In the United Kingdom, a circular sign displaying “30” imposes a maximum speed of 30 mph. The red border identifies the number as a mandatory limit rather than advice.
A 30 mph limit is common on built-up roads, especially where pedestrians, homes, shops or junctions create greater risk. The limit applies until another speed-limit sign appears or a different national limit becomes applicable.
Drivers sometimes confuse a speed-limit sign with a speed-camera warning or an advisory-speed plate. A red-bordered circular number is legally enforceable; a rectangular sign or electronic display may provide information or a temporary instruction instead.
The United Kingdom uses miles per hour on its road signs, unlike most European countries, which use kilometres per hour. A nearby question many readers ask is whether street lighting automatically means 30 mph: in England and Scotland, it traditionally indicates a restricted road, but local signs and regulations determine the actual limit.