Dallas Phasing

[roadfro]

This took me a little while to finish up, but I hope you all will find it helpful.  I've created a short video that explains in a graphic format the differences between the various forms of protected/permitted left turn controls, including Dallas Phasing.

By no means am I a whiz with video capture and online videos, so I hope it works okay. Here it is: http://www.youtube.com/watch?v=KPKjcPI5Sko

[Bryant5493]

^^
Great video. Seeing "Dallas Phasing," as well as the flashing yellow sequence for permissive turns, was pretty cool. I've never seen either of them before. Most of the signals that I've seen in Georgia -- for left turning traffic -- are the "doghouse" signals or the three-section head signals (red arrow/steady red, yellow arrow, green arrow). Additionally, I've not seen too many signals with the "yellow trap," except for the traffic signal on Cleveland Avenue at Jefferson Terrace/S. Fulton Med. Ctr. in East Point, Georgia.


Be well,

Bryant

[roadfro]

^ Thanks for the accolade on the video. It was my first effort at doing something like that.

The yellow trap can only show up at PPLT signals using lead/lag for the protected portion of the left turns. Using lead/lag phasing is common when signal coordination is a priority, so you'd have to be in a jurisdiction that focuses heavily on coordination...but such a jurisdiction would (should) have altered the phasing to avoid having the yellow trap occur.

[rawmustard]

The yellow trap can only show up at PPLT signals using lead/lag for the protected portion of the left turns. Using lead/lag phasing is common when signal coordination is a priority, so you'd have to be in a jurisdiction that focuses heavily on coordination...but such a jurisdiction would (should) have altered the phasing to avoid having the yellow trap occur.

It can also conceivably occur in situations where a side-street phase is skipped and a serviceable call exists for a lead left-turn leg, although modern controllers should be able to be programmed to prevent this. At least in Michigan, there have been a few instances where on a street crossing a railroad, the yellows would be staggered to where the approach coming towards the track would change before the one going away from the track (basically to match the opposing approach's signal placed in front of the track). In recent years, some of these instances have been fixed as well.

[Michael]

Thanks, roadfro!  I sort-of get Dallas Phasing now.  I also FINALLY understand yellow trap!  :jumping:

Now I have two more questions:

First, using your example, why would you want northbound traffic to stop, allow northbound left turns, and give southbound traffic a green?!

Secondly, why would there be a doghouse signal dedicated to a left-turn lane?  It should be shared with a thru head, like the 2009 MUTCD requires.  In my opinion, any dedicated left turn signal should either be all arrows, or have an adjacent "LEFT TURN SIGNAL" sign, like this or this (light in background of the first photo).

[roadfro]

It can also conceivably occur in situations where a side-street phase is skipped and a serviceable call exists for a lead left-turn leg, although modern controllers should be able to be programmed to prevent this.

This is true...hadn't thought of that.

My understanding is that when Dallas Phasing was first conceived, there was quite a bit of wrestling involved with signal controllers in order to get the displays to overlap correctly (with the adjacent and opposing thru signals) and not show the yellow trap. Same was true for the flashing yellow arrow situation.  Most new signal controllers (last 5 years or so) now have flashing yellow arrow programming built in.

Thanks, roadfro!  I sort-of get Dallas Phasing now.  I also FINALLY understand yellow trap!  :jumping:

I'm glad you found it helpful. When I first learned about the yellow trap in college, I didn't get right away. It partially had to do with the fact that any animation I had seen never showed what was happening with the opposing signal.

First, using your example, why would you want northbound traffic to stop, allow northbound left turns, and give southbound traffic a green?!

A situation like this is most likely to show up during a peak hour timing plan. The arterial is running lead/lag protected lefts in order to help improve signal progression along the corridor.

The geometry of this intersection may be such that the left turn demand cannot be adequately served during the protected only phase. In that case, you want to allow permitted lefts at any time possible. Allowing the NB permitted left to occur while the adjacent thru traffic is stopped gives that left turn more possible chances to complete the turn. In the Dallas Phasing and FYA examples, the NB left had about 12 more seconds to find a gap for a permitted left turn. Given that this time comes at the end of the SB thru movement, it is more likely that there will be gaps in the SB thru traffic thus increasing the chances of being able to make a NB protected left.

Secondly, why would there be a doghouse signal dedicated to a left-turn lane?  It should be shared with a thru head, like the 2009 MUTCD requires.  In my opinion, any dedicated left turn signal should either be all arrows, or have an adjacent "LEFT TURN SIGNAL" sign, like this or this (light in background of the first photo).

One reason to have a separate signal face for the left turn may be due to geometry, such as if the left turn lane is separated from the adjacent thru lanes by a short median or other buffer. Here's an example from Las Vegas: Craig Road at Rainbow Blvd (Google Map/Street View).

Probably the more likely reason for the separate left turn doghouse has to do with a semi-recent push (last 10 years or so) by many agencies to install one signal head per lane on an approach. As far as I'm aware, this isn't a mandated standard anywhere, but is recommended for a number of reasons. In any event, this would be interpreted to include the left turn lanes. Having a separate face also gives agencies more options in dealing with the lefts. For example, there's intersections around Las Vegas that use a doghouse signal but, due to safety concerns, have protected-only lefts during peak hours. In this case, the circular indications are dark during the adjacent thru phase (and a louvered circular red or red arrow is used instead).


The "Left Turn Signal" sign, such as those pictured, were developed to please agencies that did not implement arrow signals right away. The sign was necessary to distinguish the circular indications in the left turn face from those in the adjacent thru faces, which were frequently different (and in my opinion was a greater safety hazard than Dallas Phasing). They were also used for where a left turn face had green and yellow arrows but used a circular red (a signal face arrangement I've never understood).  Such signs are not needed where a full arrow display is used, as appears to be the case in the links provided.

Incidentally, the "Left Turn Signal" sign is no longer present in the MUTCD. The 2009 version did away with signal arrangements that would require the sign to be used, such as the circular red with other arrows.  The reasoning behind this is that arrow signals have been around for over 30 years now, and having circular indications in a 3-section head for protected left turns is no longer necessary and can be confusing to motorists.

[realjd]

Probably the more likely reason for the separate left turn doghouse has to do with a semi-recent push (last 10 years or so) by many agencies to install one signal head per lane on an approach. As far as I'm aware, this isn't a mandated standard anywhere, but is recommended for a number of reasons. In any event, this would be interpreted to include the left turn lanes.

I believe this practice is frowned upon in the new MUTCD. They now recommend against protected/permissive, or permissive only, left turn lanes from having their own signal head. See http://mutcd.fhwa.dot.gov/services/ppt/mutcd09training/mutcd09part4.ppt, slide 25.

They were also used for where a left turn face had green and yellow arrows but used a circular red (a signal face arrangement I've never understood)... Incidentally, the "Left Turn Signal" sign is no longer present in the MUTCD. The 2009 version did away with signal arrangements that would require the sign to be used, such as the circular red with other arrows.  The reasoning behind this is that arrow signals have been around for over 30 years now, and having circular indications in a 3-section head for protected left turns is no longer necessary and can be confusing to motorists.

It will be interesting to see what Indiana does about the red arrow (instead of circular red) rule for left turns. They have their own state MUTCD supplement where they specifically forbid red arrows until the legislature gives them a legal definition. Apparently, in the Indiana traffic law, they explicitly define what red, yellow, and green ball signals do, as well as yellow and green arrows, but they intentionally didn't define a red arrow signal as being legal in Indiana. The story goes that one of the older legislators feels that arrows indicate motion, not direction, so a red arrow was too confusing for use in Indiana. Folks there tend to resist change of any sort, so it's not surprising.

[roadfro]

Probably the more likely reason for the separate left turn doghouse has to do with a semi-recent push (last 10 years or so) by many agencies to install one signal head per lane on an approach. As far as I'm aware, this isn't a mandated standard anywhere, but is recommended for a number of reasons. In any event, this would be interpreted to include the left turn lanes.

I believe this practice is frowned upon in the new MUTCD. They now recommend against protected/permissive, or permissive only, left turn lanes from having their own signal head. See http://mutcd.fhwa.dot.gov/services/ppt/mutcd09training/mutcd09part4.ppt, slide 25.

The specific practice frowned upon is use of circular indications (particularly the green ball) in a separate signal face over the left turn lane. A separate permissive only or protected/permitted signal head is allowed for left turn lane, but it must be an all-arrow display (i.e. flashing yellow arrows).

It will be interesting to see what Indiana does about the red arrow (instead of circular red) rule for left turns. They have their own state MUTCD supplement where they specifically forbid red arrows until the legislature gives them a legal definition. Apparently, in the Indiana traffic law, they explicitly define what red, yellow, and green ball signals do, as well as yellow and green arrows, but they intentionally didn't define a red arrow signal as being legal in Indiana. The story goes that one of the older legislators feels that arrows indicate motion, not direction, so a red arrow was too confusing for use in Indiana. Folks there tend to resist change of any sort, so it's not surprising.

Well something will have to give there. The states have two years to adopt the MUTCD or get a state manual/supplement together that's in "substantial compliance". So there will be time for Indiana to change the law.

A while back I was having a debate at another forum about whether you could turn right on a red arrow in Nevada if there is no sign prohibiting it. I found that Nevada law also doesn't explicitly define what a red arrow means, but does have definitions of other arrows. That hasn't stopped us from installing red arrows statewide.

[Riverside Frwy]

Is roadfro an actual freak'in Civil Engineer or something?

I feel likea dumba$$ when it comes to roads when this guy talks....

[Michael]

^^^ 

I'm still unsure of why there are both arrows and balls on a signal for a single lane.  I think "(color) ball/arrow" means protected, while "(color) ball only" means permitted.

Also, I still don't get how give opposing traffic a green, allowing turns, but restricting thru traffic helps increase capacity.  Below is a picture from roadfro's video.  I'm assuming the sides that aren't shown have a red signal.


Note that I would use a seperate signal head with left arrows (or balls with a "LEFT TURN SIGNAL" plaque) instead of a doghouse.

[roadfro]

Is roadfro an actual freak'in Civil Engineer or something?

I feel likea dumba$$ when it comes to roads when this guy talks....

Not quite.  I have a bachelor's degree in civil engineering, and have taken several civil engineering courses working towards a master's degree in that field with a traffic/transportation emphasis. I've also had a lifelong fascination with the operational aspects of traffic signs and signals. However, I am not currently employed as a civil engineer.

When I give a technical reply like these, I don't do it to make anyone feel less intelligent but rather to share what I know...especially as it relates to traffic engineering. I've often found that many people have an opinion when it comes to traffic, but don't necessarily realize the methods behind the engineers' madness. If I can help folks understand why something works the way it does, I feel it helps them be a little more informed and hopefully they don't diss all engineers in the future.

[roadfro]

I'm still unsure of why there are both arrows and balls on a signal for a single lane.  I think "(color) ball/arrow" means protected, while "(color) ball only" means permitted.

A green arrow is the only display that definitively indicates a protected turn.  A "green ball+arrow" indication gives a protected left solely due to the presence of the green arrow.

I think the reasoning behind the 5-section display has to do with what had been allowed for use as a permissive display. For many years, the national MUTCD had only defined one signal indication that would allow a permissive left turn: the circular green. Protected/permitted displays came later, and needed to find a way to indicate both protected and permitted operation in one display that also complied with the MUTCD.

Although some areas around the country had developed other methods of displaying permitted lefts (flashing red arrow, flashing red ball, flashing green arrow, flashing green ball, flashing yellow arrow), none were defined by the national MUTCD. Up until this new version, the only flashing signal operations defined by the national MUTCD were red flash mode (emergency all-way stop), red/yellow flash mode (nighttime operation), and beacons.

Also, I still don't get how give opposing traffic a green, allowing turns, but restricting thru traffic helps increase capacity.  Below is a picture from roadfro's video.  I'm assuming the sides that aren't shown have a red signal.

In regards to capacity, this is specifically referring to the capacity of the northbound left turn, not the capacity of the entire northbound approach. More specifically, it helps more in decreasing delay than increasing capacity for that turn. The benefit comes from the extension of the permissive left turn interval. This can allow more time for permitted left turns to find a gap and complete the turn without needlessly making them wait for the next signal cycle. I'll try to explain it further.

At the point of the video shown above, the northbound left turn has already received its leading protected left turn for this cycle but the southbound lagging left turn is about to come on. The northbound thru vehicles have been stopped in anticipation for the southbound left turn, but the southbound thru vehicles still have a green. Say at this point, you have two vehicles in that northbound left turn lane. If this was a standard PPLT display, they would be seeing a circular red just like the adjacent thru traffic. Meanwhile, the opposing traffic is still going through. Conceivably, there would be gaps in the opposing through traffic at this point that the northbound left turn drivers could take advantage of, and continue to take advantage of until the cross street phase begins. Depending on the opposing traffic and length of the southbound protected left turn phase, both of the vehicles waiting in the northbound left turn lane might be able to complete their turn before the cross street phase begins. (In the animation, it's about 12 extra seconds to find a gap; in a real-world scenario with peak hour traffic, that time could be longer.) With the standard PPLT display, both of these vehicles would have to wait until the beginning of the next cycle (after cross traffic has had its turn).

Hopefully that made sense. I don't think I can make it clearer...

Note that I would use a seperate signal head with left arrows (or balls with a "LEFT TURN SIGNAL" plaque) instead of a doghouse.

In a situation where both protected and permitted left turns are desired, a separate signal head with red/yellow/green arrows would not allow the permissive movement during the southbound thru phase. Similarly, a separate signal head with only red/yellow/green circular indications cannot be used for protected turns (regardless of whether it has a "Left Turn Signal" sign or not).

As of the 2009 MUTCD, the only methods for showing both protected and permitted left turns in a single signal face are a shared 5-section face (first example in the video) or a flashing yellow arrow face (third example in the video).

[Bryant5493]

"Dallas Phasing" seems like a good idea if people will enter the gap on the green ball; however, I get behind people who won't turn into a gap on a "regular" permissive cycle -- they'll just sit and wait 'til the next cycle, treating the green ball like a red left turn arrow.


Be well,

Bryant

[CL]

This is the first time I've ever seen the "yellow trap." When I saw the animation of it, my heart skipped a beat. That is probably the most dangerous traffic signal configuration I've seen. The light turns yellow, you wait for opposing traffic to stop so you can execute the left turn, and they don't stop. Am I missing something or is this just not a good idea?

As for Dallas phasing, there are three intersections in Salt Lake City with that configuration, all installed three or so years ago (the picture below is one of those at 1300 E. Sunnyside Ave if you want to Street View it). The first time I drove through it, there was a flashing yellow left-turn signal with a red through-lane signal. I was confused about what the traffic light was telling traffic exactly... Now I get what that signifies for left-turn traffic thanks to roadfro's excellent Powerpoint. The question is, does the general public?

[roadfro]

I should clarify that "Dallas Phasing" specifically refers to the unique signal phasing scheme using the 5-section PPLT display. 4-section flashing yellow arrow displays accomplish the same thing as Dallas Phasing display, but its use is not referred to as Dallas Phasing. A technicality, I know.


Most jurisdictions have not used lead/lag left turns with PPLT displays for the sole reason of not introducing a yellow trap situation. So yes, it was far from an ideal situation.  Dallas Phasing was specifically developed for those agencies that needed to run lead/lag PPLT displays for signal timing progression purposes, in order to prevent the yellow trap.

The first flashing yellow arrows were developed in Reno, NV. The city engineer at the time (an acquaintance of my for a few years now) was instrumental in getting it implemented. From what he told me once, Reno installed flashing yellow arrows with little fanfare, and the public took to it relatively quickly. Ironically, there have been no FYA displays in Reno for at least 9 years now (regime change came, and the new city engineer had many of the installations replaced with protected-only signals).

This has been discussed in an older FYA thread, but I'll mention it again. An NCHRP study was the impetus behind getting FYA displays approved by the FHWA. That study looked at every known PPLT display that was in existence in the U.S. at the time.  They also used simulations to test driver comprehension of the various displays. The FYA display was shown to be the most understood, even more so than the 5-section displays. Apparently, it's understood well enough that FHWA didn't even develop a standard sign for it in the MUTCD (I believe the interim approval stated a sign similar to the R10-12 could be used, but this was not retained).

[US71]

Apparently, it's understood well enough that FHWA didn't even develop a standard sign for it in the MUTCD (I believe the interim approval stated a sign similar to the R10-12 could be used, but this was not retained).

The sign I've seen is "Left Turn Yield ON  <-- " (being a Yellow arrow on a black circle). I need to get a couple photos for my files.

[shoptb1]

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 "Dallas Phasing" is being used in Fayetteville, AR at the intersection of 112 & 112-S?  I can imagine accidents there as this would be a very unexpected sequence.  Instead of the green ball, a flashing yellow arrow should be used.  This is much more self-explanatory to the drivers, IMO.

[US71]

This "Dallas Phasing" is being used in Fayetteville, AR at the intersection of 112 & 112-S?  I can imagine accidents there as this would be a very unexpected sequence.  Instead of the green ball, a flashing yellow arrow should be used.  This is much more self-explanatory to the drivers, IMO.

Also at 6th St & Futrall (NB 540) for WB traffic. Those are the only 2 I've seen.
Fayetteville has NO Flashing Yellow Arrow signals at this time.

Accidents? Hell yes! See the white car in the distance? The Honda hit her, but kept going.

[roadfro]

This "Dallas Phasing" is being used in Fayetteville, AR at the intersection of 112 & 112-S?  I can imagine accidents there as this would be a very unexpected sequence.  Instead of the green ball, a flashing yellow arrow should be used.  This is much more self-explanatory to the drivers, IMO.

Accidents? Hell yes! See the white car in the distance? The Honda hit her, but kept going.

I can believe that confusion about the phasing might lead to accidents. Just from what I can tell looking at the picture, if the Honda hit that white car it was probably more from driver stupidity than confusion about the signal display. Not knowing what the signal was doing at the time, it's hard to say.

In general, from my understanding, there has been increased instances of drivers mistaking a circular green for a protected turn. This has applied not only to 5-section PPLT displays, but also at standard 3-section displays with circular indications that are mounted over the left turn lane.  This is why the 2009 MUTCD has taken the step of prohibiting a circular green indication in a signal face mounted over left turn lanes.

[shoptb1]

I can believe that confusion about the phasing might lead to accidents. Just from what I can tell looking at the picture, if the Honda hit that white car it was probably more from driver stupidity than confusion about the signal display. Not knowing what the signal was doing at the time, it's hard to say.

The problem here is that these signals (and every other signal in Arkansas) have always previously only allowed the solid green ball on a left-turn-lane mount WHEN the through lanes are also solid green balls.  It's hard to re-train all of the drivers...especially when half of them are on auto-pilot and probably using a cell phone at the time anyway.  At any rate, my point is that the most dangerous part of this all is the fact that it wasn't always this way.  It almost needs a sign, like 'New Traffic Pattern'.

[roadfro]

The problem here is that these signals (and every other signal in Arkansas) have always previously only allowed the solid green ball on a left-turn-lane mount WHEN the through lanes are also solid green balls.  It's hard to re-train all of the drivers...especially when half of them are on auto-pilot and probably using a cell phone at the time anyway.  At any rate, my point is that the most dangerous part of this all is the fact that it wasn't always this way.  It almost needs a sign, like 'New Traffic Pattern'.

Drivers making a left turn shouldn't be paying attention to the thru traffic signals if there's a separate left turn signal face. The green ball in the left turn face is the only one that matters, regardless of whether the adjacent thru traffic had circular red or circular green.  That should be inherent driving instinct that does not require driver retraining--although I realize this is a viewpoint derived in theory, not reality...

For those drivers on auto pilot or are otherwise distracted, a "New Traffic Pattern Ahead" sign won't make a bit of difference.

[shoptb1]

For those drivers on auto pilot or are otherwise distracted, a "New Traffic Pattern Ahead" sign won't make a bit of difference.

I cannot argue with you here.  

[Scott5114]

Accidents? Hell yes! See the white car in the distance? The Honda hit her, but kept going.

Did you tell the person in the white car that they got hit by the car bearing Arkansas license plate 116 LNK?

[Scott5114]

It's hard to re-train all of the drivers...especially when half of them are on auto-pilot and probably using a cell phone at the time anyway. 

Solution: "BOTTOMLESS PIT AHEAD" signs. The autopilot drivers fall into the pit and the ones who read signs avoid it!

[shoptb1]

Solution: "BOTTOMLESS PIT AHEAD" signs. The autopilot drivers fall into the pit and the ones who read signs avoid it!

We actually have those in Ohio...although they're unsigned.  I believe the locals up here call them "pot holes"