Here are some quotes from the key:level page of the OSM wiki:

The level=* key indicates the floor level where a feature (such as an amenity or a shop) is located in a building.

Highways (and other ways) can be also tagged with level=* when they are essentially bound to a floor of a building complex.

But for typical crossings of roads, waterways or railways via bridges or tunnels, layer=* should be used instead. 

Roads or other ways passing through buildings should be marked with tunnel=building_passage. They should be only marked with level if they allow access to a specific level of the building. 