I think at 12:20 in the morning, where it reasonably appears that the other party doesn't need medical assistance, one is entirely justified in not sticking around to see if he wants a piece of you for knocking him off his bike. If you are worried about running out on the situation, call the police. Explain what happened, why you left (fear for your safety) and how you can be reached. Or, offer to come to the station or meet the police at the scene if they want you to. I think that covers you just fine.