While technically illegal, the actions are intended most courteously - this is because, unlike many major cities, Pittsburgh differs in the fact that it contains more two lane roads (one lane in each direction), including bridges, than any other style of road (two or more lanes in each direction); even the major highway systems are only two lanes in each direction. As well, Pittsburgh has more two-lane road intersections missing 'left-turn lane only' lanes than most major cities.
The 'Pittsburgh left' helps maintain an efficient flow of traffic throughout the city, particularly during rush-hour; while traffic law requires the left-turning vehicle yield to on-coming traffic for safety reasons, the 'unorthodox' left-turn helps minimize delays incurred by the overall traffic system. When the left-turning vehicle yields to oncoming traffic on a two-lane road, the traffic behind the turning vehicle must wait until the turning vehicle exits the lane, and this often doesn't happen until the lights turn from green back to red (especially in rush-hour traffic). By allowing the turning vehicle to go first when the light turns green, the lead oncoming vehicle incurs a much shorter delay, and as a result both lanes of traffic can flow; it is also customary for oncoming vehicles to slow or stop at intersections during green lights to allow single turning cars to clear the opposite lanes (especially during rush-hour).
In a way, Pittsburgh drivers are more courteous not because they want to be, but because they are 'forced' to be, since the traffic lights in Pittsburgh are often ill-timed and the roadways are most inadequate. By working together, and by more evenly distributing the load of the inadequacies of the Pittsburgh roadway system, citizens have a more pleasant driving experience, particularly with making left-turns at intersections without left-turn lanes. While in a hurry like any good city dweller, Pittsburgh drivers tend to have more tolerance to delays and inefficient roadways, lending credence to why there hasn't been a large demand for major improvements in recent time (simply put - they're so used to the crap, they don't mind it anymore!)
A major distinction between the 'Pittsburgh Left' and the 'New York Left" - drivers in Pittsburgh more than likely perpetuate (and tolerate) this driving infraction because it is a necessary part of "making do" with what little in adequate driving Pittsburgh drivers truly have; in contrast, New York drivers make similar left turns (gunning through intersections at the beginning of green to beat the lead car in oncoming traffic) because they weren't beat enough as kids (i.e. snot-nosed punks)
Holly: Oh my god, did you see that? The light turned green, and he just turned before you could go! I mean, sure, he had his left-blinker on, but that doesn't mean he gets to go first...
Chris: Uh, yeah, they do that all the time. I usually don't go if I know they want to turn, don't want to get in an accident...
Holly: What a jerk! I hope a cop pulls him over and gives him a ticket...
Chris: Uh, what? Have you even driven in Pittsburgh? No, you're from Columbus, with left-turn lanes galore. Cutting me off like that, I lost like 2 seconds - but if he'd let me go, the people behind him could have ended up waiting FOREVER. I usually let people like him do that, because I'd hope they'd let me do that, too... I shouldn't have to be the bad guy just because I need to make a left turn and the fricken' Pittsburgh city planners didn't think about EFFICIENT roads when they OVERpaid a bunch of union workers to lay asphalt over EVERY FREAKIN' OLD horse trail they could find...
Holly: Wait, people do that all the time?
Chris: Yeah, it's called a "Pittsburgh left".
Holly: Wait, I thought it was called a "New York left"?
Chris: No - New Yorkers make illegal left turns through intersections at green lights like that because they're assholes. Pittsburgh-ers do it because our city has, quite possibly, the worst roadways and traffic management systems in the country, despite the higher-than-normal taxes I pay every month. With all the corrupt officials, the only difference between Pittsburgh and New Jersey is 1) the number of old people, 2) number of dialysis centers, and 3) Pittsburgh doesn't smell like shit.
Holly: Wow - well, if you hate it so much, why do you live here?
Chris: I plan on moving out as soon as possible. I'm gonna miss the polite drivers and gun laws, though - can't beat 'em!