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What Are Traffic Lights?

Traffic lights are traffic control devices placed at intersections. They use a standard color-coded light system to tell drivers when it is safe and legal for them to proceed through the intersection. California red lights keep drivers safe by reducing the likelihood that two vehicles will try to go through the intersection at the same time. They also improve the flow of traffic and help keep pedestrians safe by providing them a safe interval in which to cross the road.

A cool historical fact: the first traffic light was invented in 1868 in England. A policeman operated the light by hand, until it exploded a year later, injuring the unfortunate officer. Today, traffic lights no longer require a human being to operate them. Fortunately, they are also much less likely to spontaneously combust and harm innocent bystanders.

Before you were even old enough to get a learner's permit, you probably knew what each color of light meant: red for stop, green for go, and yellow for "slow down and prepare to stop." However, depending on how your parents drove, when you were very young you might have believed a yellow light meant “Go faster!” Did you ever wonder who decided which colors to use for traffic lights? The basic color code is the same across most of the world, and it was derived from the colors of signal lights used on railroads. In fact, William Potts invented the first 3 color traffic light (the London light only had 2 colors, red and green) in 1920 in Detroit using railroad signal lights and wires. Looking even further back, the convention of using "red" to mean "stop" and green to mean "go" may have come from naval tradition. Ships had navigation lights on each side of the vessel: a red light on the port (left) side of the vessel and a green light on the starboard (right) side. If two ships were on intersecting courses, the ship on the left was supposed to give way to the ship on the right. So, the ship that saw the red light on the other vessel’s left side would know to change course.

Today, California red lights are placed at busy intersections, where it would be to dangerous or inefficient to have an uncontrolled intersection. The California Vehicle Code describes how drivers should react when faced with a red light:

A driver facing a steady circular red signal alone shall stop at a marked limit line, but if none, before entering the crosswalk on the near side of the intersection or, if none, then before entering the intersection, and shall remain stopped until an indication to proceed is shown, except as provided in subdivision. (California Vehicle code Section 21453)

However, after you have come to a complete stop, if the way is clear you can turn right or make a left turn onto a one-way street unless signs are posted saying otherwise.

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