why are passengers in a double Decker bus advised to sit down and not to stand while moving?

Question: why are passengers in a double Decker bus advised to sit down and not to stand while moving?
- Keywords:
  bus,
  movement
Asked by aabb266 to Beatrice, Charles, George, Kamal, Patricia on 17 Jan 2017.
- Kamal Bhattacharya answered on 17 Jan 2017:
  
  because when the bus breaks you may fall and hurt yourself 🙂
  Just to qualify, the fundamental principle is Newtons law that say ever action has a reaction. And in fact, the force at with the reaction is the same as the force causing the action. So when you stand in the bus and the breaks hit (Action), the force stopping the bus abruptly goes directly opposite to the driving direction. You will be pulled towards the driving direction (reaction) and might hurt yourself badly if you don’t hold on to something.

Question: why are passengers in a double Decker bus advised to sit down and not to stand while moving?