Could they set it up so that if the bus is waiting at a red, only the transit signal goes green giving it priority over left turners? I've always thought it silly that we spend money on "transit signals" which do nothing more than duplicate the general signals above them.
Yes. But that phase would take approximately 12 seconds of the signal cycle (~6 seconds green, 3 seconds amber, 3 seconds all-way red). So that really constricts the design of the signal cycle, requiring either a long cycle (potentially long wait times) or phase lengths which approach minimums (less responsive to changes in conditions). Given cycle lengths are typically 60-120 seconds, 12 seconds is actually a fairly big deal.
A more efficient way of accomplishing the same thing would be to skip the left-turn phase if there is a vehicle at the stop bar on the busway, instead moving it to the end of the through phase.
The main disadvantage of that design is that since the length of left turn phases would be calculated in advance of the actual phase, cars could end up waiting a full cycle to turn left if they arrive after the length has been calculated. But that's a pretty small price to pay for increased transit speed.
More detailed explanation:
The left turn phases should end at the same time, so that there is no wasted time before the cross phases start. Therefore, the time the turn phases start varies based on demand, as opposed to the current setup where the start time is fixed, and the end time varies.
Let's say the maximum left turn phase length is 20 seconds (14 seconds green, 3 seconds amber, 3 seconds all-way red). The length of the left turn phases would be calculated 27 seconds in advance of the end of the through phase (up to 20 seconds for the turn phase, 4 seconds amber, 3 seconds red for through along Hwy 7), with the actual turn phases beginning individually as calculated.
