I mostly cruise about 13mph; this is because I am old and fragile and don't want to die just yet. In praise of this speed, however, let me point to my constant companion in my automotive research job, the fatality/speed graph:
Note the sharp increase from 20 kph (~12 mph) to 30 kph (~18 mph).
Now this doesn't mean we're risking death -- those numbers are for car collisions, which are more like going full speed into a wall than like falling off a plank, and those folks weren't wearing helmets. But it gives you a good idea of how much the hazard level starts to ramp up as you push toward 20mph. Roughly, energy goes as the square of speed, injury goes as the square of crash energy, and in a crash all that energy has to go somewhere.