Irrational traffic signals?
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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[Originally published in April, 2013.]