Md. Code, Local Gov't § 12-618

(a) The County Commissioners of St. Mary’s County may regulate bridges in the county that are not in a municipality and that have not been designated or maintained as a part of the State or federal highway system relating to: (1) vehicle weight; (2) vehicle parking; (3) vehicle abandonment; (4) building or maintenance by a private or public utility relating to the performance of the utility’s purpose; (5) the building or maintenance of driveway connections where connections are provided; and (6) vehicle speed. (b) If required by an ordinance enacted under subsection (a) of this section, the County Commissioners of St. Mary’s County shall provide appropriate traffic control devices. (c) For a new bridge, the County Commissioners of St. Mary’s County may regulate relating to: (1) engineering; (2) construction; and (3) acceptance of the bridge into the county road system.