Each grantee shall provide and maintain in connection with the zone—
(b)
Adequate transportation connections with the surrounding territory and with all parts of the United States, so arranged as to permit of proper guarding and inspection for the protection of the revenue;
(f)
Adequate enclosures to segregate the zone from customs territory for protection of the revenue, together with suitable provisions for ingress and egress of persons, conveyances, vessels, and merchandise;
(June 18, 1934, ch. 590, § 12, 48 Stat. 1001.)