Stop!
File a criminal report.
In most States not stopping at the scene of an accident, as a witness or participant, constitues a Hit and Run.
As such, your insurance agency may cover the full expense, including deductible, as this is a criminal offense. Did you get any information on the vehicle such as plate, make, model, year, color etc?
You do not have to be physically involved in an accident to be at fault and you are required to remain at the scene, exchange information etc. Anything less constitutes a Hit and Run, or at the very least, Fleeing The Scene.
Call the cops!