The idea that red means stop and green means go has influenced our lives in more ways than just traffic signals（信号）．We have been taught from a young age that the color red means danger and green means it's okay to move forward．But why were those colors chosen for traffic lights in the first place？For something we have to look at every day，why couldn't they have been prettier colors？

Well，it's important to know that before traffic lights for cars，there were traffic signals for trains．At first，railway companies used red to mean stop，white to mean go，and green to mean warn．As you could imagine，trains ran into a few problems with white meaning go．For example，one train mistook a bright star for a white light and something unexpected happened．Because of that，railway companies finally moved green meaning go and used yellow to mean warn．And it's been that way ever since．

Red has always been a color that mean danger，long before cars were even around．This most likely started because it's the color with the longest wavelength（波长） so it can be seen somewhere farther than

other colors．

But，believe it or not，yellow was once used to mean stop．Back in the 1900s，some stop signs were yellow because it was too hard to see a red sign in a poorly lit area．Finally，highly reflective materials（高反射材料） were developed and red stop signs were born．Since yellow can be seen well at all times of the day，school zones，some traffic signs，and school buses continue to be printed yellow．

So next time you are impatiently waiting at a traffic light，don't get so mad at them，they've certainly come a long way．

根据材料内容选择最佳答案．

56．What were traffic signals for before traffic lights for cars？ 　　

A．	Trains
B．	Boats．
C．	Planes
D．	Bicycles

57．Which of the following colors meant go at first？ 　　

A．	Red．
B．	Yellow．
C．	White．
D．	Green．

58．Why does red mean danger？ 　　

A．	Because it's the brightest color．
B．	Because it can be seen far away．
C．	Because railway companies say so．
D．	Because it has the shortest wavelength．

59．Why are school buses painted yellow？ 　　

A．	Because yellow is a prettier color．
B．	Because yellow once meat danger．
C．	Because yellow can be seen well in daytime．
D．	Because yellow means it's OK to go forward．

60．What does the text mainly tell us？ 　　

A．	How important traffic lights are．
B．	Which color has the longest wavelength．
C．	What red，yellow and green can be used for．
D．	Why traffic lights are red，yellow and green．