁goods ▁and ▁services ▁by ▁international ▁visitors ▁travel ing ▁in ▁the ▁United ▁States ▁tot aled ▁$ 1 0 . 9   b ill ion ▁during ▁February ▁ 2 0 1 3 . ▁ ▁The ▁travel ▁and ▁tour ism ▁indust ries ▁in ▁the ▁United ▁States ▁were ▁among ▁the ▁first ▁economic ▁se ctors ▁neg atively ▁affected ▁by ▁the ▁September ▁ 1 1 , ▁ 2 0 0 1 ▁attacks . ▁ ▁In ▁the ▁U . S ., ▁tour ism ▁is ▁among ▁the ▁three ▁largest ▁employ ers ▁in ▁ 2 9 ▁states , ▁employ ing ▁ 7 . 3   mill ion ▁in ▁ 2 0 0 4 , ▁to ▁take ▁care ▁of ▁ 1 . 1 9   b ill ion ▁tri ps ▁tour ists ▁took ▁in ▁the ▁U . S . ▁in ▁ 2 0 0 5 . ▁As ▁of ▁ 2 0 0 7 , ▁there ▁are ▁ 2 , 4 6 2 ▁registered ▁National ▁Historic ▁Land marks ▁( N HL ) ▁recognized ▁by ▁the ▁United ▁States ▁government . ▁As ▁of ▁ 2 0 1 8 , ▁New ▁York ▁City ▁is ▁the ▁most ▁visited ▁destination ▁in ▁the ▁United ▁States , ▁followed ▁by ▁Los ▁Angeles ,