While driving I've noticed that there are many roads in the game world that are blocked off by floating stop signs. The roads also don't show up on the GPS. For example (click for larger versions):
Those barriers are completely impassable for the player, but not for AI cars:
What is the purpose of these barriers and the inaccessible roads? Will they open up later in the game, or perhaps in a future patch? Are they just places for the AI cars to spawn/despawn without the player seeing?