Nothing more to say than watch and be freaked by the idea of a water moccasin snake on your windshield. The only thing worse than this for me would be if it were inside the car. Mind you, that snake is pretty terrified too. They should pull over and let that thing get off of there.
This snake is obviously NOT a water moccasin. The long slender body and broken pattern suggests rat snake. I suspect a gray rat snake (depending on what part of the country this video was taken).