Q: Normally, at a stoplight with left turn arrows, those left arrows turn green for both directions at the same time. However, at certain lights–Stadium and Liberty and Stadium and Pauline are examples–you get a green left arrow for one direction, then the main green, then the green left arrow for the other direction. Why are they wasting our time waiting while only one side can turn?

A: Traffic signals are timed to maximize flow throughout the entire city. Turning traffic sometimes is delayed to give through traffic a better chance of moving through multiple signals without stopping.

Next time you’re waiting, you might pass the time studying whether the left turn lanes get a green light before through traffic (termed “leading left”), after it (“lagging left”), or staggered before and after (“lag-lead”). Irrational as it may seem, the “lag-lead” pattern you describe sometimes is optimal when opposing turn lanes feed intersections at differing distances.

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