Policeman's safe cycling demonstration goes very wrong, very fast

You know exactly what's going to happen in the clip below the minute you start watching it.
All the elements are there: the crowd of onlookers, the steep flight of concrete steps, and the policeman in the helmet and the high-vis yellow jacket, who was apparently on hand to demonstrate cycling safety to children. It was only ever going to go one way.
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"Police were staging a safe way for kids to go down the stairs on th