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Dallas Phasing

Started by US71, January 20, 2010, 09:45:48 PM

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US71

I don't know if this has been discussed before, but I found what I consiederd to be a dangeroue traffic signal.

The 2 through lanes have the standard R-Y-G signals and the left turn lane has the standard R-Y-G-YA-GA signal.

I suppose this is to keep traffic flowing at a busy intersection, but I noticed after the Protected Left Turn sequence, the Left Turn signal went to Steady Green, while the thru lanes were still Red. Oncoming traffic, however, must have had steady green AND a protected left as traffic was going straight thru as well as turning.

I've only seen this once before and thought it was a dangerous sequence: if the other lanes are Red, but you have Green, wouldn't you assume you had a protected turn? Yes, you should notice there is oncoming traffic, though.

I witnessed an accident today where the person turning left thought she had a protected left on a steady Green and ran into a car passing through the intersection. No one was hurt, fortunately.

Am I wrong in thinking this is a dangerous & confusing signal sequence?

[Modified thread title.]
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agentsteel53

was there a sign that said "yield to opposite traffic on solid green circle" or similar?
live from sunny San Diego.

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Ian

This is a frequent install to be found in Pennsylvania (or at least in my area). Sometimes, there can be 2 thru lanes and 1 turn lane and there will be 1 thru signal and one doghouse signal. Here is an example of the one you explained...


And here is the one with 1 thru and 1 doghouse signal...
http://maps.google.com/maps?hl=en&ie=UTF8&ll=39.9126,-75.435326&spn=0,359.977319&t=h&z=16&layer=c&cbll=39.913072,-75.43701&panoid=-JrCKQs5Y6EFUAf2FXZoiA&cbp=12,70.98,,0,0.83
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US71

Quote from: agentsteel53 on January 20, 2010, 09:53:27 PM
was there a sign that said "yield to opposite traffic on solid green circle" or similar?

Should have been, but I don't remember. I'll have to check the photos I took
Like Alice I Try To Believe Three Impossible Things Before Breakfast

Bryant5493

I'm not sure if I'm misunderstn
Quote from: US71 on January 20, 2010, 09:45:48 PM
I don't know if this has been discussed before, but I found what I consiederd to be a dangeroue traffic signal.

The 2 through lanes have the standard R-Y-G signals and the left turn lane has the standard R-Y-G-YA-GA signal.

I suppose this is to keep traffic flowing at a busy intersection, but I noticed after the Protected Left Turn sequence, the Left Turn signal went to Steady Green, while the thru lanes were still Red. Oncoming traffic, however, must have had steady green AND a protected left as traffic was going straight thru as well as turning.

I've only seen this once before and thought it was a dangerous sequence: if the other lanes are Red, but you have Green, wouldn't you assume you had a protected turn? Yes, you should notice there is oncoming traffic, though.

I witnessed an accident today where the person turning left thought she had a protected left on a steady Green and ran into a car passing through the intersection. No one was hurt, fortunately.

Am I wrong in thinking this is a dangerous & confusing signal sequence?


I'm not sure if I'm misunderstanding your post or not, but, generally, there's a protected phase and a permissive phase at intersections where the five section head (doghouse) signals are installed. If the left lane has a green arrow, then oncoming traffic shouldn't be coming through the intersection on the left turning traffic's protected phase. On a steady green, left turning traffic has to yield to through traffic and right turning traffic, unless right turning traffic has a yield sign.

If the left lane signal was green, through traffic was red and oncoming traffic had a steady green and protected phase, then that's a poorly designed traffic sequence or a glitch in the system.


Be well,

Bryant
Check out my YouTube page (http://youtube.com/Bryant5493). I have numerous road videos of Metro Atlanta and other areas in the Southeast.

I just signed up on photobucket -- here's my page (http://s594.photobucket.com/albums/tt24/Bryant5493).

US71

It's not a Doghouse, just a standard 5 light signal

  R
  Y
  G
<Y-
<G-
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Bryant5493

Oh, okay. It's a vertical signal with five lights. Gotcha. The first time I saw one of those was when I first started going to Georgia State. It was at the intersection of Decatur St. SE and Peachtree Center Ave. SE in Downtown Atlanta.

When the tornado came to Atlanta, the signal was replaced with a doghouse signal.


Be well,

Bryant
Check out my YouTube page (http://youtube.com/Bryant5493). I have numerous road videos of Metro Atlanta and other areas in the Southeast.

I just signed up on photobucket -- here's my page (http://s594.photobucket.com/albums/tt24/Bryant5493).

jdb1234

#7
I have seen one of those on 21st Street at the ramp to I-20/59 South in Birmingham.  It must be a new signal because the casing is black.  

I remember two vertical signals with 5 lights at the interchange between US 280 and AL 149 near Mountain Brook.  Those have been replaced recently.  

roadfro

#8
Sorry in advance...this is a bit of a technical reply...

Quote from: US71 on January 20, 2010, 10:43:59 PM
It's not a Doghouse, just a standard 5 light signal

Fundamentally, there's no difference between a 5-section cluster ("doghouse") and a 5-section vertical (or horizontal) signal head. Signing and operation are the same.

Most agencies usually use the doghouse on a mast arm or span wire, and use the vertical display for supplemental heads on the far-side pole mount. I've seen some agencies use the vertical display on the mast arm (and Texas frequently uses the horizontal here), but I imagine most like the cluster because it keeps the similar colors aligned to the same height both within the face and relative to adjacent signal heads.

Quote from: US71 on January 20, 2010, 09:45:48 PM
I don't know if this has been discussed before, but I found what I consiederd to be a dangeroue traffic signal.

The 2 through lanes have the standard R-Y-G signals and the left turn lane has the standard R-Y-G-YA-GA signal.

I suppose this is to keep traffic flowing at a busy intersection, but I noticed after the Protected Left Turn sequence, the Left Turn signal went to Steady Green, while the thru lanes were still Red. Oncoming traffic, however, must have had steady green AND a protected left as traffic was going straight thru as well as turning.

I've only seen this once before and thought it was a dangerous sequence: if the other lanes are Red, but you have Green, wouldn't you assume you had a protected turn? Yes, you should notice there is oncoming traffic, though.

I witnessed an accident today where the person turning left thought she had a protected left on a steady Green and ran into a car passing through the intersection. No one was hurt, fortunately.

Am I wrong in thinking this is a dangerous & confusing signal sequence?

By "steady green", I'm assuming you mean a circular green or green ball. If I've interpreted that and the rest of your post correctly, I think what you're describing here is a variant of protective-permissive left turn control known as "Dallas Phasing". This is a signal phasing scheme developed and widely used in Dallas, TX; it has seen a decent amount of use in Las Vegas area as well.

In Dallas Phasing, the circular green of the left turn signal operates independently of the circular greens of the adjacent through signals. This allows the left turn to display a circular green and allow permissive left turns when the opposing direction has a green signal for the through traffic and a green arrow for the left turn. This mode of operation is designed to prevent the "yellow trap" situation that can occur with protective-permissive left turns operated with lead/lag left turn phasing at the intersection.  This method of operation does allow for some increased efficiency for the left turn while also allowing lead/lag phasing to be used on the approach, which is often a big help for signal progression on an arterial.

When Dallas Phasing is used, there are a couple conditions that have to be followed in order for it to work properly. First, the 5-section left turn display must be an exclusive signal face, i.e. it must be meant to control the left turn lane only and not the adjacent through lanes. Second, the circular portions of the signal face need to have visibly limiters so that they are only visible to the left turn lane and not the through traffic (usually accomplished with louvers). Third, the standard "Left Turn Yield on Green (symbolic green ball)" (R10-12) sign should be replaced with the "Left Turn Signal (line) Yield on Green (symbolic green ball)" (R10-21) sign. With all these factors in place, the through traffic should not mistake the left turn signal for the through signal.

There is nothing inherently wrong or dangerous about a PPLT display using Dallas Phasing, if the appropriate safeguards are in place. The confusion is more attributable to the fact that it's a fairly unexpected sequence, as a left turning driver typically won't expect to have a circular green when the adjacent through traffic has a circular red. This is likely one of the reasons why Dallas Phasing isn't more widespread.

The greater problem with the accident situation mentioned is that many drivers see a circular green over the left turn lane and automatically assume that they have a protected left turn. This happens frequently with 5-section PPLT displays (whether they run Dallas Phasing or not), but can also happen with a standard 3-section display. The only time you have a protected movement is if an arrow is involved, and not all drivers realize this. This misinterpretation of the circular green in regards to permissive left turns is what prompted a lot of changes to left turn controls in the 2009 MUTCD. Now, 5-section left turn faces can only be used if its circular indications are illuminated simultaneously with whatever is displayed on the adjacent through signal faces.  Circular green and yellow indications are no longer allowed in exclusive left turn signal heads...having been deprecated in favor of the flashing yellow arrow display. The flashing yellow arrow display more easily implements the principles of Dallas Phasing, using a 4-section display that doesn't require louvers or other shielding.
Roadfro - AARoads Pacific Southwest moderator since 2010, Nevada roadgeek since 1983.

Bryant5493

^^ Wow, learn something new everyday.


Be well,

Bryant
Check out my YouTube page (http://youtube.com/Bryant5493). I have numerous road videos of Metro Atlanta and other areas in the Southeast.

I just signed up on photobucket -- here's my page (http://s594.photobucket.com/albums/tt24/Bryant5493).

jjakucyk

Sounds like a perfect location for an inline-4 yellow trap signal.  Incidentally, Illinois uses the inline-5 signals almost exclusively, as opposed to doghouses.   

Michael

If I understand Dallas Phasing correctly, a doghouse signal is dedicated to the left turn lane.  Protected mode (opposing red) has both the green ball and arrow lit.  Permitted mode (opposing green) has just the green arrow lit.  Am I correct?

US71

#12
For the sake of clarification, here is a photo of the signal in question:


A Left Turn is permitted, but not protected while oncoming WB traffic has steady Green. BUT thru traffic going EB still has Red.
Like Alice I Try To Believe Three Impossible Things Before Breakfast

Brandon

^^ That's screwy.  The arrows should be used, not the ball.
"If you think this has a happy ending, you haven't been paying attention." - Ramsay Bolton, "Game of Thrones"

"Symbolic of his struggle against reality." - Reg, "Monty Python's Life of Brian"

US71

#14
Quote from: Brandon on January 21, 2010, 02:00:00 PM
^^ That's screwy.  The arrows should be used, not the ball.

But, the turn is not protected.

This still seems wrong to me, though. It should be a Left Arrow OR Steady green on all signals... or possibly a Flashing Yellow Arrow (which Fayetteville doesn't have).


Like Alice I Try To Believe Three Impossible Things Before Breakfast

rawmustard

Quote from: Brandon on January 21, 2010, 02:00:00 PM
^^ That's screwy.  The arrows should be used, not the ball.

That's Dallas phasing, though, as roadfro explained. It's effectively become verboten with the 2009 MUTCD, however. Situations like this were precisely why the flashing-yellow arrow signalface was developed.

roadfro

Quote from: Michael on January 21, 2010, 12:31:22 PM
If I understand Dallas Phasing correctly, a doghouse signal is dedicated to the left turn lane.  Protected mode (opposing red) has both the green ball and arrow lit.  Permitted mode (opposing green) has just the green arrow lit.  Am I correct?
Not correct on the second count. In permitted mode, the circular green is lit -- as indicated in the photo above.

Quote from: US71 on January 21, 2010, 01:54:12 PM
For the sake of clarification, here is a photo of the signal in question:

A Left Turn is permitted, but not protected while oncoming WB traffic has steady Green. BUT thru traffic going EB still has Red.
This would appear to be using Dallas Phasing. They're not using the correct sign, which isn't a big deal. But from what I can tell on the photo, the circular green in the left turn signal head is not visibility limited from the adjacent through traffic. That is a major safety concern and a violation of the 2003 MUTCD standard for PPLT displays.


I'm working on putting something together to illustrate the differences between Dallas Phasing and other modes.  I might have it done this afternoon...
Roadfro - AARoads Pacific Southwest moderator since 2010, Nevada roadgeek since 1983.

Brandon

Quote from: rawmustard on January 21, 2010, 02:13:28 PM
That's Dallas phasing, though, as roadfro explained. It's effectively become verboten with the 2009 MUTCD, however. Situations like this were precisely why the flashing-yellow arrow signalface was developed.

That's something I've never seen used in Illinois, Wisconsin, Michigan, or Indiana.  Michigan used to use a flashing red ball for the left turn permissive phase (now being replaced with the flashing yellow arrow), but that's it.  If I were to look at that signal in the photo, I'd assume there's some malfunction with the signal (based on my experience).
"If you think this has a happy ending, you haven't been paying attention." - Ramsay Bolton, "Game of Thrones"

"Symbolic of his struggle against reality." - Reg, "Monty Python's Life of Brian"

Duke87

There must be a better way of accomplishing that than with a green ball. How about a flashing red arrow?

Flashing red acts like a stop sign, so this perfectly fits the condition: for left turns, stop, go when clear.
If you always take the same road, you will never see anything new.

agentsteel53

Quote from: Duke87 on January 21, 2010, 04:54:11 PM
There must be a better way of accomplishing that than with a green ball. How about a flashing red arrow?

Flashing red acts like a stop sign, so this perfectly fits the condition: for left turns, stop, go when clear.

sometimes you can take the turn on the fly, though, without needing to come to a complete stop
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froggie

Same thing I'd think...some sort of malfunction.

If it was intention, it's clearly in violation of the MUTCD, as other posters noted.

And a good candidate for the Flashing Yellow Arrow, if the intention IS to allow left turners to go if clear while the through traffic has the red phase.

US71

Quote from: froggie on January 21, 2010, 05:00:49 PM

If it was intentional, it's clearly in violation of the MUTCD, as other posters noted.

And a good candidate for the Flashing Yellow Arrow, if the intention IS to allow left turners to go if clear while the through traffic has the red phase.

I'm consulting with Gridlock Guru to see if his contacts with AHTD can justify this signal sequence (there are at least 2 others in town I've seen, but never photographed) or if there any plans for a FYA.
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Scott5114

For what it's worth, the signal at S. 19th Street and I-35 in Moore, OK does the same thing, but in a doghouse. Baffled me too. Thought about posting here and writing to the Moore DPW... hard to believe this is actually an intentional phasing...
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roadfro

#23
Quote from: Duke87 on January 21, 2010, 04:54:11 PM
There must be a better way of accomplishing that than with a green ball. How about a flashing red arrow?

Flashing red acts like a stop sign, so this perfectly fits the condition: for left turns, stop, go when clear.

There is...flashing yellow arrow.

In addition to flashing yellow arrow displays, the 2009 MUTCD also added provisions for regular use of flashing red arrows. These are to be used in rare circumstances where permitted phasing can be used, but engineering studies/judgment indicates each driver must come to a complete stop before proceeding through the left turn (more severe cases like a sight distance limitation). For 99% of permissive left turn cases, a full stop is not necessary--flashing yellow arrows are better suited for this as they are treated more like a yield situation.
Roadfro - AARoads Pacific Southwest moderator since 2010, Nevada roadgeek since 1983.

roadfro

#24
Quote from: US71 on January 21, 2010, 05:17:58 PM
I'm consulting with Gridlock Guru to see if his contacts with AHTD can justify this signal sequence (there are at least 2 others in town I've seen, but never photographed) or if there any plans for a FYA.

The sequence is perfectly valid, according to the 2003 MUTCD. The problem is the lack of louvers in the separate left turn signal head to block the circular green from being seen by the adjacent thru traffic (that part is not compliant).

With revisions in the 2009 MUTCD, this sequence is no longer valid.  However, I don't see any compliance dates associated with this. So this can mean that the existing phasing can be retained until the signal is removed, modified or reconstructed--at which time a flashing yellow arrow display would be appropriate.
Roadfro - AARoads Pacific Southwest moderator since 2010, Nevada roadgeek since 1983.



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