Traffic and Railroad Signals

light signals used to regulate road and rail traffic.

A prototype of light traffic signals—a semaphore-type device—was installed in London in 1868. The first electric traffic signals were manually controlled and had green and red lights; they appeared in the early 20th century in the USA, in Cleveland, New York, and Chicago. The first three-color traffic signal was installed in New York in 1918; the type was introduced in Moscow in 1930. The use of railroad signals dates to the early 20th century.

Traffic signals used to control road traffic have three colors arranged in the same sequence from top to bottom in all countries (red, yellow, and green), as stipulated by the International Convention on Road Signs and Signals (1968). Traffic signals are installed on posts or suspended overhead at street and highway intersections, pedestrian crossings, and similar locations. They may be equipped with additional sections that display a green arrow. Independent traffic signals may be installed at an intersection to control the traffic in certain directions. Pedestrian signals have two colors—red and green. At complicated intersections trolley-car traffic may be controlled by a special four-signal illuminated board. Such displays are sometimes used to control bus or trolleybus traffic.

As of 1974, most traffic signals are controlled by automatic devices, or controllers, first introduced in the USA in the early 1920’s. Control systems for road traffic also use computers and devices that calculate the traffic and actuate the signals accordingly.

Railroad signals permit or prohibit the travel of rolling stock or indicate that the speed of travel is to be lowered. Such signal lights are installed on posts or suspended at stations and along track sections between stations. To ensure the correct guidance of a train under conditions of poor visibility or high speed, a locomotive signal light is installed in the cabin of the locomotive. The light automatically repeats the indications of the signals installed at stations and along the track sections between stations. The signals use the colors red, yellow, green, blue and white. A railroad signal usually serves a single section of the track equipped with automatic or semiautomatic blocking (seeRAILROAD AUTOMATION AND REMOTE CONTROL). In Order to increase the volume of information that can be transmitted by railroad signals, the railroads of the USSR use combinations of several lights and signals or use signals that can be operated in two modes—steady and blinking.

Traffic and railroad signals usually consist of a housing with a shield, a visor, and an optical system. Most signals are of the lens type and have a separate optical system for each signal indication. Searchlight-type signals have a common optical system with rotatable colored filters.