The Pittsburgh left is a colloquial term for the driving practice of the first left-turning vehicle taking precedence over vehicles going straight through an intersection

Intersection (road)

An intersection is a road junction where two or more roads either meet or cross at grade . An intersection may be 3-way - a T junction or fork, 4-way - a crossroads, or 5-way or more...

, associated with the Pittsburgh, Pennsylvania area. The practice is also referred to as a Boston left. It is an illegal and controversial practice.

Process

The Pittsburgh Left involves two cars facing one another waiting at a traffic light

Traffic light

Traffic lights, which may also be known as stoplights, traffic lamps, traffic signals, signal lights, robots or semaphore, are signalling devices positioned at road intersections, pedestrian crossings and other locations to control competing flows of traffic...

 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

Reciprocal altruism

In evolutionary biology, reciprocal altruism is a behaviour whereby an organism acts in a manner that temporarily reduces its fitness while increasing another organism's fitness, with the expectation that the other organism will act in a similar manner at a later time...

 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 is legal everywhere in the United States.

This practice is so common that straight-going drivers in the area are accustomed to pausing a moment before proceeding on green, for their own safety.

Signals

Generally, a wave of the hand in the direction of the turn or a flashing of headlights by the driver going straight will indicate permission for the left-turning driver to execute the turn. Conversely, the driver navigating the turn may attempt to signal the other driver for permission to do so, with similar hand motions or headlight signals. Furthermore, simply a delay by the straight-bound car can often initiate the signal, and give the left turner time to go and make the turn, especially in large intersections.

Legal basis

The Pittsburgh Left has no basis in law. Failing to yield to oncoming traffic while navigating a turn is a traffic violation, and is prohibited in the Commonwealth of Pennsylvania.

Origin

The Pittsburgh Left has an assumption that no cross traffic will run the light that is changing to red. Since the left is made as soon as the light goes green, if some cross traffic is running the red, there is a conflict. This assumption was valid before the mid 1980s (http://mutcd.pahighways.com/signals.html) when Pittsburgh had a non-standard traffic light sequence. The green light would become simultaneous green/yellow, then just yellow, and finally red. Thus, drivers had twice as much notice that the signal was changing from green to red and therefore could better judge their stop. Currently, most large intersections have four-way red for several seconds, meaning that the on-coming traffic will be verifiably at a full stop before the left-turner proceeds.