Signals with conflicting movements

[jakeroot]

It makes sense. If there is a left turning car in a shared left-straight lane, and the light isn't split phased, the through traffic would get stuck behind (and quite annoyed at) the left turning car who is waiting for a gap in oncoming traffic.

That's understandable, but how often in these situations does the oncoming traffic occupy the intersection for the entire length of the cycle? In the intersection above (one I go through quite a lot), there is only a car coming the other direction about every other time I approach it. And more often than not, they're turning left. As I always turn left here, I just turn at the same time as the other car, and if there's a car behind each of us, they can either continue straight at the same time, or also turn next to each other. I can only think of one occasion when I didn't make a light, and it was because of pedestrian activity that prevented more than a couple of cars from turning at once.

Most of the split-phased intersections are fairly busy around here, since there was enough turning traffic to justify the arrangement. Here are/were a few in my area off the top of my head (some have been replaced) and the likely reason for them being split phased:

State St/N. Temple, SLC. High volume of traffic turning west from northbound State.
...
Columbus St/500 N, SLC. High volume of traffic turning north from westbound 500 N.
...

The first example is understandable because of the dual left.

The second example...that's really silly. If most of the traffic is SB to EB and WB to NB, they could just as easily install a fully permissive setup along 500 N, with a right turn filter for WB to NB traffic (like the one already there). Split phasing becomes necessary when both directions have high amounts of through and left turning traffic. If there's not a lot of left turns, fully permissive phasing works best because both directions can run at the same time, with traffic yielding. If someone gets stuck behind someone turning left, oh well, but that's how turning left from any road without a turn lane works anway, so I don't see what the big deal is.

[US 89]

It makes sense. If there is a left turning car in a shared left-straight lane, and the light isn't split phased, the through traffic would get stuck behind (and quite annoyed at) the left turning car who is waiting for a gap in oncoming traffic.

That's understandable, but how often in these situations does the oncoming traffic occupy the intersection for the entire length of the cycle? In the intersection above (one I go through quite a lot), there is only a car coming the other direction about every other time I approach it. And more often than not, they're turning left. As I always turn left here, I just turn at the same time as the other car, and if there's a car behind each of us, they can either continue straight at the same time, or also turn next to each other. I can only think of one occasion when I didn't make a light, and it was because of pedestrian activity that prevented more than a couple of cars from turning at once.

Most of the split-phased intersections are fairly busy around here, since there was enough turning traffic to justify the arrangement. Here are/were a few in my area off the top of my head (some have been replaced) and the likely reason for them being split phased:

State St/N. Temple, SLC. High volume of traffic turning west from northbound State.
...
Columbus St/500 N, SLC. High volume of traffic turning north from westbound 500 N.
...

The first example is understandable because of the dual left.

The second example...that's really silly. If most of the traffic is SB to EB and WB to NB, they could just as easily install a fully permissive setup along 500 N, with a right turn filter for WB to NB traffic (like the one already there). Split phasing becomes necessary when both directions have high amounts of through and left turning traffic. If there's not a lot of left turns, fully permissive phasing works best because both directions can run at the same time, with traffic yielding. If someone gets stuck behind someone turning left, oh well, but that's how turning left from any road without a turn lane works anway, so I don't see what the big deal is.

Honestly, most traffic at my second example just goes straight through on Columbus. Southbound in the morning, northbound in the evening. But from the east, there are two lanes, and there is enough right turning traffic to warrant a lane dedicated to that movement, resulting in a shared left/straight lane. UDOT sees this configuration as automatic split phase criteria. And I agree with you there, split phasing there is stupid.

And the first example is also a pedestrian heavy intersection.