Stoplights are red, yellow, and green, because traffic officials, early on copied the code system railroad engineers devised for track systems controlling the trains. The goal of the railroad engineers in crafting this code was to prevent often fatal train collisions, by giving the trains advance warning. Therefore they did not take their task lightly in selecting the symbolic colors for the signals. Red, the color of blood, proved a logical choice for the stop signal, as for thousands of years, this color forbade danger. The color alone, railroad engineers reasoned, should give people cause to pause, to abide by the signal, and to stop or suffer the consequences of death and destruction. Engineers used the trial and error method in selecting the other colors. The first trial in the 1830s, that of choosing green for the caution signal, and clear for the go signal, failed miserably. Clear as a choice for the go signal, varied slightly from the light cast from typical street lamps, or from the glare of the sunlight, and, thus could quite easily be mistaken for the go signal...after the fact. This failure prompted the railroad engineers to alter their color selections to red for stop, green for go, and yellow for caution. Railroad engineers, not traffic engineers, should be credited for the lives saved in the interim, by their system of coding warning signals red, yellow, and green.   track systems: 編碼系統             trial and error: 反復試驗 clear: 明亮的燈