Was in the car when the woman in front who was driving punched the guy in the passenger seat.
Thought it was probably them messing around until she did it a second time and then a third time where she proceeded to try and punch him about 4 times while driving.
The guy was right against the door/window with his arm across his body to protect himself. He didn't attempt to stop her or retaliate.
Absolutely nothing I could say or do while driving behind them 
Made me wonder if I would have been brave enough to say anything if not driving.
It is better to say something so that the person being hit knows there is help and the person doing it knows others are disgusted with their actions and risk getting hurt or just walk on and dial 999?