Portsmouth City Council implemented a 20 mph limit (through signage alone) on almost all residential roads not part of a trunk network. Most of these roads had median traffic speeds below 24 mph.
Limits were placed on some roads with average traffic speeds above 24 mph to avoid confusion and inconsistency in different speed limits over a short area, An interim evaluation of the implementation of 20 mph speed limits in Portsmouth found a significant reduction in traffic speed on these roads, as well as roads with before speeds under 24 mph, conflicting with current DfT guidance.
Living Streets also provides a descriptive case study on Portsmouth's implementation of 20 mph speed limit. Refer to 20 mph policy briefing document on the Living Streets website for more information.