I go 40 km/h regularly on one of these things, it feels slower than it sounds. I've also fallen at that speed, wearing no protective gear other than a helmet. Not the end of the world, no different than when I fell off a roadbike going roughly the same speed.

If you're worried about scrapes throw on a motorcycle jacket. But if you know how to fall, 40-50km/h is manageable without gear -- aside from the mandatory helmet.