This is due to inertia of rest of upper part of the passengers body when the bus or train suddenly starts moving, the lower part of the passenger's body starts moving along with the bus or train while upper part of the body has a tendency to remain in the state of rest due to inertia of rest and hence falls backward.
Unless a city has the budget and the demand for a set-in-stone rail line, a bus system is the more economical choice. The rail systems of the early 20th century peaked in popularity around 1910, but by 1930, over 230 rail companies had either gone out of business or converted to buses.
Bus companies and paint companies deliberated to create one school bus model that could be used for all districts, and a crucial part of that process was picking a universal color. They, of course, picked yellow – a specific shade of yellow, in fact, which you'll see on all school buses to this day.
Watch YouTube or a MovieWatching movies or a tv show is a relaxing way to spend the duration of your bus trip. Don't forget to bring your headphones so you don't disturb other bus riders!
To avoid swaying, choose a seat close to the centre of gravity of the bus, midway between the front and back wheels. As the bus turns a corner, this point will travel in a smooth circle.
When a bus abruptly starts moving, the passengers tend to slide backward due to inertia, as the passenger's inertia tends to oppose the bus's forward motion. Hence, the passenger tends to fall back when the bus accelerates forward.