I normally average 15-16 mph on rides. If you have a lot of stop signs or red lights, it'll cut your speed down a fair bit.
Top speed- I don't often think to check, you know? I've hit over 40 mph on long downhills, hit 25-35 mph or so on flat straights with no wind. It depends a lot, if you know you need to keep riding, you have to leave a little in the tank.