Double left turns with permissive phasing

[jjakucyk]

How about something like this.  Dual left turn lanes with no protected phase at all, just standard green balls, timer-actuated too, no detectors.  https://goo.gl/maps/qbdNs3WEjQv  This would date to the late 1960s when I-71 was built in downtown Cincinnati, and this overpass is way over-engineered at 6 lanes (probably for pre-interstate traffic that was all diverted).  Plus Elsinore coming down the hill from the top right is pretty lightly trafficked so there's usually only one or two cars at a time coming from that direction anyway. 

[JMAN_WiS&S]

https://www.google.com/maps/place/St+Croix+Trail,+Oak+Park+Heights,+MN+55082/@45.0355219,-92.7925001,3a,75y,142h,90t/data=!3m4!1e1!3m2!1swY5Tktp77WnFBZTL-Z5yjA!2e0!4m2!3m1!1s0x52b2ca0483785575:0xd9f7d99b33d1f516

[jakeroot]

https://www.google.com/maps/place/St+Croix+Trail,+Oak+Park+Heights,+MN+55082/@45.0355219,-92.7925001,3a,75y,142h,90t/data=!3m4!1e1!3m2!1swY5Tktp77WnFBZTL-Z5yjA!2e0!4m2!3m1!1s0x52b2ca0483785575:0xd9f7d99b33d1f516

I think the dual permissive lefts work better with flashing yellow arrows. Not sure why. Glad Minnesota's DOT doesn't outright ban their use (engineering judgement permits any sort of signal combination -- ).

How about something like this.  Dual left turn lanes with no protected phase at all, just standard green balls, timer-actuated too, no detectors.  https://goo.gl/maps/qbdNs3WEjQv  This would date to the late 1960s when I-71 was built in downtown Cincinnati, and this overpass is way over-engineered at 6 lanes (probably for pre-interstate traffic that was all diverted).  Plus Elsinore coming down the hill from the top right is pretty lightly trafficked so there's usually only one or two cars at a time coming from that direction anyway. 

Interesting that the road wasn't scaled back after the 71 freeway was finished. Pretty cool nonetheless. I think this might be unique in being the only dual permissive left that has no protected phase.

[tradephoric]

https://www.google.com/maps/place/St+Croix+Trail,+Oak+Park+Heights,+MN+55082/@45.0355219,-92.7925001,3a,75y,142h,90t/data=!3m4!1e1!3m2!1swY5Tktp77WnFBZTL-Z5yjA!2e0!4m2!3m1!1s0x52b2ca0483785575:0xd9f7d99b33d1f516

I think the dual permissive lefts work better with flashing yellow arrows. Not sure why. Glad Minnesota's DOT doesn't outright ban their use (engineering judgement permits any sort of signal combination -- ).

Those left-turn signal heads should be bagged during construction. 

[jeffandnicole]

https://www.google.com/maps/place/St+Croix+Trail,+Oak+Park+Heights,+MN+55082/@45.0355219,-92.7925001,3a,75y,142h,90t/data=!3m4!1e1!3m2!1swY5Tktp77WnFBZTL-Z5yjA!2e0!4m2!3m1!1s0x52b2ca0483785575:0xd9f7d99b33d1f516

I think the dual permissive lefts work better with flashing yellow arrows. Not sure why. Glad Minnesota's DOT doesn't outright ban their use (engineering judgement permits any sort of signal combination -- ).

Those left-turn signal heads should be bagged during construction. 

If you look carefully, the individual lights have been gift-wrapped. 

[tradephoric]

^Good catch, i didn't notice that the individual lights were wrapped.  I'm just use to seeing the entire signal head bagged.  IMO, having the entire signal head bagged makes it more obvious to drivers that the signal head is not in use.

[Mohkfry]

https://www.google.com/maps/place/St+Croix+Trail,+Oak+Park+Heights,+MN+55082/@45.0355219,-92.7925001,3a,75y,142h,90t/data=!3m4!1e1!3m2!1swY5Tktp77WnFBZTL-Z5yjA!2e0!4m2!3m1!1s0x52b2ca0483785575:0xd9f7d99b33d1f516

I think the dual permissive lefts work better with flashing yellow arrows. Not sure why. Glad Minnesota's DOT doesn't outright ban their use (engineering judgement permits any sort of signal combination -- ).

How about something like this.  Dual left turn lanes with no protected phase at all, just standard green balls, timer-actuated too, no detectors.  https://goo.gl/maps/qbdNs3WEjQv  This would date to the late 1960s when I-71 was built in downtown Cincinnati, and this overpass is way over-engineered at 6 lanes (probably for pre-interstate traffic that was all diverted).  Plus Elsinore coming down the hill from the top right is pretty lightly trafficked so there's usually only one or two cars at a time coming from that direction anyway. 

Interesting that the road wasn't scaled back after the 71 freeway was finished. Pretty cool nonetheless. I think this might be unique in being the only dual permissive left that has no protected phase.

Nope, here's another one not far from where I live.

https://www.google.com/maps/@41.5036083,-87.4714439,3a,75y,96.18h,75.06t/data=!3m6!1e1!3m4!1sBgz-MViN_o5JHW7SF88lsw!2e0!7i13312!8i6656!6m1!1e1

[jeffandnicole]

^Good catch, i didn't notice that the individual lights were wrapped.  I'm just use to seeing the entire signal head bagged.  IMO, having the entire signal head bagged makes it more obvious to drivers that the signal head is not in use.

Yeah, I had to zoom in pretty close to determine what happened.  If you look real carefully, you can see the red light is lit underneath the bag.

Here's somewhat of a similar situation: https://goo.gl/maps/2UHbNjsBQND2 . Train lights look like they're in use.  But when you look real close, you see bags are covering the lights.  https://goo.gl/maps/cAgUzNKoMLM2  Oh...there's no tracks either!  https://goo.gl/maps/qDEzsgMLnsF2

[thenetwork]

My first encounter with Dual Lefts was in Toledo, OH back in the 80s. They were all over the city (and still are), and most had protected left arrows.

Since Toledo has always been good in synch-ing their traffic signals -- even adjusting the cycles in morning and afternoon rush hours -- there would be no need for them to go to a FYA or Doghouse. IMHO.

[jakeroot]

Slightly thread revival warning:

Tucson is known for their dual permissive turns, as noted previously. They've even gone so far as to install 5-section heads for one of their "SPUIs", though there's a catch: it's not completely finished. The flyover has not been installed, nor has the road been completed beyond the "service road".

Anyone know of any other permissive SPUIs, regardless of the number of turn lanes?

The road with the dual turn is Broadway Blvd, and the road with the future flyover is AZ-210. Here's a couple of images:

[cl94]

Anyone know of any other permissive SPUIs, regardless of the number of turn lanes?

Nope. The baby half-SPUI at the VT 9/279 intersection is protected, as is every SPUI in New York and every one I know of in Ohio.

[lordsutch]

Anyone know of any other permissive SPUIs, regardless of the number of turn lanes?

Southbound Kirby Rd to TN 385 eastbound in Memphis used to have a permissive doghouse signal for a while (a year or two) after it went on a diet from two left turn lanes to one, but it's now back to a protected-only signal. As far as I know, that's the only permissive signal ever installed at one of 385's SPUIs.

[Revive 755]

Here's another one: https://goo.gl/maps/nwaCg11bp5H2

There's slightly more of the permissive dual lefts in Chicagoland than triple lefts.  There's also:

* Thorndale Avenue at Park Avenue in Itasca (until IL 390 is finished), with permissive dual lefts for both eastbound and southbound:  Streetview (DuPage County's jurisdiction)

* Sunset Ridge Road at Skokie Boulevard in Northbrook (IIRC under Cook County's jurisdiction) Streetview

I think there is one more in Chicagoland that I can't place as of this typing.

As for single point interchanges with permissive left turns:

* Grand Avenue at the Amstutz Expressway/IL 137 in Waukegan (probably doesn't count since the ramps are controlled by stop signs) Streetview

* Nebraska Highway 2 at US 77 in Lincoln (Streetview)

As for the poll in this thread:  Yes, I think permissive dual lefts should be allowable, especially when they are across an opposing through movement with a low volume.  Missouri has it in their manual that with engineering judgement, a dual left across a single opposing through can be used (see about halfway down the page here).

[jakeroot]

As for single point interchanges with permissive left turns:

* Grand Avenue at the Amstutz Expressway/IL 137 in Waukegan (probably doesn't count since the ramps are controlled by stop signs) Streetview

* Nebraska Highway 2 at US 77 in Lincoln (Streetview)

I almost wrote a sentence about excluding the Amstutz stop signs, but I figured I'd wait to see if anyone would bring them up. Didn't think it would be reply #2!

There's another permissive SPUI in Billings, Montana. Mullowney Lane at I-90 (streetview):





Slightly related: can anyone think of any reason why nearly all single-lane SPUIs are protected? Sometimes permissive turns are disallowed because of overlap, but SPUIs are designed to run with opposing left turns. If anything, they're built for permissive signals! I get why the majority are protected-only (most are dual turns, which are almost always protected), but the single-lane SPUIs would be great candidates for permissive signals.

[Jet380]

Slightly related: can anyone think of any reason why nearly all single-lane SPUIs are protected? Sometimes permissive turns are disallowed because of overlap, but SPUIs are designed to run with opposing left turns. If anything, they're built for permissive signals! I get why the majority are protected-only (most are dual turns, which are almost always protected), but the single-lane SPUIs would be great candidates for permissive signals.

My guess would be the intersection geometry, SPUIs tend to be on the larger side and have very wide sweeping left turns. This would make it take longer for turning vehicles to clear the opposing lanes and reduce the acceptable gap to the point where it could be too difficult to judge. I'd also imagine that SPUIs are normally built at busy locations with high enough turning movements to justify fully-protected phasing.

[jakeroot]

I did manage to find some dual permissive right turns outside of North America. Here's one a couple from South Africa:

Bryanston Drive @ William Nicole Drive, Sandton, Gauteng:



There are actually several in a row:

Sloane St @ William Nicole Drive, Sandton, Gauteng:



EDIT 2: Looks like South Africa actually has quite a few dual permissive turns. In fact, for quieter intersections, it appears to be the normal phasing style.

EDIT 3: No bullshit, triple permissive turn. Damn South Africa...

Sandton Drive @ the M81, Sandton, Gauteng:



EDIT 4: As you might have expected at this rate ... quadruple permissive right turn...

Republic Rd @ the M81, Sandton, Gauteng:



Many of these appear to be split-phased, but if you click around, you can almost always find the permissive phase.

[swbrotha100]

I think dual permissive left turns should be rare, and not common like it is in places like Tucson. If they're going to be used, at least use FYA signals for them. At least one intersection in Chandler AZ has this setup with the FYAs (Alma School Rd and Ray Rd).

[mrsman]

Many of these appear to be split-phased, but if you click around, you can almost always find the permissive phase.

Split-phased should not count as permissive, because those are always protected.  It just happens to be that the protected left turn phase is equal to the straight-through traffic phase.

[jakeroot]

Many of these appear to be split-phased, but if you click around, you can almost always find the permissive phase.

Split-phased should not count as permissive, because those are always protected.  It just happens to be that the protected left turn phase is equal to the straight-through traffic phase.

Well of course they aren't permissive. I was trying to say that the signals, in several of the street view images, appear to be split phased, but when you click around, you can see that they are not. Several of South Africa's multi-lane permissive right turns have the second or third right turn lane as an option lane, so you'd think they'd be split phased.

[Jardine]

Even with an obvious artillery cannon blowing up cars to deter folks attempting lane changing while dual left turning, and/or the folks dual left turning onto streets with more than 2 lanes and randomly going down whatever lane they want regardless of whether or not anyone is beside them on either side and they are also attempting a similar and non-compatible random lane selection themselves (all the while mowing down pedestrians) I'd say this is an extremely poor idea for places like Sarpy County Nebraska and Rockford Illinois.

And note, as we inch closer to the society envisioned in the film Idiocracy, this is going to get worse.

[jakeroot]

Even with an obvious artillery cannon blowing up cars to deter folks attempting lane changing while dual left turning, and/or the folks dual left turning onto streets with more than 2 lanes and randomly going down whatever lane they want regardless of whether or not anyone is beside them on either side and they are also attempting a similar and non-compatible random lane selection themselves (all the while mowing down pedestrians) I'd say this is an extremely poor idea for places like Sarpy County Nebraska and Rockford Illinois.

This perceived danger (vehicles changing lanes during a turn) seems to be the biggest fear among those who oppose dual permissive turns. My thought is this: are vehicles changing lanes during the permissive phase more often than during the protected phase?



Visibility really should be the argument against dual permissive turns. From studying the various dual turn lanes around the world, I've come up with two solutions to this issue:

1) Offset the dual turn lanes whenever possible. This requires slightly more ROW, but the cities that use this style seem to report fewer issues. Tucson is the best example. Note the chevrons between the left turn lanes and the through lanes. The FHWA's "Signalized Intersections: Informational Guide" has a couple pages dedicated to Tucson's offset dual turns lanes, and notes a "potential issue is sight distance for the left-turning vehicles", and that "[t]he City of Tucson addresses this concern by offsetting the far lane by 1.2 to 1.5 m (4 to 5 ft) so that it has the same sight distance as a single left-turn lane" (PDF pg 338).



2) Use secondary stop lines. These are almost unheard of in the US. They seem to be used more often in Europe, but more important to this conversation, South Africa has several dual permissive turn lanes with these secondary stop lines. In cities with drivers who pull pretty far forward into the intersection (cool with a single turn lane, but a little annoying with two turn lanes), this helps prevent the inside lane from being totally blind. As well, cities with more hesitant drivers will be more apt to pull forward to wait to turn, which helps with throughput. Because this secondary stop line is more of a "yield line", I'd recommend using sharks teeth, instead of the broken white lines used in South Africa:

[swbrotha100]

I personally think the permissive dual left turns should be a rarity instead of the norm in places like Tucson. I think one thing people who have never been to Tucson should realize that most of the Tucson/Pima County area uses lagging left turns at their signalized intersections.

[tradephoric]

I personally think the permissive dual left turns should be a rarity instead of the norm in places like Tucson. I think one thing people who have never been to Tucson should realize that most of the Tucson/Pima County area uses lagging left turns at their signalized intersections.

That's a great point.  With lagging left turns drivers have an opportunity to clear the intersection at the end of the thru phase.  Permissive dual left turns are somewhat more acceptable if drivers are guaranteed a protected left at the end of the thru phase. 

[jakeroot]

I personally think the permissive dual left turns should be a rarity instead of the norm in places like Tucson.

But why? Is there any evidence to suggest that Tucson needs to do away with their permissive dual left turns? Tucson's website is highly protective of the permissive phase:

Protected Left Turn Arrows

Protected left turn signals include a red arrow along with the normal green and amber arrow. They allow left turning drivers to proceed only on the green arrow. This turning method is very inefficient and generally not used in Tucson. Adding inefficiencies to signal timing reduces overall capacity and increases congestion. With increased congestion comes the potential for an increase in certain types of accidents.

Permitted/Protected Left Turn Arrows

This is the most common turning method used in Tucson at locations having left turn arrows. During the permitted "green ball" part of the cycle, vehicles are allowed to turn when there are adequate gaps in opposing traffic. This type of left turn phasing is designed to help minimize delay by eliminating the need for the red arrow and allowing vehicles to turn on the green ball after opposing traffic has cleared. By not having the red arrow, motorists do not have to sit and wait to turn left even when there is no opposing traffic, a situation that often occurs during periods of low traffic volumes. The signal still provides a green left turn arrow for those not able to turn during the permitted phase.

I think one thing people who have never been to Tucson should realize that most of the Tucson/Pima County area uses lagging left turns at their signalized intersections.

On leading vs lagging signals, with the advent of flashing yellow arrows, protected/permissive left turns should begin with the permissive phase. If there's only one car, they'll get through either in the middle of the cycle, or at the end. And if several more cars show up to turn left, the lagging protected left turn will be able to get the rest of them through the intersection, rather than making them wait for the next leading left turn, which would be after the cross-traffic gets their phase.

My point being, Tucson is pretty ingenious, having adopted lagging pro/per signals. Though I'd like to see them transition over to FYAs, so there isn't a red phase in the middle of the left turns.

[cl94]

I think PPLT situations should always have the permissive phase at the end. Cuts down on cycle time, as traffic conditions might not warrant a permissive phase for the vehicles waiting to turn. It is not very common in much of the northeast, but NYSDOT Region 1 uses lagging phases extensively, especially if only one direction gets a protected phase. I do not know if the region's FYA intersections are leading or lagging.