In the UK Highway Code, a built-up area is a settled area in which the speed limit of a road is automatically 30 mph (48 km/h). These roads are known as 'restricted roads' and are identified by the presence of street lights.
Built-up area (Highway Code) Wikipedia
In August 2006, the Department for Transport issued new guidance relating to speed limits. It states that:
Section 82(1)(a) (of the Road Traffic Regulation Act 1984 (RTRA 1984)) defines a restricted road in England and Wales as a road which is provided with “a system of street lighting furnished by means of lamps placed not more than 200 yards apart”. Section 81 specifically makes it an offence for a person to drive a motor vehicle at a speed of more than 30 mph on a restricted road.
"Street lighting" is further defined as follows:
As set out in paragraph 45, it is generally recognised that a ‘system’ of street lighting could be three or more lamps spaced not more than 183 metres apart. However, street lighting (for the purposes of determining whether or not a road is a restricted) is not necessarily limited to street lamps, but may extend to lighting provided by authorities or parish councils.
To avoid confusion that could arise if some 30 mph zones were 'street-lit' and some were not, DfT guidance goes onto say:
Direction 11 of The Traffic Signs Regulations and General Directions 2002 (TSRGD 2002) defines the requirements for the placing of speed-limit repeater signs. This states that speed-limit repeater signs cannot be placed along a road on which there is carriageway lighting not more than 183 metres apart and which is subject to a 30 mph speed limit. The Department will not make exceptions to this rule.
This leads to the conclusion that:
This means it should be assumed that, unless an order has been made and the road is signed to the contrary, a 30 mph speed limit applies where there are three or more lamps throwing light on the carriageway and placed not more than 183 metres apart.
Most road traffic law pertaining to speed limits is contained in the Road Traffic Regulation Act 1984 (RTRA 1984). Other relevant legislation includes the Highways Act 1980, where Sections 90A-F cover road humps and Sections 90G-I cover other traffic-calming works. Part VI of the RTRA 1984 deals specifically with speed limits, with Sections 81-84 dealing with different speed limits and the speed-limit order-making process.