A typical geared MA60 column on the A12 in East London. These are now being replaced with iridium based lanterns.
Street lighting is exactly what it says on the tin. Streets are lit for a variety of reasons, the primary two that are cited are to improve road safety and reduce the fear of crime.
Historically using torches, then gas lamps, modern electric street lighting has been available since the late 19th century. Technology has made street lights automatically turn on at dusk and off at dawn, and to protect the circuit against individual failure.
There are numerous type of street light design available and these are generally chosen by a lighting engineer at the relevant highway authority. Since the late 1980s, the trend has been away from SOX (the yellow coloured) lighting and towards 'white' light, such as SON and Metal Halide. Recently LEDs have become commercially viable and are being trialled in numerous locations with mixed results. Various brackets have been used for mounting, originally using metal, then concrete principally throughout the 1960s and 1970s, to more flexible modern designs.
Streetlights are also what determines a speed limit on the UK road network. A system of lighting means the default speed limit is 30mph unless signs state otherwise.
The early motorways such as the M1, M6 Preston Bypass and the M62 over Saddleworth Moor were mostly unlit, as street lighting was (and, indeed, is) not designed to replace headlamps, but to illustrate other hazards. The first main stretch of motorway to open fully lit was probably the M4 from Chiswick to Langley in West London in 1965.
By the end of the 1960s, a series of fog-based fatalities resulted in lighting being retrofitted throughout the existing motorway network. However, by 2011, a number of motorways, such as the M58, M65, and M66 were having lighting removed on environmental and cost grounds.
The following terminology can be encountered when discussing street lighting.
Catenary lighting has lanterns suspended from an overhead wire column, mounted on regular posts. It was popular in the 1960s and 1970s, and could be seen on the M1 through the home counties and the western section of the M25 around Heathrow. It fell out of favour due to being incredibly difficult to maintain and having a poor spread of light. Very few installations are left. Some catenary lighting can still be seen in London, such as the A3200 Southwark Street and the A4 between the Hogarth Roundabout and the M4.
Though steel was a popular choice for mounting columns, new postwar regulations in the late 1940s meant that other materials had to be used. Concrete was regularly used from the 1960s through to the 1980s for the base of a street lantern. Beyond this, it has gradually been phased out, although a few exceptions, such as West Sussex County Council, continued to install new concrete columns up to the 1990s. Concrete is starting to disappear completely now; North Yorkshire County Council went through an extensive replacement scheme in 2010.
MA lights use low pressure mercury as their principal element. They were considered problematic due to inefficiency with lighting, with very bright light that made it hard to see pedestrians, and consequently are now considered obsolete, and generally being replaced with SON lanterns instead.
SOX and SON
SOX (Sodium with OXide) and SON (variant on "Sun") refer to the type of element used in the filament in a sodium based lamp. At a particular frequency, sodium vapour emits a very strong light, which makes it ideal for street lighting. The two can be distinguished by the colour of the emitted light - SON emits peachy orange, while SOX emits a plain yellow. One of the problems with SOX is that the sodium eventually reacts with the material of the lantern container, which ultimately results in its failure.
SOX was used for low-pressure lanterns and contained a mix of solid sodium with argon and neon, giving off an amber coloured light. They first appeared around 1964 and were widely used until the 1990s. SON refers to a newer type of high-pressure sodium that contains mercury. SOX's colour spectrum makes it ideal for road lighting as it is non-fatiguing for human eyes and penetrates fog and mist more effectively than SON and other types. It is also considered more environmentally safe as its emissions are easy for astronomers to filter out, and are less likely to disturb other life. However, it is SOX's colour that has caused it to go out of favour by lighting professionals. Gradually, SOX lanterns are being "end of lifed" and replaced with SON, though some councils still specify SOX for casual replacements and some new installations.
Both SOX and SON lighting is becoming obsolete due to recent innovations in LED technology. The use in street lighting is still fairly new, but a common theme on all designs is avoiding the distinctive colour of SOX lighting for a shade of white. Notable installations include the M4 through Berkshire, the M3 south of Winchester and the M6 through Birmingham.
The articles below give further technical details about street lighting.