When I build a bike, I have three things in mind. Safety, ease of operation, and Safety! My trikes go much faster than 15 Mph. I like 15-20 Mph as a general use range. In full racing gear I have been almost 40 mph. From an engineers point of view 20Mph is twice as fast as it was designed to go. If you remember the three wheel ATV's of the past they were very unstable. On my bike you can go get a reasonable amount of anything and travel safely. It's not a sport bike, but a way to travel without a lot of work. I am working on a trailer hitch to carry even more. It is a fun safe way to get around if operated within it's limits. Sometimes life is about slow. Can't smell the roses @ 100 Mph.
I don't know. Being in Houston we don't have very many hills. It's pretty strong. I don't think hills would be an issue tho. 
