Willis Lamm's
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History of Traffic Signal Design |
Many Colors - Standardization of Signal Displays |
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. |