From the Super Mario Wiki
Interstate 310 is a highway that crosses through most of Diamond City. Several notable buildings, including Wario's House, Diamond Taxi, and Toy Express, are located on the highway. Despite this, the highway itself is never really interacted with; its most notable appearance is in WarioWare: Twisted!, when Dribble and Spitz are driving down it when their taxi breaks down. In WarioWare: Smooth Moves, 18-Volt crosses it to get to a game shop.