The first traffic lights are installed, outside the Palace of Westminster in London. Resembling railway signals, they use semaphore arms and are illuminated at night by red and green gas lamps.

Traffic lights, traffic signals, or stoplights – also known as robots in South Africa, Zambia, and Namibia – are signaling devices positioned at road intersections, pedestrian crossings, and other locations in order to control the flow of traffic.

The Palace of Westminster is the meeting place of the Parliament of the United Kingdom and is located in London, England. It is commonly called the Houses of Parliament after the House of Commons and the House of Lords, the two legislative chambers which occupy the building. The palace is one of the centres of political life in the United Kingdom; "Westminster" has become a metonym for the UK Parliament and the British Government, and the Westminster system of government commemorates the name of the palace. The Elizabeth Tower of the palace, nicknamed Big Ben, is a landmark of London and the United Kingdom in general. The palace has been a Grade I listed building since 1970 and part of a UNESCO World Heritage Site since 1987.

A railway signal is a visual display device that conveys instructions or provides warning of instructions regarding the driver's authority to proceed. The driver interprets the signal's indication and acts accordingly. Typically, a signal might inform the driver of the speed at which the train may safely proceed or it may instruct the driver to stop.

Railway semaphore signals are an early form of fixed railway signals. The semaphore system involves signals that display their different indications to train drivers by changing the angle of inclination of a pivoted 'arm'. Semaphore signals were patented in the early 1840s by Joseph James Stevens, and soon became the most widely used form of mechanical signal. Designs have altered over the intervening years, and colour light signals have replaced semaphore signals in most countries, but in a few they remain in use.