The problem is that sometimes it is difficult to tell whether a light is a yellow street light or a yellow traffic light. The left photo shows a view down a San Jose street with an arrow pointing out two green traffic lights. In contrast, the right photo shows the same view with yellow traffic lights. Notice how the yellow traffic lights look similar to the street lights. Imagine you are driving along at night and don't notice that the yellow lights ahead are yellow traffic lights. You might be very surprised when they turned red. I wonder if this causes traffic accidents?
The risk of mistaking the trafficlight for a streetlight is easily eliminated if the red or green light remains lit as the yellow light goes on. There is another benefit in this: green and yellow light indicates that it will soon turn red, red and yellow tells you to get ready... - UW
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