1. You need make immediate stop without blocking the traffic. However, if the accident is serious, forget about the traffic.
2. Get other parties information.
3. Dont chase the other party.
4. 911 is for emergency. 311 is for none emergency police assistance. In your case, 311 is adequate.
You can file a police report and same time contact the bus company and look for the bus. At specific time, specific intersection, it is easy for them to figure out which bus possible cause the incident. If the bus has scrach then the case closed.