Origin
Before the mid 1980s, Pittsburgh had a non-standard traffic light sequence. The green light would become simultaneous green/yellow, then just yellow, and finally red. While some believe this sequence reduced the chances of cross-traffic running the red light immediately before the anticipated green, and thus encouraged early left turning, this practice is also observed in other regions with no history of simultaneous green and yellow. Some large intersections have four-way red for several seconds as a safety buffer, providing an opportunity for an illegal and unsafe left turn after cross-traffic has physically stopped but before the green light gives legal right of way.
Read more about this topic: Pittsburgh Left
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