Going 30 on anything that resembles a bike in geometry and construction is not a leisurely activity one engages in casually. 40 is pushing it, 50 you know is unsafe. 70 is a publicity stunt in a controlled environment. Yes, I know road bikes (with or without electricity) hit high speeds in certain situations, but those situations are away from traffic or potential traffic, a big downhill in the middle of nowhere or a race course or something like that.

This is a self solving problem. It doesn't need a law. Nobody is going lightening fast on anything that resembles a bike except in the few situations where a) they can b) doing so is less sketchy than not (e.g. going faster than the bike "likes" is can be less sketchy than going too slow for traffic).

The fact that there is a huge pent up demand for "totally not a moped" type stuff really goes to show you the degree to which not laughing these people out of town has already perverted the personal transportation market.

Edit: The above are Philly speeds. Add 5-10mph for Miami or somewhere else wide, smooth and flat with great visibility. But the cars are going faster there too and the pedestrians are fewer and/or more removes so I don't think it actually matters.