Coasting down switch-backs at whatever speed the car traffic on the road feels like, and not giving any car incentive to overtake by matching the speeds.
Traffic regulations are fundamentally about tail events. Most of the time, vehicular traffic is boring.
The right metric is kinetic energy. But, it'd be unpopular for speed limits to depend upon vehicle mass. Or, in the case of bicycles vehicle and rider mass since the rider mass isn't negligible.
However, that shark is jumped as soon as the classification criteria involve operator-dependent properties like maximum speed of a bicycle. It has no operator-agnostic answer.
So maybe we can enforce that part a little better.
If I tried to drive a car on pedestrian path at even 10kmh I'd be chased by police helicopter and appear on national news.