Are diverging diamonds a fad?

[tradephoric]

How annoying is it to wait at a red light at 2AM when no traffic is coming?  That scenario happens routinely for drivers exiting the freeway at DDIs, SPUIs, and Parclo A4s.

That scenario exists for drivers approaching nearly every traffic light everywhere; more so for those not on the main thru street.

The scenario doesn't exist for drivers exiting the freeway at a Parclo B4.  All drivers exiting the freeway at a Parclo B4 make a simple right turn onto the arterial and experience minimal delays.  There are other advantages to the Parclo B4 interchange, specifically maintaining good signal progression along the corridor (which also helps reduce delays).   

If it exists in the first place. 

Besides, there are a host of things to look when engineering an intersection.  You're in bed with the B4, but a real engineer is going to look at a whole lot of scenarios to determine if it's the best option.

I'm in bed with interchange designs that minimizes total network driver delay.  In the specific example of I-75 & Holland Road, the Parclo B4 appears to achieve that.  You might be in bed with inefficient, red light stopping designs like SPUIs and DDIs.  No thank you.

[NE2]

I'm in bed with sheep.

[tradephoric]

You're in bed with the B4, but a real engineer is going to look at a whole lot of scenarios to determine if it's the best option.

The microsimulation study found that the Parclo B4 resulted in lower delays and fewer stops when compared to Parclo A4s and DDIs (when tested under a variety of conditions).  I posted SYNCHRO models at I-75 & Holland Road with 2035 peak hour volumes, to visualize the Parclo B4 interchange with MDOT's preferred alternative.  I don't see traffic breaking down in the Parclo B4 model.  What am I missing?  All you can come back with is "a real engineer is going to look at a whole lot of scenarios to determine if it's the best option" .   Now that is profound. 

Convince me why the Parclo B4 isn't superior to MDOT's preferred alternative.
https://www.youtube.com/watch?v=9y-8okb2VOA
https://www.youtube.com/watch?v=7C_1qd34e4Y

[jeffandnicole]

I take it vehicles are also going 65 mph on the ramps, based on the videos you are showing us.

And these videos are showing very low-level traffic volumes.  I don't see any congestion in either scenario.  Present to us rush hour traffic scenarios, not 10am "it's after rush hour but before people start going out and shop" scenarios.

Also, how is traffic affected down the roads?  The videos are showing the effects of traffic in the interchange only, not on a broader scale.

[kphoger]

The point remains:

Signal progression is only a key factor when a significant portion of the surface street traffic is through traffic.

Scenarios in which DDIs are supposed to be considered are those in which a significant portion of the surface street traffic is turning onto the expressway. At least in theory, were talking about apples and oranges.

Are DDIs only chosen for locations that meet the criteria it was designed for? No. Perhaps that is where the breakdown is. So let's turn the table:  Can you envision a scenario in which the amount of left-turning surface-to-expressway traffic is so great that the DDI ends up outperforming the B4 due to the former's free-flowing ramps?

[tradephoric]

I take it vehicles are also going 65 mph on the ramps, based on the videos you are showing us.

I'm sure you could find more than just the speed of the ramps being off if you looked closely.  If I was a consultant getting paid $1.4 million dollars for a traffic study these models would look perfect and the results probably wouldn't differ by much (the Parclo B4 would still be displaying smooth traffic flow with minor delays). 

And these videos are showing very low-level traffic volumes.  I don't see any congestion in either scenario.  Present to us rush hour traffic scenarios, not 10am "it's after rush hour but before people start going out and shop" scenarios.

The models were based on 2035 peak hour volumes.  I used the peak volumes detailed in the MDOT report.

Also, how is traffic affected down the roads?  The videos are showing the effects of traffic in the interchange only, not on a broader scale.

The green bands for each model should give you a good idea how the adjacent signals interact with the interchange signals.   Parclo B4 leads to smooth signal progression with much larger green bands compared to the Parclo AB4 (MDOT's preferred alternative).

MDOT preferred alternative:


Parclo B4:

[tradephoric]

So let's turn the table:  Can you envision a scenario in which the amount of left-turning surface-to-expressway traffic is so great that the DDI ends up outperforming the B4 due to the former's free-flowing ramps?

Left-turning surface-to-expressway traffic at a DDI still must cross opposing lanes of traffic before entering the freeway... they just do so at a signal upstream of the free-flowing on-ramp.  I don't see why a DDI would outperform a B4, even if the left-turning surface-to-expressway traffic is high.  They have diverging diamonds models in SYNCHRO and it might be interesting to compare SYNCHRO models of DDIs and Parclo B4s with various traffic volume scenarios.
 
If left-turning traffic onto the freeway is the heaviest movement, then consider a folded interchange (which is basically a B4/DDI hybrid).  Unlike the DDI which diverge all traffic on the surface street, the folded interchange only diverge traffic that actually will be entering onto the freeway.   

http://sabra-wang.com/media/TheFoldedInterchangeAnUnconventionalDesignfortheReconstructionofCloverleafInterchangesKeithRiniker.pdf
https://www.youtube.com/watch?v=ZZZOBPyHuCM

[tradephoric]

The I-695 at US 40 folded interchange study (linked in the previous post) looked at the average delay per vehicle for a number of different interchange alternatives.  The existing Parclo B4 had 15 second average delay per vehicle compared to roughly 75 second for the DDI.

[tradephoric]

Scenarios in which DDIs are supposed to be considered are those in which a significant portion of the surface street traffic is turning onto the expressway. At least in theory, were talking about apples and oranges.

The DDI at Bangerter Hwy and Utah State 201 is a good example of a DDI that probably should never have been constructed.  Bangerter Hwy is a major 6-lane boulevard that does get a lot of through traffic....it's the Telegraph Road of Salt Late City.  Here is my feeble attempt at modeling the DDI on SYNCHRO:

https://www.youtube.com/watch?v=LsHEy461w3o

Here is the same DDI in action.  Thru drivers on Bangerter Hwy who get a green at Camera 1 come straight to a red light at Camera 2.  This bad progression continues throughout the entire video.  The beginning of the platoon on Bangerter Hwy is being led straight into the dilemma zone!  The DDI is often described as an innovative interchange... that's effed up innovation.
 
https://www.youtube.com/watch?v=RhazZu4xMeA

[johndoe]

This topic has sort of gone a million ways, here are a few thoughts:

-the paper posted earlier doesn't have any volume scenarios where turns from the cross street are greater than through volumes
-the "folded interchange " might be great in some situations, one drawback is that it requires more bridge area

I feel like a broken record, but assuming the same geometry is either always good or always bad is foolish.  Each interchange is unique in both traffic and project goals.

[tradephoric]

I'm not totally opposed to DDIs.  Here is a scenario where a DDI works well...

https://maps.google.com/maps?ll=39.156138,-76.74507&spn=0.003428,0.003406&t=h&z=18

The one thing i don't like about this design is that drivers can't make a simple right on red when exiting the freeway.  Again, just adding delay.  If the sight distance at DDIs is such that you can't allow a driver exiting the freeway to make a simple right on red, that's just one more reason not to like DDIs.

https://maps.google.com/maps?ll=39.15584,-76.74326&z=19&t=h&output=classic&dg=brw

[jakeroot]

I'm not totally opposed to DDIs.  Here is a scenario where a DDI works well...

I want to know what was wrong with the roundabouts that were there before.

OT: How often are modern roundabouts removed and replaced with signalized junctions?

[vdeane]

The one thing i don't like about this design is that drivers can't make a simple right on red when exiting the freeway.  Again, just adding delay.  If the sight distance at DDIs is such that you can't allow a driver exiting the freeway to make a simple right on red, that's just one more reason not to like DDIs.

I suspect it's to make sure that drivers don't accidentally look at the wrong traffic (since they need to look to the far side of the road rather than the near one, and drivers not from the area may not know to do that) and get T boned.

[tradephoric]

The one thing i don't like about this design is that drivers can't make a simple right on red when exiting the freeway.  Again, just adding delay.  If the sight distance at DDIs is such that you can't allow a driver exiting the freeway to make a simple right on red, that's just one more reason not to like DDIs.

I suspect it's to make sure that drivers don't accidentally look at the wrong traffic (since they need to look to the far side of the road rather than the near one, and drivers not from the area may not know to do that) and get T boned.

That's a legitimate reason to have a "˜no right turn on red' sign at that DDI, but it's still annoying getting stuck at a red light at 2AM waiting to make a right turn.  There are other interchange designs that would allow a simple right turn on red... *cough, cough* the Parclo B4.

[hbelkins]

OT: How often are modern roundabouts removed and replaced with signalized junctions?

Not often enough! 

[jakeroot]

OT: How often are modern roundabouts removed and replaced with signalized junctions?

Not often enough! 

[tradephoric]

An advantage of signalizing the Parclo B4 off-ramps is that acceleration/deceleration lanes aren't needed.  In the examples below, the Parclo B4 only has to accommodate 4-lanes of traffic over the bridge deck as opposed to 6 with the DDI (since there are no accel/decl lanes).  The Parclo B4 maintains good signal progression and all traffic exiting the freeway are allowed to make a simple right turn on red.  The DDI you get bad signal progression and drivers exiting the freeway experience delay (exiting freeway drivers making a left must wait for a green light, exiting freeway drivers making a right could potentially see a "no turn on red" sign as previously discussed). 

DDI = Drivers Delayed Indefinitely

https://youtu.be/yU3B9kRBSLI
https://youtu.be/NQVlEchM3ck

[vtk]

DDI = Drivers Delayed Indefinitely

Your zeal exceeds the substance of your arguments.

[tradephoric]

DDI = Drivers Delayed Indefinitely

Your zeal exceeds the substance of your arguments.

"Drivers Delayed Indefinitely" is still less dramatic than "Death Diamond Interchange".

[6a]

DDI = Drivers Delayed Indefinitely

Your zeal exceeds the substance of your arguments.

"Drivers Delayed Indefinitely" is still less dramatic than "Death Diamond Interchange".

'A haiku on the subject of the Diverging Diamond'

A Parclo 4B
What an interchange for me
Holy cow, 4B

[tradephoric]

Are drivers exiting the freeway making a left onto the arterial allowed to make a "˜left-turn on red' at a DDI?  If not, why not?  Drivers are being needlessly delayed when they are not allowed to merge into gaps along the arterial.  This video is the best argument against the DDI.  Drivers along a major 6-lane arterial get a green light only to come straight to a red at a signal 800 feet away.  This bad progression continues throughout the entire video.  How anybody could be impressed with the operation of this DDI is beyond me.

https://www.youtube.com/watch?v=RhazZu4xMeA#t=19

[vtk]

Are drivers exiting the freeway making a left onto the arterial allowed to make a "˜left-turn on red' at a DDI? ... Drivers along a major 6-lane arterial get a green light only to come straight to a red at a signal 800 feet away.

Left turn on red? In Ohio, yes, unless there's a sign prohibiting it.

As for your six lane arterial, the only traffic from the artierial that will hit both lights at a DDI is the through traffic.  Usually, through surface traffic is not the main concern at a DDI.  If there's a lot of traffic on this six lane arterial that is not getting on the freeway one way or another, then I agree a DDI was a poor choice for this specific location.  This reasoning does not extend to condemn all DDIs.

[jakeroot]

Here is a great place where I think a DDI would work. Canyon Road @ WA-512, south of Seattle. Canyon Road is a major arterial, really throughout its length, but definitely moreso from the interchange below and to the right (south in real-life, up is east). From the image, you can tell from just the width of the arterial that Canyon Road is a lot busier on one side of the interchange, thus most of the traffic at the interchange is turning traffic; you can see from the image the cat-tracks of vehicles and the major movements. Canyon road spans up to 8 lanes south of the interchange pictured, but north, it drops to two (just past the next intersection). Coming from the left side of the image (approaching from the north), traffic backs up for a few blocks because of the incredibly long phase for both the off-ramp and the left turn onto the onramp. Because these are the major movements, the through signal is very short. There isn't a lot of traffic coming from the north, but it backs up so much because of the short signal phase.

FWIW, Canyon Road filters commuter traffic in both directions, morning and night. There's a large industrial park south of the interchange in Frederickson (namely, Boeing, and the factory that builds the walls for the Alaskan Way Tunnel) that both employ thousands of workers. Thus, there's really no dominant movement at any point during the day.

Also, because of the large industrial nature of the areas south of the interchange, there's a lot of trucks coming through this interchange (just look at the satellite shot), many of which are incredibly long, carrying airplane fuselages. So, especially taking into account the interchange geography (tight land use on both sides) and the fact that many large trucks use the interchange, I'm not sure a loop ramp would work here.

http://goo.gl/8KcZlI

[tradephoric]

Here's another DDI along a heavily traveled corridor (Pleasant Hill Road in Atlanta). 
https://maps.google.com/maps?ll=33.952249,-84.129961&spn=0.001462,0.001725&t=h&z=19

This DDI replaced a tight diamond interchange.  The space limitations at this interchange limited what interchange design could be selected (a Parclo B4 was obviously out of the question... the loop ramps wouldn't fit).  An argument can be made that a DDI is more efficient than a tight diamond.  However, is it a huge success to replace an inefficient tight diamond interchange with a slightly less inefficient DDI?  Each design stops both directions of travel along the corridor, leading to excess driver delays due to poor signal progression.  The DDI is only arguably better than the tight diamond it replaced. 

It's not always practical to convert a tight diamond to a Parclo B4 (Jake just gave another perfect example).  However, when the space is available shouldn't the Parclo B4 be the preferred interchange alternative?  MDOT was considering replacing the full cloverleaf interchange at I-75 & Holland Road with a DDI, but there was enough public resistance that they scrapped the plans.  I just don't understand the fascination with the DDI to where you see full cloverleafs (that have the space for efficient Parclo B4s) are being converted to inefficient DDIs. 

I keep pushing the Parclo B4 because it's one of the few interchange designs where the on/off ramp signals only stop one directions of travel along the corridor.  That's huge for reducing driver delay.

[vtk]

ODOT doesn't like Parclo B interchanges.  I think, more specifically, they don't like loop exit ramps, because apparently some drivers don't decelerate early enough and lose control on the loop.  This disfavor has only existed for about the last 10 years or so.

[A DDI] stops both directions of travel along the corridor

You keep saying that, but it's not really true.  Each signal at a DDI stops one direction or the other – not both directions – at any given time.  Taken together, there almost certainly is some time when neither direction can go through both signals, but in the cases where DDIs are usually chosen, it is decided that this doesn't really matter.

A DDI is usually built where most of the surface road traffic approaching the interchange gets on the freeway, and most of the surface road traffic leaving the interchange has just exited the freeway.  Therefore, it is of little use to talk about signal progression along "the corridor" because, essentially, you have two distinct surface road corridors that end at that interchange.

Would I be wrong if I say a typical sample of traffic approaching a DDI on a surface road is about 40% entering the freeway to the right, 40% entering the freeway left, and 20% continuing on the surface road?  Since nobody can answer before I continue typing the post, I'm going to go with those numbers for an example.  I think they're good numbers for a hypothetical DDI.

So you've got this arterial boulevard with a big pack of cars coming towards it, which has ridden a green wave for miles.  Great.  They hit the DDI, and 40% of the cars immediately turn right to enter the freeway.  No delay for them.  The other 60% gets to the first traffic light.  Now if I'm setting up the timing for this DDI, I'd probably have it set so this first light is green a little less than half the time, but I'd also make it so the end of this green phase coincides with the end of the inflow of green wave cars, or maybe just a little later.  So the first few green wave cars have to stop for a few seconds, but then the light turns green, and the rest of the pack gets through before it turns red again.  A small fraction of the cars have experienced a very short delay.  Not perfect, but really not a big deal.  Now these cars cross the freeway, and two thirds of them enter the freeway on the left.  The remaining cars, which are staying on the surface boulevard, encounter the next light.  If it's set up like my earlier DDI Example 1, they will hit that light green, and continue on the boulevard, though slowing down for the swervy DDI might cause them to miss the light at the next mile-road.  Or, maybe they get stopped at the second DDI light.  As far as I'm concerned, they've changed to a different signal progression corridor, which doesn't have to be synchronized from the first one – though it probably will be, if this is a whole Detroitoid grid with progression on all the mile-roads.  And anyway, we're talking about just 20% of the original incoming wave of cars.

But there's a good argument for why the second signal at a DDI doesn't have to be timed to let through all the cars coming from the first signal: there's always cars coming from that other signal.  The other signal is always sending it either left-turning freeway-exit traffic, or boulevard through traffic.  It's going to cause approximately the same total delays for cars in those streams no matter when it turns red.  So each signal in a DDI really only needs to be timed to accommodate the waves of cars coming towards the interchange from the boulevard on its own side of the freeway.  In that sense, a DDI can easily be tuned to cause minimal interference with signal progression along a corridor that ends there.

Maybe I'll do another time-distance diagram for a DDI accounting for the slowdown required by the swervy geometry.  It probably won't be until at least Tuesday, if I do it at all.

Additional thoughts:

Have you considered a Parclo A4 where traffic exiting the freeway and wishing to turn left must actually turn right and then U-turn?  After all, that's how left-turn traffic from minor side streets is handled on a Detroitoid boulevard.  A SPUI can be similarly modified.

If you insist on considering the boulevard on both sides of the freeway as a single corridor for signal progression purposes, large volumes of turning (exiting and entering) traffic at the freeway – the conditions where a DDI is favored – will be problematic in any case.  With any interchange design that brings exiting traffic to a traffic signal, the stream of traffic leaving the interchange on the boulevard will be practically constant.  Even a parclo B4 without signalized exit ramps (which may be problematic in other ways with heavy exiting traffic volumes) will even out the waves of traffic to a lesser degree.  Timing of traffic signals on one side of the freeway will be almost irrelevant to timing of signals on the other side because of this.