Explanation

"In some built-up areas, the speed limit may be reduced to 20 mph. Where there are traffic-calming features, the speed limit may be signed as a 20 mph zone with no repeater signs within the zone. Where there are no traffic-calming features, the 20 mph speed limit is signed at each end of the street and with repeater signs in between. Driving at a slower speed will help give you the time and space to see and deal safely with hazards such as pedestrians and parked cars."