The evolution of the traffic light can be traced back some 200 years, but before becoming the reliable system we know today it has had to recover from a number of design flaws and evolutionary dead-ends – as well as all-out explosions. In this exclusive extract from his new book Traffic Signals, Alistair Gollop (pictured), senior ITS consultant at Mott MacDonald, presents what is perhaps the most complete history of traffic signal design ever compiled. It is just the first chapter of the book, which is a comprehensive guide to traffic signals, from first principles and design issues, to equipment and testing, commissioning and assessments. It is a complete introduction to the subject and likely to be of interest to traffic management practitioners of all levels.
A lot of people are surprised to find out that the history of traffic signalisation pre-dates the advent of motorised vehicles. The technologies that lie at the root of this actually emanate from research work undertaken by the British Admiralty, for Communications and Maritime Navigation.
To enable the Admiralty in London to communicate quickly with the naval ports along the south coast of England, a chain of optical telegraph stations were erected in the late 18th century.
The operation of the telegraph was further improved by work undertaken by General Pasley in the early 19th century, who observed the system perfected in France by Claude Chappe, which resulted in the adoption of the semaphore style of telegraph from 1816 in the UK. (a restored Chappe telegraph station in St Marcan, France, is pictured)
In later life, General Pasley became Inspector General of Railways and during this time, in response to rising accident rates, suggested the use of the semaphore signal as a means of improving communication with locomotive drivers. The first of these was erected by Charles Gregory of the London and Croydon Railway at New Cross in 1842.
With the introduction of steam ships in the mid-19th century, there was a huge increase of collisions at sea, resulting in many ships being lost. Following the work of a Parliamentary Select Committee which first looked at the issue in 1831, a number of studies were carried out. These included investigating the use of coloured lights to make the direction a ship is travelling in more apparent to other vessels after dark. It was found that oil lamps with clear, red and green lenses could be viewed from the greatest distance, with minimal risk of misinterpretation. The outcome of this study was the recommendation that red and green lights should be used as navigation sidelights on vessels, which were adopted universally in 1858.
These two bodies of work, which were leading edge technologies at the time, resulted in two sets of developments:
• The widespread adoption of semaphore signals to control traffic on the railways
• The use of red and green lights as visual warning signals
In the mid-19th century, traffic congestion in London was getting worse and, in response to a suggestion made by a Parliamentary Select Committee, the first traffic signal in the world was installed in Bridge St, adjacent to the Houses of Parliament, in December 1868. This was undertaken to enable MPs to cross over this busy street.
The signal, which was promoted by railway engineer J P Knight (pictured with his design), who lived nearby in Bridge Street, was over 20ft (6m) high. When the semaphore arm was extended horizontally it meant ‘stop’, and when lowered to 45-degrees it meant ‘proceed with caution’. At the top of the pole were red and green gaslights, which were used to augment the arm at night. The operation of the signals was controlled manually by a police officer turning a handle. Unfortunately, the signal didn’t last for long because on 2 January 1869, leaking gas in the signal caused it to explode. The police operator was injured in the incident, resulting in the installation being removed.
After this, the only other recorded traffic signal installation in the UK was part of the bridge interlocking system which controlled the lifting operation at Tower Bridge. The system (pictured below) was designed and manufactured by the railway signal firm of Saxby and Farmer, who had also made the earlier signal at Bridge Street. Included in the system were semaphore signals (also fitted with green and red gas lights) for both river and highway traffic, which worked in relation to the operation of the current bridge position. Part of the mounting bracket for the original signal is still visible today. Since opening in June 1894, the traffic signals have continued to control traffic flow on the bridge which arguably makes Tower Bridge the location which has been signalised for the longest period of time in the world.
No further attempts seem to have been made at mechanised traffic control, until the growth of motorised traffic in the U.S. lead to a need to control junctions in the early 20th century.
A plethora of differing signal systems were developed over the coming years, but these mainly fell into two families, semaphore arms and light signals. One of the most famous of the semaphore type of signal was patented in 1922 by inventor Garrett Morgan. His signal consisted of rotating arms, red and green lights and a bell (which warned of an impending change). Although this system was hand-cranked, it had the added sophistication of including an all-stop period between opposing traffic streams to allow the junction to clear. However, it soon became apparent that semaphore styles signals were an evolutionary dead end.
The first electric traffic light was invented by Lester Farnsworth Wire and installed in Salt Lake City in 1912. Lester was the head of the traffic division of the Salt Lake City Police Department. His signal (pictured right) had two lamps, one red and one green, and was installed in a large wooden box with two six-inch holes on each side. It was operated by a patrolman who used a two way switch to change the light’s colours. However, in this instance, the same signal was displayed to all approaches. The colour it illuminated (red or green) signified traffic could flow on a particular approach, i.e. north and southbound or east and westbound.
William Potts, a Detroit policeman, invented the first three-colour lights in 1920. His four-way signal head used railway signal lamps and was designed to be suspended over the centre of a junction to control traffic from four approaches. It was the first signal to resemble the operation we know today.
In England, the first manually controlled electric traffic lights were installed in Piccadilly, London, in 1925 and the first automatically controlled signals were installed in Princes Square, Wolverhampton in 1927. Today, this junction has specially painted black and white signal poles to commemorate its historic significance.
During the 1930’s, experiments were carried out in the use of vehicle actuation to make the traffic signals responsive to vehicles using a junction. An early attempt at this utilised a microphone placed at the side of the road, which would make the lights respond to motorists sounding their horns (left). Although this system worked, it was very unpopular with people living nearby.
Later experiments used electrical pressure mats and pneumatic tubes which were extensively used up to the 1970’s when they were replaced with inductive loops for permanent installations. However, pneumatic tubes are still used for temporary count sites, where they are fitted to the surface of the carriageway.
The first vehicle actuated site in the UK was installed in 1932, at Gracechurch St / Cornhill in London. Unfortunately, history repeated the events of 1869 when the controller blew up. In this case though, gas had seeped into the controller cabinet base, from a nearby leaking gas main, and was sparked by the electrical apparatus in the controller. However, on this occasion, the accident did not block the course of progress and within a couple of year’s vehicle actuated controllers were being used widely across the country.
In a second extract from Traffic Signals, to be published here on Traffic Technology Today next week, Alistair Gollop will look at what the future may hold for such systems.
Do you know what these terms refer to and the differences between them?
MAN, VA, FXT, CLF, UTC, SCOOT and MOVA
Traffic Signals: An introduction to signalised junctions and crossing facilities in the UK has the answers.
In addition to the history of pioneering signal development, Traffic Signals looks at the way in which modern signals operate and the equipment commonly used in current traffic control systems in the UK. It also looks at how signalised junctions and crossings are designed, explaining the fundamental design principles, and how these are used by modern software modelling tools to predict traffic operation.
Although it uses mainly examples for the UK, Traffic Signals will be of interest to traffic management practioners around the world.
Included within Traffic Signals is a handy set of standard detail drawings which are commonly used when specifying and designing projects.
Order your copy here