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

Explanation

When the luggage is placed on the bus roof, it is hot fixed to the bus. When the bus starts suddenly, the luggage due to inertia of rest tends to remain in the state bf rest and as a result, some of the luggage might fall towards the backside of the bus. When the moving bus suddenly stops, due to the inertia of motion, the luggage Inight slide past the bus and fall in the front. To avoid it, uggage is tied up with a rope, so that the luggage becomes part of the bus.