If you allow the user base to report speed limits it would be more accurate than having a static database of speed limits. Also there should be an option for "Variable Speed Limit" for places where the speed limit changes depending on traffic (for example, I-90 and I-5 in Seattle, WA have variable speed limits that range from 30 to 60 mph). There should also be a temporary construction speed limit option in areas where there is construction--you can pop up a verification screen so users to confirm whether the construction speed limit still applies once or twice each day.