A suitcase wheel teaches the difference between a smooth floor and an easy journey.

A wheel works beautifully until it meets a kerb, a cobble, a step or a gap, and then it becomes a weight with a handle. The floor inside a terminal tells you nothing about the pavement outside it. Anything that rolls turns a journey into a series of surfaces, which is exactly the experience of anyone using a chair, a trolley or a pram.

Next time you drag a case across a town, pay attention to where the route stopped working. Those points decide whether a whole set of people can get where you just got, and they are worth mentioning to whoever maintains them.