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Traffic signals direct the flow of traffic, usually at a road junction, a pedestrian crossing, or a level crossing. They are primarily installed in order to minimise congestion and improve road safety, and have a key advantage over other engineering solutions such as grade separation as being comparatively cheap and compact. They have been commonplace in the UK and Ireland since the first modern-style automatic system was introduced in Princes Square, Wolverhampton in November 1927.
In the United Kingdom – as well as in Austria, Belarus, Bosnia and Herzegovina, Bulgaria, Croatia, the Czech Republic, Denmark, Estonia, Finland, Germany, Hungary, Iceland, Latvia, Lithuania, Macedonia, Malta, Moldova, Montenegro, Norway, Poland, Russia, Serbia, Slovakia, Slovenia, Sweden, Switzerland, and Ukraine – a standard four-phase signal sequence is used consisting of the consecutive display of green (proceed), amber (stop, unless it is unsafe to do so), red (stop), and red-and-amber lights (stop, but prepare to proceed). In other European countries the red-and-amber phase is omitted, the lights changing straight from red back to green.
The standard sequence became so well known following its introduction that by the end of the 1920s, when light-controlled pedestrian crossings were being installed in Manchester, the phases were not documented as it was assumed the public would understand them. The Metropolitan Police, in endorsing the design, pointed out that the simple colour-light scheme could be understood without any knowledge of English, and would therefore be of benefit to foreign drivers.
Modern traffic signals evolved from the coloured-light indications which accompanied railway semaphore signalling, introduced in the 19th century. Red was used to indicate "stop" as it was already widely known to represent danger.
Traffic signals were regarded as something of a necessary evil during the early to mid 20th century, but by the 1960s, the need to install more signals at pedestrian crossings was becoming more urgent. The first attempt at this was the Panda Crossing, introduced by then transport minister Ernest Marples in 1962, but this suffered criticism for being over-complicated and confusing to users. Consequently, a new design, the now familiar Pelican Crossing, appeared in 1969.
Toucan Crossings followed in the 1990s, for pedestrians and cyclists, so called because "two can cross".
In the 2000s, a new type of pedestrian controlled signal, the Puffin Crossing started to be installed in favour of Pelican Crossings. The principal difference between the two is that the control and the signal is on the same unit facing the user. Also includes on-crossing detectors to reduce unnecessary delays.
In May 2015, Queens Circus Roundabout in Battersea was reconfigured to include a fully signalised cycle route with special cycle aspect signal heads mounted at cyclists eye level. These heads have subsequently been rolled out on some of London's cycle superhighways, where examples can be seen on Farringdon Street.
SGE were one of the most significant manufacturers of traffic signals from the 1930s onwards. Their design could generally be identified by the text "STOP" on the red signal, and completely separate housing for each lamp. Another major manufacturer at the time was ATE/ATM, who also began installing signals in the 1930s.
Traffic signals took on a more modern appearance with the introduction of the Mellor head from 1970, and this rapidly replaced the SGE design, including a complete retrofit on pelican crossings in 1973. A key advantage of the Mellor design was that it allowed easy installation of fixed-phase timing, whereas previous installations tended to be vehicle activated. Mellor heads were commonplace until the 2000s, when newer signal heads with modular designs started to replace them.