Why doesn't a left-turn arrow come up first?
Motorists often find the sequence of the left-turn arrows to vary from intersection to intersection. They usually expect the left-turn arrows to always precede the through green ball. This is probably related to the fact that older traffic control equipment was only capable of sequencing the left-turn arrows first.
During certain times of the day, the left-turn arrow in one direction comes up at the beginning (leads) of the main street green while the left-turn arrow in the opposite direction comes up at the end (lags). This left-turn sequence is called "lead-lag" operation.
The "lead-lag" left-turn sequence is used by engineers to provide green-to-green progression between signals and prevent the main street traffic from having to stop and wait for the opposing left-turn arrow to terminate. The left-turns are sequenced to correspond to the arrival of the group of cars from the upstream signal.