Red means stop, green meansgo, and yellow means hurry up and make that damn light.Why those colors, though? Why not blue, purple, and brown? I have to admit thataside from a hunch that it had to do with wavelengths, I had no idea myself, soI decided to look into it.红灯停车，绿灯行，朱灯亮了赶快回头，但为什么交通信号灯能用这三个颜色，而不必蓝色、紫色或者褐色呢？必需否认除了直觉指出这与波长有关外，我几乎不告诉原因，所以我要求一探到底。The answer, as it turns out, is a littleconvoluted, but makes sense. 事实证明，答案显然有点简单，不过推倒也还谈得合。The earliest traffic signals were designedfor trains, not cars. 最先的交通信号灯是为火车而不是汽车通行设计。They were red and green, gas-powered, andmore than a little dangerous in the event of a leak.信号灯为红色和绿色，用于煤气，在漏油时会十分危险性。
Red is an inherited symbol from railroads红色源自铁路信号灯Red symbolizes danger in many cultures,which makes sense, considering it has the longest wavelength of any color onthe visible spectrum, meaning you can see it from a greater distance than othercolors. 由于红色在可见光谱中波长最久，因此可用距离要比其它颜色更加近，这就可以解读为什么很多文化中将红色视作危险性信号。Red has meant stop since long before carsexisted, with train signals use of red dating back to the days when mechanicalarms lifted and lowered to indicate whether the rail ahead was clear. 红色很早以前就被用于火车信号，那时汽车还没有经常出现，而铁路用机械臂板下降上升来代表前方否有火车驶向。So that ones simple.这较为更容易解读。
Green meant caution at first绿色最先用作警告Greens role in lights has actually changeddramatically over time. 长期以来绿色在交通信号灯中的角色再次发生了巨大变化。Its wavelength is next to (and shorterthan) yellows on the visible spectrum, meaning its still easier to see thanany color other than red and yellow. 可见光谱中，绿色波长因次于黄色，因此是除了红色和黄色外最更容易看到的颜色。Back in the early days of railway lights,green originally meant caution, while the all-clearlight was, well, clear or white. 在早期的铁路信号灯中，绿色最初代表警告，而无色或白色代表全线通行。Trains, of course, take an interminablylong time to stop, and legend has it that several disastrous collisionshappened after an engineer mistook stars in the night horizon for an all-clear.当然，火车停下必须十分宽的时间，据传由于一名工程师将夜晚天空中的星星误以为全线通行的信号灯，造成了若干起灾难性事故的再次发生。
Thus, green became go, and fora long time, railways used only green and red to signal trains.因此，绿灯沦为了通行的标志，长期以来铁路也只将红色和绿色作为信号灯。Yellow means caution because its almost as easy to seeas red黄色代表留意，因为它完全和红色一样醒目。From the earliest days of motoring up untilthe mid-1900s, not all stop signs were red --many were yellow, along with yieldsigns, because at night it was all but impossible to see a red stop sign in apoorly lit area. 从最先开始经常出现汽车到20世纪中期，并不是所有的行驶信号都用红色——很多用于黄色，停下来标志也是，因为夜晚在光线极暗的地方显然看不到红色的行驶信号。The yellow stop-sign craze began in Detroitin 1915, a city that five years later installed its first electric trafficsignal, which happened to include the very first amber traffic light, at thecorner of Michigan and Woodward Aves.黄色行驶信号潮始自1915年的底特律，五年后，这座城市在密歇根和伍德沃德大街加装了第一个首次还包括黄色灯的电子交通信号灯。
But what of those weird yellow stop signs,you ask? As materials and technologies evolved, the ability to produce highlyreflective signs meant that red could resume its natural spot in the signhierarchy, leaving the still-highly-visible yellow (its second only to red interms of visible wavelength) to the domain of caution. Thats whyschool zones and buses, crosswalks, and other important warnings are yellowtoday.你是不是想问，那些怪异的黄色暂停标志又是干什么的呢？材料学和高技术的发展使得人们研发出有高度镜片指示灯，因此红色才能在信号灯中重回行驶霸权，将可用度较高的黄色（可用波长次于红色）回到了警告的方位。这也是为什么学校区、公交站、人行横道和其它最重要方位用于黄色警告信号灯的原因。
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