[Izkata]

That doesn't solve it at all, you're thinking of a different problem. I got clued in from another comment that only mentioned it offhand, but this is the problem the "Pittsburgh left" tries to solve:

The traffic is a single lane, and you want to turn left. Light turns green, and you wait for the oncoming traffic to clear. All traffic behind you is stopped on a green because they can't get around you.

[dabbledash]

Thanks, that’s a great point. That really is a terrible situation.

And I’m sure the risks associated with the “Pittsburg left” are significantly mitigated when everyone expects it.

[c22]

Even with multiple lanes this can be an issue as people behind you try to merge right to pass you which in busy traffic increases the chance of accidents and causes slowdowns.