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.)