Suppose you are driving on the freeway and there is a road that is built on top of the road that you are currently driving on. For example, a transit that takes cars up to their exit or maybe it's just a regular road that was built on top of the freeway. Is there a specific name for such roads? Could I refer to such roads as a "bridge"?
Would it be acceptable to say, "A car was parked underneath the bridge" in such a situation or is there another word for such roads?
Adding to Nathan's answer, the word you're looking for is overpass (or flyover, if you're in the UK and other Commonwealth countries). For example, in India, where I reside, flyover is what we call such constructions where there is a road that passes over another road.
From the Cambridge Dictionary:
overpass and flyover have the same meaning: bridge that carries a road or railway over another road.
The roads aren't really built on top of other roads, which would imply there's no space in between; they're built over them.
And that's the word that's used in the name: "overpass", a road that passes over.
As the other answers say, the more specific terms are "overpass" and "flyover". But it's perfectly reasonable to call such a structure a "bridge". (At least, in British English; the term may be less common in American English and other dialects.)
I feel I should clarify: the road is still called a road, whether it is on a bridge or not. The structure which carries the road can be called a bridge, a flyover or an overpass (as per this question they would all be valid words in this situation). The road itself is continuous, i.e. the road before the bridge, on the bridge and after the bridge is all one road; you do not have a road followed by a bridge followed by a road.
So "A car was parked underneath the bridge" is perfectly valid. "A car was parked underneath the road" would not be easily understood. "A car was parked underneath the bridge which carried the main road" is valid, though it is quite a long phrase!
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".
A specific name for roads that pass over each other is "roads linked up and down". I'm pretty sure that this is British.
