单项选择题

Even during the horse and carriage days, traffic in big cities was often heavy. Police officers had to be stationed full time directing traffic at busy intersections.
The world’s first traffic light came into being before the automobile was in use, and traffic consisted only of pedestrians, carriages and wagons. On December 10, 1868, the first traffic lights were installed outside the British Houses of Parliament in London, by the railway engineer J. P. Knight. They resembled railway signals of the time, with signaling arms and red and green gas lamps for night use. The gas lantern was turned with a lever at its base so that the appropriate light faced traffic. Unfortunately, it exploded on 2 January 1869, injuring the policeman who was operating it.
The modern electric traffic light is an American invention. As early as 1912 in Salt Lake City, Utah, policeman Lester Wire invented the first red-green electric traffic lights. On 5 August 1914, the American Traffic Signal Company installed a traffic signal system. It had two colors, red and green, and a buzzer, based on the design of James Hoge, to provide a warning for color changes. The design by James Hoge allowed police and fire stations to control the signals in case of emergency. The first four-way, three-color traffic light was created by police officer William Potts in Detroit, Michigan in 1920.
Timers on traffic lights were first seen in Taipei, China, and then brought to the U.S. after an engineer discovered its use. Though uncommon in most American urban areas, timers are still used in some other Western Hemisphere countries. Timers are useful for drivers to plan if there is enough time to attempt to cross the crossroad before the light turns red and the amount of time before the light turns green.
The colors of the traffic lights are psychologically associated with the message they are meant to transmit. Red is classically seen as a color representing danger or caution. (There are countless phrases and idioms that use "red" as a message of the bad or unknown--"in the red" and "seeing red.") Green, on the other hand, is a reassuring color in most cultures--the color of nature and growth; of harmony, freshness, and fertility. Green has a strong emotional correspondence with the idea of safety, and was intuitively chosen to guide pedestrians responsibly through an intersection. it can be learned that the first traffic light ______.