This an interesting topic. I definitely believe that 15mph is too slow.
However I have some comments to make:
1. Some people here say they are not signing since they dont feel they need to go faster. Even if I believed that 15mph is enough for me, I would not impose this opinion of mine on others, in other words, I would say, "yes raise the limit to 20 or even 25mph, but I would still keep it to 15mph for myself because thats the speed I feel comfortable at."
Do these same people believe that a fit cyclist who rides a normal bike at 25mph is too fast and dangerous?
2. The lawmaker should explain to us, the public, the logic behind the 15mph limit. Where does it come from? What was the scientific process through which this specific number came into being?