Va. Code § 46.2-881
It shall be unlawful to drive any motor vehicle, trailer, or semitrailer on any public bridge, causeway, viaduct, or in any tunnel, or on any interstate at a speed exceeding that indicated as a maximum by signs posted thereon or at its approach by or on the authority of the Commissioner of Highways.
The Commissioner of Highways, on request or on his own initiative, may conduct an investigation of any public bridge, causeway, viaduct, tunnel, or interstate and, on the basis of his findings, may set the maximum speed of vehicles which such structure or roadway can withstand or which is necessitated in consideration of the benefit and safety of the traveling public and the safety of the structure or roadway. The Commissioner of Highways is expressly authorized to establish and indicate variable speed limits on such structures or roadways to be effective under such conditions as would in his judgment, warrant such variable limits, including but not limited to darkness, traffic conditions, atmospheric conditions, weather, emergencies, and like conditions which may affect driving safety. Any speed limits, whether fixed or variable, shall be prominently posted in such proximity to such structure or roadway as deemed appropriate by the Commissioner of Highways. The findings of the Commissioner shall be conclusive evidence of the maximum safe speed which can be maintained on such structure or roadway.
Va. Code § 46.2-881