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Traffic lights are red, yellow and green. They are very important signals (信号). Traffic lights are usually placed at busy street corners to help control traffic. Sometimes, they are on streets near schools and shops, too. Most young children are taught what each of the colored lights means, so everybody can stay safe.

The first traffic signals were used for trains. The signals were put alongside railway tracks (轨道). The color red was used to mean “stop” because it warned people of danger. A red light was also the easiest color to see from a distance. Train drivers could see the red “stop” light ahead and had enough time to slow down and stop their trains. A green light was used to mean “take care” and a white light meant “go”.

However, there were many dangerous accidents on the train tracks. Some train drivers thought a star shining in the night sky was a white light that meant “go”. So the signals were swapped. The “go” light was changed to green and the “take care” light was changed to white.

During these times, people travelled by train or in carts pulled by horses. Years later, when cars were built, people began to drive them because they could travel much faster than horses. However, the road rules for drivers were not very clear and many people were hurt crossing the roads. Yellow warning signs had to be placed beside the roads to remind drivers to take care and allow people to cross safely.

Today, towns and cities all over the world have traffic lights. Red and green lights match the signals used on the railways. Yellow lights are easy to see, so they are used to warn drivers that they should slow down and prepare to stop.