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Below are some of the most popular questions asked about traffic signals and signal systems.


Q: What sizes do traffic signals come in?

A: Typically there are two. They are measured by their approximate lens diameter... 8" and 12".


Q: How much does a typical traffic signal weigh?

A: Signals are generally made out of two different kinds of material; cast aluminum, and polycarbonate. Aluminum signals weigh about 30 lbs. for an 8 inch head and about 50 lbs. for a twelve inch head. Polycarbonate signals weigh quite a bit less, especially if they are using plastic lenses and aluminum reflectors (or LEDS!). Typical weights are 15lbs. for the 8 inch signals and 30lbs. for the 12 inch heads.


Q: What are the measurements of a typical traffic signal?

A: The 8" traffic signals are approx. 30" tall X 9-1/2" wide X 6" deep.  The 12" are 42" tall X 13-1/2" wide X 8" deep. These are approximate measurements of the main body of the signal without visors.


Q: What kinds of lamps do traffic signals use?

A: Lamps for American and Canadian traffic signals are of the medium screw base type. They are similar in shape and size to a household light bulb, except that they are clear and have a "U' shaped filament. Most 8" signals use 67W and 69W lamps with 8000 hrs. of life. 12" signals use 150W and 167W lamps that are usually rated by their lumens.


Q: What are those "shade things" over the lenses called? What are they for?

A: They are called "visors" and they help keep the sun from reflecting light into the lens and reflector and causing "sun phantom" or false display of an opposing color. There are three kinds of visors. the "cut away" type is the oldest, but covers the least amount of lens. The "tunnel" type is the most common. It looks like an inverted "U" and covers all but the most bottom of the lens. This visor is more practical because it is not "bird nest friendly" and it keeps snow from accumulating onto the lens. The "full circle" type is primarily used in California, and is best at keeping out reflective light. It also is useful in the mounting of louver-type devices in controlling traffic in a specific lane. This visor has the drawback of accumulation of snow (or cups, shoes, and birds) blocking the lens.


Q: What are those black border things around the signal?

A: Those are called "back plates". Back plates are used to increase the target value of the signal, or in other words, make the signal more visible by blocking out the background around it. This way when you are approaching the signal, especially at night, other colored or flashing lights (from buildings or neon signs) that normally would be a distraction behind the signal, will be decreased.


Q: What is the typical time that the yellow light is displayed before the red comes on?

A: The timing of the yellow varies with the particular jurisdictions philosophy. Many use a formula that figures in stopping distance and driver reaction times. Other cities use the prevailing speed limit as their guide (i.e. 35 mph = 3.5 sec, of yellow, 40 mph = 4.0 sec. and so on). The M.U.T.C.D. states that all yellow times should be at least a minimum of 3 sec. but not greater than 6 sec.


Q: Why were the colors red, yellow, and green chosen for traffic signals?

A: Basically the colors came from the ones used by the British railway semaphore signals. The railways would always use red (danger) in the lanterns to warn of danger ahead. Green was originally white to signal the "all clear" meaning. However, railroads later discovered that a broken red lens in a signal could give a false clear indication...with disastrous consequences. Green then was chosen because of it's cooler contrast to the other colors. .Yellow came a bit later in America as a cautionary indication in two light signals by combining the red and green lamps. From a distance, the mixture of colors produced this new color. Later, this color was given it's own spot between the red and green. Oh, and a side note, traffic signals have red on the top...completely opposite railroad signals, which have green on top! Not sure exactly why this is, but it was established that way from the birth of the first 3-color signal.

More info can be found here:

Common Questions