Why is it advised to tie any luggage kept on the roof of a bus with a rope?

Solution:
Luggage on the top of the bus is a loose fixture and not a compact part of the bus. Thus, when a speeding bus brakes suddenly, the luggage continues moving forward because of inertia of motion and is likely to fall off the bus. Conversely, if a stationary bus accelerates suddenly, the luggage continues in the same state because of inertia, and hence is left behind relative to bus, such that it falls backward. To avoid the falling of luggage, it is tied with a rope.