Is that down a big hill? Naturally local phenomenon can be at any part of a variance, but it seems very odd. Even on a large stretch of leveled road, few bikes get close to 15 mph and the only time I have seen one go faster is from a professional biker, on a racing bicycle, doing a training run, and I would assume that their training paths don't generally include cargo and sidewalks.

The average bike speed, as far as I know, is average 10 mph on city bike roads. Seeing a bike going 20 on a city sidewalk is like seeing a car going 150 on narrow city roads. I am sure it happens, but it is illegal, very dangerous, and not something most people will ever see in a life time. I think it is just fair to treat both forms equal and address it for what it is.