It's a Monday morning. There's a tube strike and the bus is absolutely rammed. All the seats are taken and people are cramming into every nook and cranny they can find.
Commuter after commuter climb the stairs searching for a seat — in vain. They slink away back to the shadowy lower deck beneath. Then one brave soul decides to stay there. Sure, there are no seats, but it's a lot less squished than downstairs, he thinks to himself.
At which point the driver plays that message over the tannoy: "Please do not stand on the upper deck or stairs." The commuter retreats back to the sardine-like hell beneath. But why isn't standing on the top deck allowed?
It's more than just sadism on the part of the drivers that makes staying vertical upstairs is a big no-no. Standing on the top deck of a bus in Britain is illegal and it's all to do with physics.
It has to do with a the centre of gravity of a bus — the point at which weight is dispersed and all sides are in balance. On an empty bus, the centre of gravity is quite low, as the bottom part of the bus is heavier.
When people get on the bus, the centre of gravity changes dependent on where they are. If most people are on the lower deck, then the centre of gravity remains low. As more people climb the stairs, the centre of gravity inches up with them. If these people were all standing, the centre of gravity would reach even greater heights.
But what's wrong with a bus having a high centre of gravity?
When a bus is stationary or going in a straight line, absolutely nothing. When it comes to turning, that's a whole other matter. A higher centre of gravity makes a vehicle more unstable. An unstable vehicle going around bends is a disaster waiting to happen; it could easily topple.
Is it likely that one commuter standing on the top deck could make this happen? Not really, the centre of gravity would only slightly rise. If everyone were to simultaneously stand up, however... well, we don't recommend trying it.
So next time the bus driver reminds you not to stand on the top deck, do everyone a favour and listen to the rules.