Chapter 70:01:02:05 – Codington County speed zone. A 50 miles per hour (mph) speed zone on State Trunk Highway 81 in and around Watertown is being reduced to 45 mph, and this 45 mph speed zone will extend further south on State Trunk Highway 81. This reduced speed zone coincides with the construction of a roundabout at the intersection of U.S. Highway 81 and 20th Avenue Southeast in Watertown and will enhance the safety and function of the highway.