It's 100% fake and even from looking at it a completely unoriginal idea, Enclave in San Fran? The Legion won? Why is there a D.C. roadsign in San Fran? We know the Shi's logo yet there was not even a teaser. Very basic and from an unimaginative mind.
Oh and no mention from official Bethesda sources even though the trailer is months old?
The only sign with D.C. on it was the road sign with San Francisco, Washington, and Vegas mile markers, which would make sense considering the locales of the last games. In other words, if this was made by Bethesda, that sign in a trailer ( not ingame ) makes a whole lot of sense.
And Bethesda saying nothing about the trailer points more to it being real than it being fake. If it was fake, I'm sure they'd say "That's fake, guys.". But they haven't.
Also, I see nothing there indicating it's done in Gamebryo and not the new Creation engine, as there's really nothing there outside of one car and some signs. The cars will most likely look the same regardless which engine they're in, as they're art assets. And I don't forsee Bethesda changing the actual art of the game.
So until Bethesda says something, I will consider this trailer to be as real as any other until Bethesda either says it's fake or the supposed faker admits it.