The following are post roads: (1) the waters of the United States, during the time the mail is carried thereon; (2) railroads or parts of railroads and air routes in operation; (3) canals, during the time the mail is carried thereon; (4) public roads, highways, and toll roads during the time the mail is carried thereon; and (5) letter-carrier routes established for the collection and delivery of mail. ( Pub. L. 91–375 , Aug. 12, 1970 , 84 Stat. 766 .) Statutory Notes and Related Subsidiaries Effective Date Section effective July 1, 1971 , pursuant to Resolution No. 71–9 of the Board of Governors. See section 15(a) of Pub. L. 91–375 , set out as a note preceding section 101 of this title . CFR Title Parts 39 601