Of course as things stand you can go 20 mph, you just have to do it under your own steam, like the 'ordinary' cyclist. I've had my 2006 Giant Twist Comfort up to 41 mph... it helps to have a long, steep hill to ride down though. Since electric assist bikes are almost always at least slightly heavier than non-power assisted bikes, they'll require more effort to go over 15 mph on the flat or up hill than comparible non-power assisted bikes but rider fitness has alot to do with things. Many years ago my brother won the local community bike race on my old 3 speed hub geared bike - he was competing with many riders on sleeker, more high-tech 10 speed bikes (told you it was a long time ago). I've been overtaken a few times by riders without power assistance, once by a lycra-clad rider on a racing style bike going up a hill. Since we can use shared use and cycling lanes, it's important that our electric assist bikes will blend in with MOST cycles and don't terrorise pedestrians we cycle by - a 15 mph top powered speed helps to achieve this.