In such a situation, the best thing to do would be to, KEEP DRIVING, get on your cell phone, call 911, and report the location of the accident and that an ambulance is needed. Then arrange to meet a cop car ELSEWHERE to make your statement.
Tough one and each situation is obviously unique. As they say, when seconds count, emergency services are minutes (and longer) away from arriving. It also depends on the environment i.e. in this case, in order to drive away the driver might have had to plow through a crowd. Sounds like a no-win situation to me.