20mph (~30km/h) in my state, 28mph (~45km/h) if it has a speedometer. If it assists above that, you need it to be registered (license plate) and have a motorcycle license.
I think that's fair. That basically restricts you to driving on side streets with cars, or on the side of the road on higher speed roads. Most shops only sell the 20mph bikes, so it's really a non-issue.