Some sound advice already posted here. There are some points I would like to emphasize:

First, DON’T put yourself in a situation where you could become another victim and make the situation more complicated. This should be common sense but you would be surprised how many people allow “tunnel vision” or “the candle moth syndrome” to affect their decision making process and put themselves and others into life threatening situations because they want to help. I have seen just as many trained rescuers without experience do this as laypersons. The best advice I can give anyone is STAY OUT OF TRAFFIC! Time after time I see bystanders trying to direct traffic or standing in moving traffic lanes as cars wiz by at highway speeds. Don’t forget that even HUGE fire apparatus lite up like Time Square parked on roadways get hit regularly. You safety is your priority… not the victims already involved in the incident.

Second, as anyone who has taken CPR knows, early activation of the 911 system is the key step in any emergency to assure the proper response gets dispatched and on the way quickly. No matter what tools or training you have, even if you are an on duty fire fighter/EMS worker/PD officer notifying the call center that there is an emergency is your first priority. Before you call 911, take a deep breath and calm down. A frantic 911 with (frequently wrong) spotty information can cause more confusion. By calming down and observing what is going on you can give the 911 operator a clear view of what is going on allowing them to make the right dispatch decisions. If you keep driving while you call 911, report where the accident happened not where you are 5 minutes later when the operator ask you (seriously, this happens ALL the time).

Lastly, sometimes your best action is to do nothing but make the 911 call. As someone else noted, know your limitation. What level of training do you have, what tools do you have with you, are you trained to use those tools you have? I know that is not in the ethos of the member’s of this forum, but sometimes you are going to be ill equipped and ill prepared to do anything but make a situation worse. The hard part is to know when to do something and when to just make the call. Taking that deep breath and staying calm can help you make that decision based off of facts not emotions.