Process
The Pittsburgh left involves two cars facing one another waiting at a traffic light or other stop signal: one turning left and one going straight. (The signal in question has no dedicated left-turn advance green.) The left-turning car will anticipate the green light, although ideally not actually entering the intersection on the red.
As the light turns green, the left-turning car will proceed (with momentum) left ahead of the opposing traffic, frequently with the signaled assent of the opposing traffic. By accepting a modest delay in going straight, the opposing traffic has saved the left-turning traffic waiting an entire light cycle to turn left, as well as saved an equivalent amount of time for all the cars that otherwise would have been stuck behind the left-turning car. In situations where there is so much oncoming, straight traffic that a left turn would not be otherwise possible during any part of the light cycle, the Pittsburgh left can allow a line of left turning traffic to proceed incrementally.
As this modest delay is incurred by the opposing driver, with the hopes of similar treatment at some later date, it represents reciprocal altruism on the part of all participants.
Frequently, the on-coming cars will accelerate slowly enough to allow the turn to be completed without anyone slowing down or being delayed at all. This means that a "Pittsburgh left" can, in places other than Pittsburgh, be the equivalent of simply judging the oncoming traffic to be slow and distant enough to turn in front of it: something that in and of itself is not illegal.
Read more about this topic: Pittsburgh Left
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