if you're commuting on roads, it's all about pace for me. my commute is about 12 miles and i average around 16 mph. my route has a little of every thing as the longest single segment is only 1.5 miles. a couple segs are less than 1000 ft. i have a hill on the way home that gets me 30 mph, but the same hill on the way to work brings me an 8-9 mph. i have about a mile of bike path and some bumpy ass residential streets as well. throw in a handle full of potential signalized intersections and a long flatter sloped inclines. i'm happy with a 16 mph average, but everyday i work harder to get that up. more of a self challenge than a need. so all in all, everyone that has answered is right.
what's a good speed? if you're going as fast as you can go, it's a good speed. in a week, your good speed will change.