Variations on the DDI

[kernals12]

Most Roadgeeks know what a Diverging Diamond Interchange is, but for those who don't:


What it does is force drivers crossing over a freeway to briefly crossover to the wrong side of the road. This allows left turns to be made without crossing the opposing traffic. They've been spreading like wildfire in the last 15 years due to the safety and traffic flow benefits they offer.

But there have been ideas for improving on the DDI.


A relatively simple one is to add left turn loops and make it a Diverging Partial Cloverleaf. The benefit of this is it eliminates the weaving of left turns.



Another idea is to get rid of the crossover for drivers going straight but keep for those who are turning left. The designer describes it as a Splintered Untwisted DDI. This gets rid of a current problem with DDIs in that they can't allow traffic go straight in both directions at the same time. But it does require a wide (read: expensive) bridge structure


And another idea is to grade separate the crossovers. The patent holder describes it as a "Double Crossover Merging Interchange". This makes the DDI into a free-flow interchange and thanks to the way that left turns off the freeway rejoin the mainlines, it eliminates weaving. But of course, flyovers are very expensive. Also, if you take two of these, you get a diverging windmill interchange.

These are all very interesting designs and I hope we see them on the road some day.

[vtk]

Neat!

[jeffandnicole]

Much more expensive though.

[kphoger]

Much more expensive though.

...and cost is a big reason departments opt for the DDI in the first place.

[kernals12]

Much more expensive though.

...and cost is a big reason departments opt for the DDI in the first place.

No it's not, a normal diamond interchange is cheaper. The DDI is favored because it gets rid of left turns across the opposing flow of traffic, which vastly improves safety and capacity.

[froggie]

^ Yes, cost is still a deciding factor.  SPUIs are more efficient and higher capacity than DDIs but DDIs can be retrofitted into existing diamond interchanges much more easily.

[Henry]

patent holder describes it as a "Double Crossover Merging Interchange". This makes the DDI into a free-flow interchange and thanks to the way that left turns off the freeway rejoin the mainlines, it eliminates weaving. But of course, flyovers are very expensive. Also, if you take two of these, you get a diverging windmill interchange.

I'd love to see this built! It would be fun to use at locations where one freeway does a crossover on itself before meeting another. (cough cough...I-77 at I-85)

[kernals12]

patent holder describes it as a "Double Crossover Merging Interchange". This makes the DDI into a free-flow interchange and thanks to the way that left turns off the freeway rejoin the mainlines, it eliminates weaving. But of course, flyovers are very expensive. Also, if you take two of these, you get a diverging windmill interchange.

I'd love to see this built! It would be fun to use at locations where one freeway does a crossover on itself before meeting another. (cough cough...I-77 at I-85)

It also seems to require less right of way than most other free flowing interchanges.

[vtk]

patent holder describes it as a "Double Crossover Merging Interchange". This makes the DDI into a free-flow interchange and thanks to the way that left turns off the freeway rejoin the mainlines, it eliminates weaving. But of course, flyovers are very expensive. Also, if you take two of these, you get a diverging windmill interchange.

I'd love to see this built! It would be fun to use at locations where one freeway does a crossover on itself before meeting another. (cough cough...I-77 at I-85)

Michigan used to have one where both freeways did crossovers. US 131 at I-96 maybe? I'm pretty sure it was changed like ten years ago or something.

[GaryV]

patent holder describes it as a "Double Crossover Merging Interchange". This makes the DDI into a free-flow interchange and thanks to the way that left turns off the freeway rejoin the mainlines, it eliminates weaving. But of course, flyovers are very expensive. Also, if you take two of these, you get a diverging windmill interchange.

I'd love to see this built! It would be fun to use at locations where one freeway does a crossover on itself before meeting another. (cough cough...I-77 at I-85)

Michigan used to have one where both freeways did crossovers. US 131 at I-96 maybe? I'm pretty sure it was changed like ten years ago or something.

US-131 crosses over itself at I-196, but I-196 doesn't cross over.  Lot of left exits and entrances.  And that high-in-the-air ramp from sb to wb is kind of scary; you're looking into the 4th floor windows of the old school to the right.

[kphoger]

Much more expensive though.

...and cost is a big reason departments opt for the DDI in the first place.

No it's not, a normal diamond interchange is cheaper. The DDI is favored because it gets rid of left turns across the opposing flow of traffic, which vastly improves safety and capacity.

A simple litmus test is this:  Does MoDOT construct them frequently?  If the answer is YES, then cost is definitely a factor.

[US 89]

UDOT has built a couple interchanges that resemble something between a DDI and CFI, with a crossover only for one on-ramp. The I-15 interchanges at SR 93 and at SR 103 both use this design. Not quite sure what to make of them.

[CoreySamson]

I once designed and built a folded DDI in C:S, but it was messy and didn't look good. Also, it was expensive.

[johndoe]

UDOT has built a couple interchanges that resemble something between a DDI and CFI, with a crossover only for one on-ramp. The I-15 interchanges at SR 93 and at SR 103 both use this design. Not quite sure what to make of them.

Thanks for posting, I only knew of 1 of those 2.  I think it was highlighted in an industry magazine; how'd you stumble across them?  Anyone know of more examples?  Texas has a few similar to that (although the crossover happens a little differently so not sure it counts as DDI...) a couple in San Marcos and Bandera Road in San Antonio.

Missouri has another sort of CFI-DDI combo and two "half-DDI half roundabout".

That "splintered" idea is shown in this presentation: http://www.cowyite.org/presentations/Smith%20-%20DDI.pdf

Here's a thread of another variation, where they added the frontage road "through" movements in Florida: https://www.aaroads.com/forum/index.php?topic=27649.0

And yes... I promise cost is a big reason a lot of these things happen.  Bridges are very expensive, so less is more!

[US 89]

UDOT has built a couple interchanges that resemble something between a DDI and CFI, with a crossover only for one on-ramp. The I-15 interchanges at SR 93 and at SR 103 both use this design. Not quite sure what to make of them.

Thanks for posting, I only knew of 1 of those 2.  I think it was highlighted in an industry magazine; how'd you stumble across them?

Driving through them. I go through the 93 interchange fairly often and knew that one was coming before it got built, as it was part of a bigger project that reconstructed I-15 through the southern third of Davis County. The other one I did not know of before I happened upon it about two years ago while clinching state routes in the area.

[kphoger]

Missouri has another sort of CFI-DDI combo ...

Are you referring to this one?

[johndoe]

Missouri has another sort of CFI-DDI combo ...

Are you referring to this one?

Yep.  Not sure if others of those have been built, but Florida and Georgia have at least considered them:
https://www.yourobserver.com/article/public-outreach-meeting-planned-i-75-bee-ridge-construction
http://www.dot.ga.gov/BuildSmart/Studies/ManagedLanesDocuments/OPSFinalReport-Appendices.pdf

(googled things like "displaced left interchange" etc)
https://www.researchgate.net/publication/273887124_Study_of_Alternative_Design

[ErmineNotyours]

Tacoma's I-705/I-5/SR 7 interchange has the through lanes south of I-5 switch place to make ramps shorter.  I always thought of that as a controlled access DDI.

[kphoger]

Tacoma's I-705/I-5/SR 7 interchange has the through lanes south of I-5 switch place to make ramps shorter.  I always thought of that as a controlled access DDI.

That's basically like "El Sarape" interchange in Saltillo, Coahuila.

[TheGrassGuy]

Do any of these variations actually exist IRL?

[tradephoric]

With the Folded interchange through traffic on the arterial street aren't forced to cross opposing lanes of traffic leading to an unnecessary and potentially dangerous conflict point.  However this design only works if you have enough ROW for loop ramps. 

https://www.youtube.com/watch?v=gR-VOJlKjmc

[CoreySamson]

That's basically like "El Sarape" interchange in Saltillo, Coahuila.

I have 2 intersections like that back-to-back in one of my C:S cities.

[tradephoric]

The DDI at Timpanogos Hwy and I-15 has experienced significant suggestion since opening in 2011.  As a result UDOT has decided to tear it down and convert it back to a traditional interchange.  Truly taxpayer dollars at work here... building this "innovative" interchange only to tear it out less than 10 years later.  Is this the first DDI casualty in America? 

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

[Rothman]

The DDI at Timpanogos Hwy and I-15 has experienced significant suggestion since opening in 2011.  As a result UDOT has decided to tear it down and convert it back to a traditional interchange.  Truly taxpayer dollars at work here... building this "innovative" interchange only to tear it out less than 10 years later.  Is this the first DDI casualty in America? 

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

Suggestion is intolerable.

[US 89]

The DDI at Timpanogos Hwy and I-15 has experienced significant suggestion since opening in 2011.  As a result UDOT has decided to tear it down and convert it back to a traditional interchange.  Truly taxpayer dollars at work here... building this "innovative" interchange only to tear it out less than 10 years later.  Is this the first DDI casualty in America? 

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

It should be noted that the removal was done in conjunction with a major reconstruction of that portion of I-15. That included the installation of about three miles of one-way frontage road...which doesn't really work with a DDI.