I would call 9-1-1 immediately. No way am I going to drive off and risk getting found out later on and probably getting a worse punishment. It's always better to tell the truth, no matter how difficult or scary it is.
i wouldn't have to "turn myself in" because i wouldn't have committed a crime. i would call the police and let them know what happened though. then i would do whatever was needed to make sure that i wasn't wrongly procecuted for it.
I would call the police immediately, and I would feel horrible for taking someone's life--but I would have to own up. It isn't about getting away with something--the guilt would follow you everywhere and it would be right to report it.