Willis Lamm's
Traffic Signal Collection

  History of Traffic Signal Design
Part Seven

  Many Colors -
Standardization of Signal Displays

The yellow light

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In the early days of motor vehicle traffic a yellow light meant "caution." The motor vehicle laws in most states required motorists not to enter an intersection under a flashing "caution" light unless it was safe to proceed. Early caution beacons had yellow on all four sides, a display that would certainly result in a pile-up nowadays. However early motorists were expected to enter the intersection carefully and yield to any traffic already entering the intersection. Similarly many traffic signals would be set to all yellow flash during late hours when traffic was light.

Traffic signals in some jurisdictions showed all yellow displays during signal changes. Traffic was not to enter the intersection under a yellow light unless it was unsafe to stop at the moment when the light first went from green to yellow. Traffic standing at what was the red light would be ready to proceed once the intersection was clear.

A more common early display provided yellow lights in all four directions but the yellow overlapped the previous movement display. This way motorists coming up to the light right at signal change could tell whether the light was changing from red to green or from green to red.

Pedestrians out in crosswalks could also be aided by the yellow light when a signal changed from red to green. Oftentimes the only side of a center mounted signal that they could see would be the side facing stopped traffic. The yellow light provided them with a warning that the light was changing.

Also, these early signals only required only three control leads - red (and opposite green,) yellow (all four directions) and green (and opposite red.) This configuration made for a simpler controller and less wiring.

Manufacturers started offering what they called "split amber" or "split yellow" controllers. These controllers provided separate leads to the main street and cross street yellow lights. As a result, the light only displayed yellow when changing from green to red.

The argument for split yellow displays was that an increasing number of motorists would rush the green light when a signal displayed yellow when changing from red to green. If they were approaching the intersection from a distance some drivers tended to pick up speed when the light turned yellow in anticipation of the green light, putting other vehicles and pedestrians in the intersection at risk.

Early 4-way signals were usually wired so that the red display on one street was wired to the green display on the cross street, and vice-versa. This design required the least amount of contacts in the controller and the least amount of wiring. An early signal only required three power leads and a neutral.

Split yellow displays and a trend toward single face adjustable signals helped produce a rethinking of wiring concepts and soon each color in each direction had its own contact in the controller and its own power lead. As a result the overlapping of a green light by the yellow light has all but disappeared from the scene, although a handful of older signals can be found that still display green when the yellow light comes on.

The overall meaning of the yellow light changed over the years. Originally a yellow light in a beacon or late night all yellow flash from a traffic signal indicated a dangerous intersection that should be entered only with due caution. A steady yellow light on a traffic signal meant that the signal was changing and it was not safe to enter the intersection. Eventually the all-yellow flash was abandoned. Beacons equipped with all yellow lenses had two red lenses installed for the cross street. Older signals that displayed all yellow "late night flash" were changed to flashing green on the main street and flashing red on the cross street.

It eventually became clear to traffic engineers that America was on the move. People were driving great distances and some standardization with respect to traffic signal displays was required so that drivers from one region could clearly understand the signal displays in another region. Thus the current indication standards were developed - green, yellow from 3 to 5 seconds (depending on traffic speed), red, back directly to green. There were no color overlaps except for certain signals that displayed a combination of through and protected left turn indications. Yellow indications on signals were redefined as "clearance intervals" (times for green light traffic to clear the intersection.) Flashing yellow was redefined to mean that traffic had the right of way but cross traffic could be expected, or to denote situations such as a traffic signal ahead, dangerous curve, and other road hazards.

Continue to Standardization of Signal Placement


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