Mo. Rev. Stat. § 234.180

The state highways and transportation commission of the state of Missouri is hereby authorized and directed to include as a part of the state highway system the whole of any toll bridge across any river forming a common boundary to this state and any other state or states, now or hereafter constructed, maintained, operated or owned by any person, firm or corporation, and to construct, maintain and operate, as a part of said state highway system, an approach within the boundaries of the state of Missouri to such bridge and highways connecting the same with established highways.