Varying Left Turn Cycle Phasing During the Day

[The High Plains Traveler]

I suspect the relative merits of leading versus lagging protected left turn cycles have been discussed here before. While I have generally seen leading cycles as the norm, I have also seen the merits of lagging protected turn cycle, especially on a signal with protected-permissive cycles. But, CDOT has done some interesting changes to signals along U.S. 50 between Pueblo and Pueblo West to take advantage of both. [Obligatory geographical note: "C" in CDOT = Colorado].

By way of brief background, Pueblo West is a bedroom community to Pueblo, and thus there is considerable eastbound traffic in the morning and westbound in the evening. (I'm one of those). U.S. 50 is a 65 mph, four-lane expressway in Pueblo West, with two main signalized intersections that provide access to/from the highway. Entering Pueblo it drops through 55 mph to 45 mph leading to I-25. Looking outbound, near I-25, there are three lanes each way, dropping to two on the west edge of the city. This lane drop is a choke point. There are signals at five intersections from the west city limits to I-25. One has a protected-only left turn cycle for U.S. 50, while the rest have 4-section left turn lane signal heads with FYA and part-time protected-only left turns during heavier traffic periods.

CDOT has struggled to deal with traffic, especially in the afternoon. There is a SPUI at the interchange of I-25 with U.S. 50 west, with a relatively short distance to the first signalized intersection west of I-25, so there is only limited storage between the two intersections. When this is full, exiting traffic is trapped under the freeway overpass and if it can't clear in time will block U.S. 50 through traffic.

One of their main tools CDOT has used to manage traffic flow is to change the left turn cycle phasing. At the interchange SPUI, they use a lagging left turn from U.S. 50 to I-25 both directions to give the backed up westbound traffic at the first intersection time to clear before the offramp traffic enters from the SPUI. This has changed since I have lived here and I think this is a 24/7 pattern, but further west, it appears CDOT changes the left turn phasing from leading to lagging for afternoon rush hour. I've noticed that the left turns for westbound 50 in Pueblo West seem to change from leading to lagging around 4:00 in the afternoon, and by 7:00 they're back to leading.

Is this unique, or has this technique of varying the left turn cycle sequence during the day to manage traffic been used elsewhere?

[Alps]

Not unique, but not common. Often in these cases what you'll have is a signal set up to skip certain uncalled phases but be able to return to them at other times in the cycle - even if they were already called at the beginning. There is a signal near me that is normally lead left EB and lag left WB. If there are no EB left calls, it will normally just skip phase, but I have seen interesting timings. In one case, if there is only one EB through call (error, I believe) or no EB through calls, it will go WB lead left, and either terminate for the cross street or actually go to all green from there. (I think it only goes to all green by skipping the cross street if it's uncalled.) I've seen some funky things happen off-peak at the intersection in question, such that I still don't know what causes it.

[6a]

The intersection I use to go home from work (OH 317 NB to US 33 WB) uses both a leading and lagging arrow in the same cycle at certain times of day.  I've never seen it anywhere else, and it only seems to be active for a short period of time.

[kphoger]

I see leading/lagging arrows that seem to activate based on pavement sensors, but not simple timers.

[twinsfan87]

I can think of a few MnDOT signals that switch between leading and lagging left turn arrows depending on time of day. Specifically, I know that the signals on US 169 in Champlin switch between leading and lagging lefts on US 169.

[roadfro]

It's not a unique phenomenon, but it's not a method employed by many jurisdictions. Most agencies that have a regular signal timing/coordination program or personnel dedicated to signal timing would be more likely to use this than other agencies.

One reason for doing this may be whether there is signal coordination and progression in a corridor. It may be better to have certain signals have leading lefts at the beginning of a progression wave and others have lagging lefts towards the tail end of that wave. Also, as is the case in the original example, limitations in turn bay storage may prompt different cycling depending on the dominant direction of travel at different times of day.

The intersection I use to go home from work (OH 317 NB to US 33 WB) uses both a leading and lagging arrow in the same cycle at certain times of day.  I've never seen it anywhere else, and it only seems to be active for a short period of time.

Use of both a leading and lagging arrow is typically done by setting one arrow to come up every cycle, and then the arrow can be recalled before the signal serves the side street if there is no or little conflicting through traffic.

[The High Plains Traveler]

I forgot to mention this in the lead-in to this thread. One aspect to the situation I described at the beginning of this thread is that, at the intersections that have bi-directional lagging protected left turn, the direction of travel with the higher number of vehicles waiting will get the green arrow first. The video detection system appears to be capable of estimating the time needed to clear left turns in each direction of travel so that the left turn cycle for both directions will terminate at the same time, and then the cross-street green cycles can begin.

[roadfro]

^ It's not so much that the vehicle detection estimates the amount of time needed to clear the turns. It is more likely that the signal phase green times have been set in such a way that humans have calculated a certain amount of time for clearing that movement and entered settings into the controller based on that calculation. Vehicle detectors at traffic signals (loops or video) are typically designed to only sense presence of vehicles, not count numbers of vehicles.

[roadman65]

Orlando has the left turn signals either before or after the straight through green depending on the time of day.  Sometimes at four way left turn signals it has the two sides of the road opposite for both roads, and gives the illusion of a longer light wait time.   It is confusing because you pass through the intersection at one time you get the arrow soon, then another time you get it later.

[thenetwork]

Toledo, OH has had leading/lagging left turn phasing in place for over 30 years.  In most cases, it was at intersections with either double-left turn lanes and/or protected lefts.  For many of the other intersections throughout Toledo, lights became double-length cycles during rush hours to keep the number of stop & go's per driver to a minimum. 

Also credit Toledo as the city where I first experienced double-left-turn lanes.  To this day, double-lefts at 4-way intersections in Cleveland are extremely rare.  And given the way people drive in C-Town, I can see why!

[codyg1985]

The intersection I use to go home from work (OH 317 NB to US 33 WB) uses both a leading and lagging arrow in the same cycle at certain times of day.  I've never seen it anywhere else, and it only seems to be active for a short period of time.

This setup is also used in Huntsville, AL at US 72 at Moores Mill Rd for the EB US 72 to NB Moores Mill Rd left turn movement. It is in effect from 3:00 PM to 6:00 PM each weekday I believe.

[vdeane]

Take a look at this wonderful intersection: http://g.co/maps/7t4kg

It's a five phase light:
1. US 11, US 11 north to Maple St.
2. Clarkson Ave north
3. Maple St. west
4. NY 345 south
5. US 11 protected left turns

At night, phases 1 and 5 change:
1. US 11 north, US 11 north to Maple St., US 11 north protected left turn
5. US 11 south, US 11 south protected left turn.