Final answer:
Federal laws, particularly the Federal Highway Act of 1956, govern on/off ramps on interstates, while states have concurrent powers to enforce traffic laws as long as they do not conflict with federal regulations. States also have reserved powers for intrastate legislation, but federal law or constitutional rulings can sometimes override state laws.
Step-by-step explanation:
The regulations surrounding on/off ramps on interstates are governed by federal laws, specifically the Federal Highway Act of 1956, which led to the construction of the federal interstate system. These laws set the framework for the creation and maintenance of interstate highways, including the design of on/off ramps. States, counties, and other localities do have the authority to enforce traffic laws on these interstates, as long as those laws do not conflict with federal regulations. This is reflective of the concurrent powers shared between the state and federal governments, which include the responsibility for building and maintaining transportation infrastructure.
However, states retain reserved powers to pass intrastate legislation on particular aspects such as commerce and transportation within their borders. In some cases, judicial rulings at the federal level, based on federal law or constitutional interpretations, may supersede state legislation—such as the landmark case Obergefell v. Hodges regarding marriage equality. This highlights how the interstate commerce and full faith and credit clauses of the U.S. Constitution play a critical role in balancing power between federal and state jurisdictions.