交通信号灯为何用红色、黄色和绿色？
<p><span word="Red">Red</span> <span word="means">means</span> "<span word="stop">stop</span>," <span word="green">green</span> <span word="means">means</span> "<span word="go">go</span>," <span word="and">and</span> <span word="yellow">yellow</span> <span word="means">means</span> "<span word="hurry">hurry</span> <span word="up">up</span> <span word="and">and</span> <span word="make">make</span> <span word="that">that</span> <span word="damn">damn</span> <span word="light">light</span>." <span word="Why">Why</span> <span word="those">those</span> <span word="colors">colors</span>, <span word="though">though</span>? <span word="Why">Why</span> <span word="not">not</span> <span word="blue">blue</span>, <span word="purple">purple</span>, <span word="and">and</span> <span word="brown">brown</span>? <span word="I">I</span> <span word="have">have</span> <span word="to">to</span> <span word="admit">admit</span> <span word="that">that</span> <span word="aside">aside</span> <span word="from">from</span> <span word="a">a</span> <span word="hunch">hunch</span> <span word="that">that</span> <span word="it">it</span> <span word="had">had</span> <span word="to">to</span> <span word="do">do</span> <span word="with">with</span> <span word="wavelengths">wavelengths</span>, <span word="I">I</span> <span word="had">had</span> <span word="no">no</span> <span word="idea">idea</span> <span word="myself">myself</span>, <span word="so">so</span> <span word="I">I</span> <span word="decided">decided</span> <span word="to">to</span> <span word="look">look</span> <span word="into">into</span> <span word="it">it</span>.</p><p>红灯“停”，绿灯“行”，黄灯亮了“快点走”，但为什么交通信号灯要用这三个颜色，而不用蓝色、紫色或者褐色呢？必须承认除了直觉认为这与波长有关外，我完全不知道原因，所以我决定一探究竟。</p><p><span word="The">The</span> <span word="answer">answer</span>, <span word="as">as</span> <span word="it">it</span> <span word="turns">turns</span> <span word="out">out</span>, <span word="is">is</span> <span word="a">a</span> <span word="little">little</span> <span word="convoluted">convoluted</span>, <span word="but">but</span> <span word="makes">makes</span> <span word="sense">sense</span>. <span word="The">The</span> <span word="earliest">earliest</span> <span word="traffic">traffic</span> <span word="signals">signals</span> <span word="were">were</span> <span word="designed">designed</span> <span word="for">for</span> <span word="trains">trains</span>, <span word="not">not</span> <span word="cars">cars</span>. <span word="They">They</span> <span word="were">were</span> <span word="red">red</span> <span word="and">and</span> <span word="green">green</span>, <span word="gas">gas</span>-<span word="powered">powered</span>, <span word="and">and</span> <span word="more">more</span> <span word="than">than</span> <span word="a">a</span> <span word="little">little</span> <span word="dangerous">dangerous</span> <span word="in">in</span> <span word="the">the</span> <span word="event">event</span> <span word="of">of</span> <span word="a">a</span> <span word="leak">leak</span>.</p><p>事实证明，答案确实有点复杂，不过倒也还讲得通。最早的交通信号灯是为火车而不是汽车通行设计。信号灯为红色和绿色，使用煤气，在漏油时会非常危险。</p>
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