Are there any guidelines regarding when split-phasing signals should occur? Any guidelines as to the signal order of the phasing?
Let's take an example of an intersection with a major north-south street and a more minor east-west street. WB traffic is heavier than eastbound traffic, as the street to the east of the intersection is a collector with a heavy left turn movement to the main street. This WB traffic faces a left turn lane and an option left/straight lane. Because of the option lane WB traffic is completely time-separated from EB traffic by the signal. The side street EB is a much smaller movement.
Is there any priority as to whether EB should go before WB or WB should go before EB due to the different traffic demands on each segment? Is there a preference to doing one over the other?
Quote from: mrsman on September 27, 2024, 05:21:13 PMAre there any guidelines regarding when split-phasing signals should occur? Any guidelines as to the signal order of the phasing?
Let's take an example of an intersection with a major north-south street and a more minor east-west street. WB traffic is heavier than eastbound traffic, as the street to the east of the intersection is a collector with a heavy left turn movement to the main street. This WB traffic faces a left turn lane and an option left/straight lane. Because of the option lane WB traffic is completely time-separated from EB traffic by the signal. The side street EB is a much smaller movement.
Is there any priority as to whether EB should go before WB or WB should go before EB due to the different traffic demands on each segment? Is there a preference to doing one over the other?
It sounds like the split phased is being used properly here. As for as who gets it first, it's probably more often than not the heavier-traffic leg, although ibdont know if I've seen a true correlation to know that's usually the case.
There are a couple of those in this area and the side with lighter traffic goes first.
If you're asking about the split phase order for the minor intersecting streets (not the major route where signal coordination determines the order)...
My experience is that in the places I have seen in Georgia, along the same corridor the same direction always goes first, I guess because it's the simplest way to do it and therefore the least likely to get screwed up during programming/installation/etc.
In particular, the main east-west road in Athens GA has several streets intersecting with split phasing. As a kid I noticed that SB always got the green phase before NB. Then about 2 decades ago they switched it for some reason, and now they all have NB getting the green then SB.
One would think there would be a more logical way to do it.
My opinion, (and I'm not an engineer or anything) is that the direction whose left turn has a longer distance to the next signal, should be first. Because IMO it doesn't make sense to get a green left turn light first, only to have to stop at a red light in just a short distance.
As to when split phasing should be used, it is generally implemented when you have both a dedicated left turn lane and a shared left/through lane on an approach. To have this lane layout implies a heavy left turn volume that cannot be adequately accommodated with a single lane, but having two dedicated left turn lanes might also impede throughput for the through direction. (There could be other uses of split phasing, like with some very unusual intersection geometry or an adjacent bikeway/path crossing that needs its own phase, but these would be less likely and accommodated with other unique signal phasing if at all possible.) Split phasing is not ideal, as it typically adds much more time to service the cross street than if the left turns run together or if lead-lag phasing is used, leading to a longer signal cycle length.
As to which side of a split-phased street goes first, I don't think there's a rule or guideline on this. It would make sense generally for the side with lighter traffic to go first, just so that it can gap out early and serve more time to the heavier side of the split if needed. It's also possible that which split goes first could vary depending on time of day—that's especially likely if the municipality is keeping up on signal coordination and the surrounding network experiences significant differences in travel flow directions in the am peak versus pm peak.