Modern Railroad Semaphore
His proposal was accepted and, on December 9, 1868, the system was put in place at the junction of Great George and Bridge Street in London, near Parliament. The system worked extremely well… for about a month. That’s when one of the gas lines that supplied the lights began to leak. Unfortunately, the policeman who was operating the arm was unaware of the leak and ended up being severely burned when the lamp exploded. Thus, despite its early success, the semaphore traffic system was immediately dropped in England.
In the late 1920s, several “automatic” signals were invented. The first ones used the simple method of changing the lights at specific timed intervals. However, the drawback of having some vehicles stopped when there were no cars going in the other direction annoyed people.
All of these different types of lighting systems began to present a problem. Drivers could drive through different areas and encounter several different types of systems, causing confusion and frustration. So, in 1935, the Federal Highway Administration created “The Manual on Uniform Traffic Control Devices.” This document set uniform standards for all traffic signals and road signs.
In the early police officer manned traffic control systems, police officers often used red for stop and green for go, but rather than have a yellow light, they simply blew a whistle to indicate that they were about to change the signal.
Another early traffic light system, developed by Earnest Sirrine, threw out the whole red/green paradigm and instead had lit words saying “Proceed” and “Stop”.
The railroad semaphore system was originally patented by Joseph James Stevens in the 1840s.
In the U.S. and some other countries, modern traffic signal lights are either 8 or 12 inches in diameter and must be visible in every kind of weather and lighting condition.
Source : http://www.todayifoundout.com