The Red Bridge is a small scale bridge located south of the Vice City Junkyard and the Haunted Haitian Drug Factory. It was created by the developers for unknown reasons. The bridge caught attention particularly due to the rampage and red balloon that can be found near the bridge.
Astonishingly, the corrupt Vice City Port Authority is also located right next to the bridge. Due to this fact, players have theorized that the bridge could be used by the Port Authority to sneak into the city via the drug factory to smuggle drugs, which could also be the potential purpose of the bridge. Little Haiti is also known to get less police attention in the city, adding some weight to the theory.
This hasn't received any official confirmation, thus restricting it to just a theory. A more rational explanation would be that the developers wanted to shorten the path in this area, as the player cannot get into the rear of Little Haiti if they are in the southern region.