In Colorado, for roads that are not posted, speed limits are:
20 mph on narrow, winding mountain roads
25 mph in business districts
30 mph in residential districts
40 mph on open mountain highways
Speed limits should not be set higher or lower than these limits unless a traffic study has been performed to justify the change. The study includes an analysis of roadway conditions, accident records, traffic speed counts. A safe and reasonable limit is established at or below the speed at which 85% of drivers travel.