If the upper road goes in the same direction (or the 180° opposite direction) as the lower road, it is a "viaduct". For example, the "Alaskan Way Viaduct" in Seattle is a "double-decker freeway". Both decks are actually above ground, because the ground level is a parking lot. Thus, there are "cars parked under the viaduct".
This word "viaduct" is similar to the word "aqueduct". Many Roman "aqueducts" had portions that were elevated, using "viaducts".
Multi-level intersections often include "overpasses" (where one road rises up to cross another); sometimes they include "underpasses" (where one road drops down into a tunnel to cross another road).
In the Seattle area, some freeways are below ground, and have ceilings. Above the ceilings are parks, roads, and/or convention centers. These freeways are said to have "lids", as in the "Mercer Island lid". When traffic is bad, sometimes "cars are stopped under the Mercer Island lid", or "there is stop-and-go traffic from the Mount Baker Tunnel through the Mercer Island lid".
In the San Francisco Bay Area, the western span of the "Bay Bridge" has two levels of traffic. Cars go west on the "upper deck" from Yerba Buena island to San Francisco; cars go east on the "lower deck".