First, never assume someone else has called 911. Call them and give them as much information as you can. Location and direction of travel on a given road. When in doubt report the injuries as serious/life threatening.

Second, assess the situation. This coincides with the information that the 911 operator will need. Is just an ambulance needed? Are there spilled fluids, downed power lines or fire hazards that would also necessitate fire equipment? Are the victims easily accessible or are they trapped/pinned? How safe is the situation? Part of assessing may be to keep others away to prevent one emergency from escalating into multiple ones.

Give your name, info and statement to a police officer. If you cannot stay, give that to a witness who can and have them pass that on to the police so they can get your statement later. It may also help to write down what you saw as quick as you can.