It works like this: you ring your bell at other cyclists.
The official website explains all: “The idea is to create a fun experience for cyclists and locals in and around London, with the hope of establishing more of a sense of community amongst fellow cyclists.”
So, if you don’t have a bell, maybe you should think about getting one. It might not suit your full-carbon road bike, but hunt around; there are some cool models out there.
Of course, critics will say that a sense of community would be more easily achieved by conversation; at traffic lights, the office bike shed or on club rides, rather than ringing a bell at each other from a distance. Who knows?
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