Traffic Signal Timing - Left Turn Loop

[Signal Guy]

For a protected/permissive left-turn loop with volume density operation on the mainline, why does the left-turn loop (say, loop 1A), also call phase 6? And what is the purpose of the 15 second delay for phase 1 and 3 second delay for phase 6?

 

[teeman]

Not clear on the loop layout. Classic volume density relies on advance detection and not stopline loops. Regardless of the configuration, the following should hold. Since the loop is in not in an exclusive turn lane, it can't tell if the waiting cars want to go through or left. The 3s delay gives priority to the through traffic. The 15s delay allows turns to clear permissively. If the queue persists over the detector for 15s, it would then place a call for the left turn phase. It would have a tendency to act like a lagging left unless the sidestreet phase duration was more than the 15s delay.