Are diverging diamonds a fad?

[JKRhodes]

In a perfect world, no one stops at a roundabout, but I've been in plenty of scenarios where they were so clogged that it was necessary to stop for a minute or two before entering. I'm thinking of Happy Valley Rd and I-17 in Phoenix, in particular.

The DDI concept really seems to be taking hold in Utah, where I saw several completed and some under construction a few weeks ago. I ate lunch at a restaurant in American Fork where the intersection was plainly visible from my table. I watched, and never saw any traffic stack up. As another poster mentioned, the through movement gets shafted a little in the DDI. The signal was set up so the Eastbound thru movement from the bridge had a green at the same time as the northbound ramp onto the bridge. I assume the same setup was in place for Westbound from bridge and Southbound from Ramp. It was definitely interesting to watch; I even saw a funeral procession go through without a hitch, albeit with several escorting officers.

[UCFKnights]

Can anyone speak to how a DDI will handle traffic in a busy shopping area with multiple traffic signals? PennDOT is converting the US 19 at I-70/79 in Washington, PA. US 19 already has 5 signals in a little over a mile stretch (one south of the interchange, four north) through an area with four separate shopping areas.

I travel this area almost daily and I recognize that the US 19/I-70/I-79 interchange is in drastic need of updating. I am no way suggesting that a DDI is wrong here - I don't have anywhere near the training or education to make that decision. But, as a driver in this area, I worry about an often congested corridor becoming even worse.

There is one I went by where the left turn phase of the shopping center had a very extended green tied to the green in the same direction right after the DDI.

[triplemultiplex]

I drove through all of Utah's DDI's multiple times for work and they functioned beautifully in my experience. I can't wait to see them in Wisconsin.

I think of how much extra money was put into the interchange at I-43/894 and 27th Street to put in those low-speed semi-directional ramps to handle 'left' turning traffic onto the freeway.  A DDI would've been a much, much cheaper and equally effective option.  Could've used the money saved to rebuild the Howard Ave overpass to accommodate a SB auxiliary lane between the Holt Ave entrance and the Howard Ave exit.

Talk about a 'fad'; those types of ramps in the Milwaukee area are going to be one.  In addition to that example, WisDOT put them in at I-41 and Watertown Plank and another at I-94 and WI 100.  Watertown Plank; there's another one where a DDI would've been way, way, way cheaper and handled the traffic just fine.

[tradephoric]

Since the I-70/I-79 interchange is a full cloverleaf, I would question whether a DDI would work better than removing two of the loops.

I think one of the reasons of the DDI over that idea is so they can use the existing bridges over US-19 while 6-laning I-70/79.  If they left some loop ramps in, They'd have to use what will be the outer I-70/79 lanes as acceleration/deceleration (depending on what loops were left) lanes for the ramps instead.

I see your point here.  They are saving money by not having to widen I-70/79 bridge to accommodate acceleration/deceleration lanes.  I'm just ultra sensitive to designs that screws with coordination along a main arterial.  It's commonplace to drive 10+ miles without getting stopped at a red light along major arterials in Metro Detroit.  If DDI's start taking off here, the game of seeing how far you can go without getting stuck at a red light won't be as fun.  The smooth coordination seen along Big Beaver Road in this video would be impossible if a DDI replaced the full cloverleaf interchange at I-75:

https://www.youtube.com/watch?v=eB2H4bGp4Jc

[Big John]

I can't wait to see them in Wisconsin.

One is being planned for the WI 441 / US 10 / Oneida St interchange by Menasha.

[mgk920]

I can't wait to see them in Wisconsin.

One is being planned for the WI 441 / US 10 / Oneida St interchange by Menasha.

It's actually in Appleton (you just gotta love Wisconsin's failed municipal boundary law!    ).

Also, WisDOT has *firm* plans for DDIs at I-39/90/WI 26 in Janesville and I-39/90/WI 81 (Milwaukee Rd), piggybacked on the I-39/90/I-43 ('Beloit' interchange), in Beloit.

Mike

[jakeroot]

I think of how much extra money was put into the interchange at I-43/894 and 27th Street to put in those low-speed semi-directional ramps to handle 'left' turning traffic onto the freeway.  A DDI would've been a much, much cheaper and equally effective option.

That interchange looks like something designed during the Reagan administration.

[cl94]

From an engineering perspective, DDIs are superior to SPUIs in almost every case. While roundabouts are better in theory, people don't know how to use them. As previously stated, you only have to put down a little bit of concrete and change the striping and signals to get a major improvement, as both signals in the intersection are coordinated. Are they great when there are many adjacent intersections? No. But they are often many times better than the diamonds they replace. They're probably best in medium-traffic situations or when having each direction on different phases doesn't matter.

SPUIs and roundabouts have a bit of a learning curve. DDIs have less of one.

[DaBigE]

Have the safety numbers on that interchange in New Berlin been improving lately?  If so, I'd chalk that up to what I call the 'drivers ed' issue - getting people up to speed on how they work and how to use them.

The roundabouts around here, including a few with three lanes, appear to me to be working well with good safety.

Mike

I haven't seen the official latest crash numbers, but I've been told Moorland Rd is getting better. The roundabout haters still make a comment every now and again, but their minds are made up.

And for the record, one more 3-laner squeaked approval, mainly because things were too far down the road to change (funding, design, etc.): Wis 47 and CTH OO (Richmond St at Northland Ave) in Appleton.

I can't wait to see them in Wisconsin.

One is being planned for the WI 441 / US 10 / Oneida St interchange by Menasha.

It's actually in Appleton (you just gotta love Wisconsin's failed municipal boundary law!    ).

Also, WisDOT has *firm* plans for DDIs at I-39/90/WI 26 in Janesville and I-39/90/WI 81 (Milwaukee Rd), piggybacked on the I-39/90/I-43 ('Beloit' interchange), in Beloit.

Mike

IIRC, there is at least one on the drawing board for the I-43 corridor between Grafton and Milwaukee.

[tradephoric]

Have the safety numbers on that interchange in New Berlin been improving lately?  If so, I'd chalk that up to what I call the 'drivers ed' issue - getting people up to speed on how they work and how to use them.

The roundabouts around here, including a few with three lanes, appear to me to be working well with good safety.

Mike

I haven't seen the official latest crash numbers, but I've been told Moorland Rd is getting better. The roundabout haters still make a comment every now and again, but their minds are made up.

The chart below looks at crash data of 16 major roundabouts built throughout SE Michigan between 2006-2009.  While PDO accidents spike the first year after roundabout construction, injury accidents drop from day one.  By year two people seem to get use to them and the PDO steadily declines from the initial spike.  This has at least been the trend for SE Michigan.

[mgk920]

Have the safety numbers on that interchange in New Berlin been improving lately?  If so, I'd chalk that up to what I call the 'drivers ed' issue - getting people up to speed on how they work and how to use them.

The roundabouts around here, including a few with three lanes, appear to me to be working well with good safety.

Mike

I haven't seen the official latest crash numbers, but I've been told Moorland Rd is getting better. The roundabout haters still make a comment every now and again, but their minds are made up.

And for the record, one more 3-laner squeaked approval, mainly because things were too far down the road to change (funding, design, etc.): Wis 47 and CTH OO (Richmond St at Northland Ave) in Appleton.

I am well familiar with that intersection, it is the most dangerous in the entire City of Appleton.  All of the needed ROW is there for it, too - when Northland Ave (Outagamie County 'OO') was originally built as US 41 in the late 1930s, the Wisconsin Highway Commission (predecessor of WisDOT) acquired enough additional land to allow for generous 'vision corners' at the major intersections along that then new two-lane rural bypass highway.

Mike

[DaBigE]

Have the safety numbers on that interchange in New Berlin been improving lately?  If so, I'd chalk that up to what I call the 'drivers ed' issue - getting people up to speed on how they work and how to use them.

The roundabouts around here, including a few with three lanes, appear to me to be working well with good safety.

Mike

I haven't seen the official latest crash numbers, but I've been told Moorland Rd is getting better. The roundabout haters still make a comment every now and again, but their minds are made up.

And for the record, one more 3-laner squeaked approval, mainly because things were too far down the road to change (funding, design, etc.): Wis 47 and CTH OO (Richmond St at Northland Ave) in Appleton.

I am well familiar with that intersection, it is the most dangerous in the entire City of Appleton.  All of the needed ROW is there for it, too - when Northland Ave (Outagamie County 'OO') was originally built as US 41 in the late 1930s, the Wisconsin Highway Commission (predecessor of WisDOT) acquired enough additional land to allow for generous 'vision corners' at the major intersections along that then new two-lane rural bypass highway.

Mike

I'm actually working on that intersection project right now. The fixes needed to make the signals an operationally-acceptable solution were ugly from just about every perspective.

[tradephoric]

Have the safety numbers on that interchange in New Berlin been improving lately?  If so, I'd chalk that up to what I call the 'drivers ed' issue - getting people up to speed on how they work and how to use them.

Mike

I haven't seen the official latest crash numbers, but I've been told Moorland Rd is getting better. The roundabout haters still make a comment every now and again, but their minds are made up.

The Moorland Road roundabout has a central island diameter of nearly 190 feet.  This is a BIG roundabout that is approaching traffic circle dimensions.  There are other 3-lane interchange roundabouts with central island diameters that are much smaller.  For instance, the central island  of the 3-lane interchange roundabout at Sawyer Road and I-94 is only 100 feet. 

[triplemultiplex]

Utah is using them on their highest volume service interchanges which suggests they are best at moving traffic in tight spaces where loop ramps are out of the question.  I saw one under construction in Brigham City where US 91 ends at I-15 and even though there is one dominant "left turn" movement through that interchange, the DDI was still a better option, apparently, than putting in loop ramp.

This ain't going to be no fad.  Nationwide, DOT's are going to save billions over the next two decades building DDI's instead of all other types of high-volume service interchanges that require more land or more expensive bridges.  It should become the default design used in metropolitan areas, in my opinion.

[cl94]

Utah is using them on their highest volume service interchanges which suggests they are best at moving traffic in tight spaces where loop ramps are out of the question.  I saw one under construction in Brigham City where US 91 ends at I-15 and even though there is one dominant "left turn" movement through that interchange, the DDI was still a better option, apparently, than putting in loop ramp.

They are. 2011 Green Book requires very large curve radii for a decent design speed. Ontario's many parclos often have recommended speeds of 30 km/h (about 20 mph) on the loops because larger loop ramps are cost-prohibitive.

[bugo]

What about doing something like this interchange in Texas? Thru traffic on the street tunnels under the access roads so the only traffic at the lights are the access road and turning traffic from the street.

https://www.google.com/maps/place/Richardson,+TX/@32.940039,-96.745678,346m/data=!3m1!1e3!4m2!3m1!1s0x864c1ee979bea767:0x2cdb29c046270495

The Texas U-turn there.  Helps traffic at intersections with one-way access roads as traffic wanting to turn around avoid 2 sets of traffic signals.  Without knowing traffic patterns there it may be the optimal solution to a busy intersection with access roads.

The Texas-style U turn ramp is brilliant. Whoever designed it should be canonized. I live near one of these and it's nice to be able to go in the opposite direction on the frontage roads without having to make 2 left turns and without having to wait for the signals to cycle. Any highway with frontage roads should use these highly useful ramps whenever possible.

As for the Death Diamond, the designer should be drawn and quartered. Terrible, terrible design. The only reason there haven't been more fatalities are because the travel speed is very slow.

[bugo]

I need to get on a soap box for just a minute... I think the name "DDI" has already caught on and it's too late to convince people to call it something else, but "double crossover diamond" makes so much more sense.  What about this interchange diverges more than other service interchanges?  There are actually fewer diverging conflicts in a DCD than a normal diamond... I vote we all start going with "DCD" 

The term "Death Diamond", coined by yours truly, has caught on in the road community.

[thenetwork]

The Texas-style U turn ramp is brilliant. Whoever designed it should be canonized. I live near one of these and it's nice to be able to go in the opposite direction on the frontage roads without having to make 2 left turns and without having to wait for the signals to cycle. Any highway with frontage roads should use these highly useful ramps whenever possible.

ODOT "experimented" with the Texas U-Turn when it built the Lakeland Freeway stretch of I-90/SR-2 in the mid 60s. (https://goo.gl/maps/DZ6MG), but it was never used elsewhere in Cleveland (mainly because there were very few frontage roads associated with the newer freeways being built). 

I can think of a few interchanges in NEOH that should be retro-fitted with some sort of Texas U-Turn:  This fustercluck in particular (between Alexander and Broadway): https://goo.gl/maps/3ohXd

[bugo]

From an engineering perspective, DDIs are superior to SPUIs in almost every case. While roundabouts are better in theory, people don't know how to use them. As previously stated, you only have to put down a little bit of concrete and change the striping and signals to get a major improvement, as both signals in the intersection are coordinated. Are they great when there are many adjacent intersections? No. But they are often many times better than the diamonds they replace. They're probably best in medium-traffic situations or when having each direction on different phases doesn't matter.

SPUIs and roundabouts have a bit of a learning curve. DDIs have less of one.

Driving on the wrong side of the road doesn't require a learning curve? I know if I hadn't read about Death Diamonds on the internet I would have turned right at the first traffic light into oncoming traffic. They are confusing, cheap (in the bad way) and will eventually prove to be deadly.

What is the learning curve for a SPUI? It functions much like a diamond and I don't see how they could cause confusion.

[tradephoric]

What are the operational benefits of a DDI compared to a PARCLO interchange?  There are examples of full cloverleaf interchanges being converted to DDI's as opposed to PARCLO's (IE. I-79 & US19).  A Parclo B4 has 12 conflict points as opposed to a DDI that has 14 conflict points.  A Parclo B4 can achieve good coordination (since traffic signals only stop one-direction of travel) while a DDI you get bad coordination (since closely spaced traffic signals stop both directions of travel).   Why crisscross traffic on US19 (and add two-additional conflict points) when PennDOT could simply take advantage of the grade separation between I-79 & US19 and turn it into a Parclo B4? 

[tradephoric]

The DDI proposed at I-70 and US19 has roughly the same footprint as the full cloverleaf interchange that it's replacing. 

http://www.i-70projects.com/docs/I70T20InterchangeLayoutFD%2004-03-14.pdf

[cl94]

What are the operational benefits of a DDI compared to a PARCLO interchange?  There are examples of full cloverleaf interchanges being converted to DDI's as opposed to PARCLO's (IE. I-79 & US19).  A Parclo B4 has 12 conflict points as opposed to a DDI that has 14 conflict points.  A Parclo B4 can achieve good coordination (since traffic signals only stop one-direction of travel) while a DDI you get bad coordination (since closely spaced traffic signals stop both directions of travel).   Why crisscross traffic on US19 (and add two-additional conflict points) when PennDOT could simply take advantage of the grade separation between I-79 & US19 and turn it into a Parclo B4?

As was already mentioned, a DDI allows for I-70 to be widened without replacing the bridges. They're turning the auxiliary lane into an additional travel lane. Parclos are superior when space allows, but space isn't always available.

From an engineering perspective, DDIs are superior to SPUIs in almost every case. While roundabouts are better in theory, people don't know how to use them. As previously stated, you only have to put down a little bit of concrete and change the striping and signals to get a major improvement, as both signals in the intersection are coordinated. Are they great when there are many adjacent intersections? No. But they are often many times better than the diamonds they replace. They're probably best in medium-traffic situations or when having each direction on different phases doesn't matter.

SPUIs and roundabouts have a bit of a learning curve. DDIs have less of one.

What is the learning curve for a SPUI? It functions much like a diamond and I don't see how they could cause confusion.

Tell that to people when one is first installed in an area. I-270 at Sawmill Rd in Columbus rings a very loud bell. There's a large amount of uncontrolled pavement and you have to get people to turn at the right places. When people are learning, it's a nightmare if not done with an excessive amount of signage.

[tradephoric]

What are the operational benefits of a DDI compared to a PARCLO interchange?  There are examples of full cloverleaf interchanges being converted to DDI's as opposed to PARCLO's (IE. I-79 & US19).  A Parclo B4 has 12 conflict points as opposed to a DDI that has 14 conflict points.  A Parclo B4 can achieve good coordination (since traffic signals only stop one-direction of travel) while a DDI you get bad coordination (since closely spaced traffic signals stop both directions of travel).   Why crisscross traffic on US19 (and add two-additional conflict points) when PennDOT could simply take advantage of the grade separation between I-79 & US19 and turn it into a Parclo B4?

As was already mentioned, a DDI allows for I-70 to be widened without replacing the bridges. They're turning the auxiliary lane into an additional travel lane. Parclos are superior when space allows, but space isn't always available.

Your hung up on cost.  I'll ask you another question.  You say Parclo's are superior but there are different types of Parclos to choose from.  Are there any operational, safety, or cost benefits of picking a Parclo A4 over a Parclo B4?

[jeffandnicole]

What is the learning curve for a SPUI? It functions much like a diamond and I don't see how they could cause confusion.

I never understand why there's a learning curve with SPUIs at all.  At traditional intersections, when opposing traffic makes a left turn, you each keep to the left.  At a SPUI, when opposing traffic makes a left turn, you each keep to the left.  The only difference is the starting angle is a bit different. 

[Ned Weasel]

Driving on the wrong side of the road doesn't require a learning curve? I know if I hadn't read about Death Diamonds on the internet I would have turned right at the first traffic light into oncoming traffic. They are confusing, cheap (in the bad way) and will eventually prove to be deadly.

Why would you have done that?  Because the signage and pavement markings were unclear?  Because the center median didn't establish a clear division of directions of traffic?  Because you didn't understand the concept of a two-way street becoming a pair of one-way streets?  Or because your future self spoke to you from this present reality and told you to make an incorrect maneuver, just to prove a point?