Episode 25 – Highway Interchanges and SPUIs

Topics: Highway Interchanges and SPUIs


Websites and Citations:
Theme Music: Five Star Fall, Mercurial Girl, Magnatune.com
Diamond Interchanges
Single Point Urban Interchanges
Clover Leaf Interchanges
Partial Clover Leaf Interchanges


Episode 25 – Highway Interchanges and SPUIs

Hello and good day! Welcome to Talking Traffic episode 25. My name is Bill Ruhsam and I host this podcast, and its sister website, talkingtraffic.org. Today is Tuesday September 8, 2008.

Today’s episode is about highway interchanges and specifically about the SPUI. But before I get into that subject, I’d like to note that this is the 25th episode! Talking Traffic has been motoring along since August of 2007 and I’d like to thank all of my listeners and especially those who have sent in comments or suggestions for future episodes. Those suggestions have led to a good quarter of the podcasts so far. So, Those of us who are about to Podcast Salute you!

Given that it’s the 25th episode, and I have reason to suspect I might get a few new listeners this week (hello anybody popping over from Podcastle!), I thought I might re-introduce myself. As I said, my name is Bill Ruhsam and I am a Professional Engineer living and working in Metro Atlanta, Georgia. I’m also a Professional Traffic Operations Engineer with a few other certifications and licenses that end up being alphabet soup and without much applicability to this podcast. I do this because I sincerely believe that there is a chasm of understanding between the engineering community and the general public with respect to transportation. For example, just the other week I was driving with several friends in the car and one of them made a comment about the new Atlanta Ramp Meters (episode 22) which made me understand that *he* didn’t understand at all what they are for. Talking Traffic is an attempt to bridge that gap. What I don’t try to do is to sell you on the pros and cons of some of the infrastructure that is constructed. Like commuter rail. While I might discuss commuter rail and it’s general impacts and needs, I’m by no means an economic expert on the topic and I won’t try and push an agenda. The only time you will hear me advocate a particular position is when the engineering is so straightforward and the results so clear cut that it doesn’t make sense to not push for that outcome. Those episodes are rare. In fact the only one recently was Episode 22, about Ramp Meters.

Nope, TalkingTraffic is about information and education, not advocacy. I want you to understand why that green left turn arrow keeps coming up when there’s no one in that lane! Or why, after new highway construction is complete, the road is still full of cars! The answers to these questions might not reduce your frustration on the roadway, but it will help you understand the challenges faced by transportation professionals and will make you a more educated citizen and voter.

Enough about me and the podcast! Let’s get to the podcast episode itself. This one is about interchanges. Interchanges are roadway structures that occur where two roads cross at different elevations yet there must be some exchange of vehicles. If one road goes under or over the other, you need ramps or flyovers or some other intermediary structure to provide access between road A and road B. These are interchanges and they come in many shapes and sizes but only two general categories.

Category one is… All right, hold on. I’m giving you a fair warning here. This part of the episode is drier than most, but I want you to hang in there because the whole point of this is going to lead to a descritpion and analysis of the SPUI! What’s a SPUI? You’ll have to listen to find out… Anyway, Category one is the System Interchange: A system interchange is where two roads that are fully controlled for access (think interstate highways) intersect. System interchanges are designed for high-speed, no stopping exchange of vehicles from one roadway to another. Those huge, massively complicated interchanges that people love to show pictures of from above to illustrate how tangled and confusing the transportation system is? Those are system interchanges. As an aside, the more complicated a system interchange *looks* the less complicated it will actually *be* for the driver because there will be only one decision point. Once you’re on the right ramp or flyover, you’re on to your destination, no need for further maneuvering.

Anyway. The second category of interchange is the NON SYSTEM INTERCHANGE where an access controlled highway intersects with a non-access controlled highway. These interchanges usually have stop signs or signals at the bottom of the ramps to control the traffic flow, but not always. This second category is where much grief and concern comes to be because it’s where most people spend lots of time sitting and waiting to either get on or off the highway.

Inside this second category of interchange are several subtypes. Here we are going to run into a fundamental problem with podcasting: the whole Picture worth a Thousand Worlds thing. This is an audio production and instead of showing your pictures which will make you go, “Oh yeah! *That*’s what you’re talking about” I’m forced to describe things in the best manner possible. For those of you at a computer, there are images available at the talking traffic website under the show notes, but for those of you listening through ear buds or car speakers, you’re going to have to put your imaginations to work.

Ok. Interchange number one, and something you’re very familiar with if you’ve driven very much is the “Diamond” interchange. A diamond interchange is so named because if you look at it from above, the shape of the ramps make a diamond. Very simple! If that doesn’t float your imaginary boat, cast your minds’ eye into your driver’s seat and imagine that you’re on the highway needing to get off onto a cross street. You exit on a ramp on your right hand side which leads to a signal. Across the street is another ramp that leads back to the highway. This same pattern is repeated for the other direction of travel. These four ramps, two on and two off, make for a diamond interchange.

And, because engineers are never satisfied unless we can divide and subdivide things to death, there are a few subcategories of the diamond interchange. Have no fear, though, they’re all the same basic configuration. The only differences boil down to how close together the traffic signals are a the end of the ramps. In an urban situation, where there’s just not much room to build highway interchanges, the ramps can hug the edges of the highway, sometimes with walls hold up the highway bridge. These interchanges are called Tight Urban Diamond Interchanges, or TUDIs. Then there are the same basic layouts but with the signals far apart from each other, and these are called, creatively, Spread Diamonds.

Another basic interchange that I’m sure you are familiar with is the clover leaf interchange, where there are four loops laps in each corner to allow you to exit right, go around 275 degrees and end up having made a left turn onto the cross street. Now a Full Cloverleaf is used for System Interchanges, remember where one controlled access roadway is crossing another, but a Partial cloverleaf can be used for non-system interchanges in cooperation with the Diamond interchanges I was just mentioning. If you take a loop ramp, and add it to a diamond-type ramp, congratulations, you’ve now got a partial cloverleaf or PARCLO. yes, I have had discussions where I’ve used the term PARCLO. It sounds silly to me, too.

PARCLOs have many subcategories, and I’m not going to get into it now. They basically deal with where the loop ramp is.

Now, for the piece de resistance, the interchange that is shaking things up around the country, I introduce the SPUI! The Single Point Urban Interchange! This interchange is so named because it’s typically used in urban settings where space is a premium, and because all of the traffic flows through one single point. For those of you who don’t already know what a SPUI looks like, I want you to imagine that you’re sitting in your car at a normal Diamond interchange signal. You just came down the ramp from the highway and you’re wanting to turn left onto the cross street. What do you see? You see the signal in front of you, obviously. you’re facing directly across the street and looking at the on ramp for the highway. If you so chose, you could cross the street and get back onto the highway by going up that ramp. As you look to your left, you can see under the bridge (I’m assuming the main highway is bridging the cross street) and you can see a fellow car parked at the signal of the *other* set of ramps for the other direction of the main highway. Now, I will describe to you what you’d see ifyou were sitting at the intersection for a SPUI.

First off, you wouldn’t be facing perpendicular to the cross street. Instead, your car will be pointing diagonally underneath the bridge, approximately directly at the car at the other ramp on the other side of the highway. Secondly, there aren’t two signals, there’s only one. Third, at a properly designed SPUI, you will have no option to cross the street and get back onto the highway. This is critical to how the intersection functions. More on that is a second.

So, here you are, sitting at a signal, wanting to turn left onto the cross street. Your light turns green and you proceed into the intersection. But, OH MY GOD, what the hell is that other car at the other ramp doing! He’s driving straight at you! He’s…oh, he’s making a left turn too, but he’s passing toyour *right*. What is this, England? Nope, this is a correctly functioning SPUI. The purpose behind this intersection is to reduce the total delay that vehicles experience because of the traffic signals. Signals cause delay just by being there, and the more there are, the more delay is experience by drivers. If you can eliminate a signal, say by going from a Diamond Interchange with two signals to a SPUI with only one, you’ll reduce that delay by a significant fraction. SPUIs operate with concurrent left turns, as described by my scenario, both for the Ramps turning onto the side street, and for the side street turning onto the ramps. One additional phase is needed for the side street through movement and that is it. Diamond Interchanges need at least 4 phases on their traffic signals and that is where the delay savings comes from, by eliminating a phase. At the end of every phase is a sequence of yellow then all-red on the traffic signal. If you can eliminate a phase, then that lost time goes away. If you remember, I mentioned that for a correctly designed SPUI, you won’t have the option to get directly back onto the highway? Well, this is why. In order to have a ramp to ramp through movement, you’d have to add that signal phase back into the mix, totaling back to four phases, thus eliminating a great deal of the usefulness of the SPUI. If you’re going to do that, you might as well go with the Tight Urban Diamond rather than the SPUI, because you eliminate some of the extra costs that are required to build a single point urban interchange.

So, if these things are all that, why aren’t we building them everywhere? As with all things, there are pros and cons. I’ve illustrated the pros, less need for space and less delay, but the cons are significant.

for one, this is not a friendly intersection for bicycles and pedestrians. There are no pedestrian phases for crossing the street and the only pedestrian phase available for crossing the ramps is the cross-street through movement. Not good.

For another, the costs of this intersection can be higher than a tight urban diamond, despite the need for less space. A SPUI typically requires a higher and longer bridge span, and bridges are expensive.

Also, as I illustrated in my scenario, the vehicle maneuvers through this intersection are atypical and can be confusing to drivers. The intersection layout leads to a large expanse of pavement with multiple overlapping stripes. If the stripes are not enthusiastically maintained by the local agency in charge, the intersection will quickly become difficult to navigate.

So, SPUIs are a useful tool in the trafffic engineer’s tool box, but they’re not the end all be all. But they do have a cool name.

Thanks for listening to talking traffic. If you’re a first time listener, I encourage you to send me your comments or suggestions. You can send an email to Bill @ Talkingtraffic.org or just post a comment on the shownotes. If you’re a long time listener, you already know you can do that, so I won’t repeat it.

This episode is released under a creative commons 3.0 attribution noncommercial no-derivatives license. Feel free to share it with whomever you want, but please don’t change it, or sell it, and be sure to credit me and talkingtrafffic.org. For all of you who are now, or are about to be driving through Single Point Urban Interchanges, have fun, and send me pictures.

The music you hear is by Five Star Fall and can be found at Magnatune.com. Thanks for listening and we’ll see you next time. Have a great week.

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2 Responses to Episode 25 – Highway Interchanges and SPUIs

  1. Steve says:

    Good article.

    I assume that the interchange of I-85 at Clairmont Road would be a SPUI because it has the “looparound” to go back to I-85 south and I-85 south frontage road from the I-85 north onramp/frontage road?

    As a “roadgeek”, I just wanted to make sure. :)

    Thanks in advance for your answer.

  2. Bill Ruhsam says:

    Actually, Steve, I-85 at Clairmont is a Diamond Interchange. It doesn’t even qualify as a tight diamond because the ramp intersections are too far apart. ~250 to 300 feet is usually the definition of a Tight Diamond. The U-Turn loop-arounds are just an added bonus to eliminate the u-turners from the left turn movement at the signal.

    If you want a local Atlanta example of a SPUI, go to SR 400 and the Lenox Road (exit 2?). This is also an excellent example of why SPUIs are more expensive than diamonds. If you look at the overhead imagery (Google Maps), you can see the many additional bridge beams needed to hold up the roadway in order to give the SPUI the lane directionality it needs to operate correctly.

    Remember, SPUIs have one signal. If it has two, it’s a Diamond.

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