Hate Left Turns? Take A 'Diverging' Route

Tom Vanderbilt calls left turns "the bane of traffic engineers." The diverging diamond interchange, he says, is one way to get around them.

Tom Vanderbilt calls left turns "the bane of traffic engineers." The diverging diamond interchange, he says, is one way to get around them. Missouri Department of Transportation hide caption

itoggle caption Missouri Department of Transportation

Roughly 10 million motor vehicle accidents are reported on U.S. roads every year. Regular drivers may not be surprised to hear that many of them involve cars making left turns. Some drivers go to great lengths just to avoid making a difficult left.

Traffic engineers have figured out some alternatives to the standard, four-way intersection, including the Jersey jughandle and the Michigan left. But most still involve crossing directly in front of oncoming traffic.

In a piece for Slate, "Don't Turn Left," Tom Vanderbilt investigates another solution: the diverging diamond interchange. He tells NPR's Tony Cox, "Engineers for decades have dreamed of basically eliminating left turns.


Interview Highlights

On why two rights are better than a left

"Especially on a busy, multi-lane road, you're going to make a left turn ... you have to navigate across several lanes of oncoming traffic — something that humans are a little bit bad at doing, judging the speed and distance of approaching vehicles — it's sort of a stressful maneuver."

On why dedicated left-turn lanes aren't the answer

"This is when you have the green arrow that allows only you to make a left turn. This is better for the driver, but it's worse for the intersection as a whole, because every time you give that special phase, you're taking away from all the other motions in that intersection. So a left-turn phase is incredibly inefficient in the eyes of engineers."

cafachiever/YouTube

Watch a diverging diamond interchange in action.

On how the diverging diamond works

"It's one of the more innovative and experimental ideas out there... Imagine that you're on a north-south arterial road ... and you're crossing over an interstate highway that runs east-west. So you're going north and you want to go west, you need to make a left ... [In the] diverging diamond, as you're approaching the interstate, the two lanes of traffic basically criss-cross. As you go through the first traffic light, you're asked to go across what you think is the normal flow of traffic. You sort of veer to the left, and if you're going to head west on that intersection, you just go off on this ramp that's specially dedicated only for left turns. If you want to continue straight, you continue going straight, then loop back through another set of criss-crosses, back into the right lane of traffic."

On the chance you'll be driving through one soon

"There's only been about a half-dozen built to date, but there are many more either in the works or on the planning boards. There are about several dozen other innovative intersection treatments that are on people's wish lists or in the stages of being drafted."

Copyright © 2011 NPR. For personal, noncommercial use only. See Terms of Use. For other uses, prior permission required.

TONY COX, host: Every year, roughly 10 million motor vehicle accidents are reported on the roads, and it may not be a surprise to regular drivers that many of them happen during a left turn. Traffic engineers have come up with alternatives to the standard four-way intersection, like the Jersey jughandle and the Michigan left and the more ubiquitous roundabout. Yet most still involve crossing directly in front of another oncoming vehicle. It is indeed a driving dilemma, but there may be hope. In a piece for slate.com, transportation columnist Tom Vanderbilt proposes a solution: the diverging diamond exchange. Oh, let me get that right. The diverging diamond interchange.

To give you an idea of what he's talking about, we have a video of this design on our website. Go to npr.org and click on TALK OF THE NATION. You can see how it works for yourself. What's your left-turn strategy? Do you even have one? 800-989-8255. That's our phone number here. Our email address is talk@npr.org. And you can join the conversation at our website. Go to npr.org. Click on TALK OF THE NATION.

Tom Vanderbilt, joining us now from our bureau in New York. Tom, welcome to the program.

TOM VANDERBILT: Hi, Tony. Great to be here.

COX: I want to ask you this first. Why are left turns so tough to design in a safe and efficient manner?

VANDERBILT: Well, yeah. I mean, just to foreground this whole discussion, engineers, for decades, have dreamed of basically eliminating left turns. And this is something that we've heard. Famously, UPS has special routing software when they send their trucks out to try to have their drivers make as many right turns as possible. I mean, an ideal trip would be nothing but right turns.

And the question is why do two rights - why are two rights better than a left? And, I mean, it's sort of intuitive to anyone who's out there driving. You're - especially on sort of a busy, multi-lane road. You're going to make a left turn. Either you have to navigate across several lanes of oncoming traffic - something that humans are a little bit bad at doing, judging the speed and distance of approaching vehicles. It's sort of a stressful maneuver.

Or they'll turn to - they have sort of what's called a dedicated, left-turn phase. This is when you have the green arrow that allows only you to make a left turn. This is sort of better for the driver, but it's worse for the intersection as a whole, because every time you give that special phase, you're taking away from all the other motions in that intersection. So a left turn phase is incredibly inefficient in the eyes of engineers.

COX: Oh, so it makes you have to just sit there and wait forever and ever and ever for the person to make a left turn. And if they're texting or on the phone and don't see it and don't make the turn while the green light's there, you have to wait for a whole another cycle to go through.

VANDERBILT: Exactly. Or several cycles. And then just to complicate things further, you know, by the time that left - that green left arrow turns yellow and then red, they even have to build in a few extra seconds, what's called the clearance phase, to make sure every driver has gotten out of the middle of that intersection. The clearance phase has only gotten longer as drivers sort of seem to increasingly test those red lights. And so you have to wait for the intersection to clear, which is, again, adds time.

COX: So you have the idea, right? You have the idea for us?

VANDERBILT: Well - now this is, of course, is not my idea, and it only applies in certain situations. But it's one of the more, let's say, innovative and experimental ideas out there, and it's basically called the diverging diamond interchange. And this is the sort of thing, it's like - it's a bit - trying to explain it in words is sort of like trying to describe to someone how to tie a complicated knot. I mean, it's better sort of seen, but I'll give it a go.

Imagine that you're on sort of a north-south arterial road, like in a suburban environment, and you're crossing over a highway, interstate highway, and you want to take - that runs east-west. So you're going north. You want to go west. You need to take a left turn to get onto that intersection. In a normal sort of situation, you would go over the interstate and probably have a green arrow - wait for that green arrow to have to take a left turn across those lanes of traffic.

The diverging diamond, what it does, as your approach - as you go over the - excuse me. As you're approaching the interstate, the two lanes of traffic basically crisscross. And as you go through the first traffic light, you basically are asked to go across what you think is a normal flow of traffic. You sort of veer to the left. And then if you're going to head west on that intersection, you just go off on this ramp that's specially dedicated only for left turns. If you want to continue straight, you continue going straight, then loop back through another set of crisscrosses back in the right lane of traffic. So I'm sure I've already confused (unintelligible) - I've confused myself.

COX: Well, you know, it is confusing to describe. I have seen it, so I understand what you're talking about. And to look at it, you can see that it makes a certain amount of sense trying to describe it this way. We'll probably drive our listeners crazy and you as well. But we'll do the best we can to put the explanation of how this might work and where it already is in effect. But before we do that, let's take a couple of phone calls. We've got Jim standing by from Jackson, Wyoming. Jim, welcome to TALK OF THE NATION.

JIM: Good afternoon. When I go to town, I plan my route, and I make nothing but right-hand turns. Right-hand turns on the way into town, attend to all my business, right-hand turns on the way out. I can't believe how many people I see here in motor homes that will make a left-hand turn to get into a Shell station and then they have to make a left-hand turn to get back out of the Shell station.

COX: Well, Jim, what if you're in a hurry? I mean, you're taking all those rights. That slows you down, doesn't it?

JIM: Well, I know, I mean, I'm taking rights. I plan my trip accordingly, so that I make my stops where I'm only making right-hand turns.

COX: Alrighty, and that works for you. How long have you been doing that?

JIM: A long time.

COX: A long time and it works for you.

JIM: It definitely does.

COX: Thank you very much for the call. Let's go to Ken in St. Louis, Missouri. Ken, welcome to TALK OF THE NATION.

KEN: Hi. Hey, guys. Thanks for taking my call. One of the things that has happened here in St. Louis in the last couple of months is that they've incorporated a yellow - a blinking yellow light. And I just started seeing those, you know, the last couple of months. Then I'm realizing, people are like freaking out about this thing because they're sitting at the light not wondering if they should turn or go or not. Some of them, you know, end up having to honk.

And then today, I noticed I was turning left and it was blinking yellow, and the two red lights on the right to go straight were still red. And I was like, God, you know, should I not go? You know, I felt kind of weird because the traffic coming on was still going. And thinking, well, you know, it was really - it's just really confusing.

COX: I could only imagine that it would be. Thank you very much for the call. Let's take another one, then I'm going to come back to you, Tom. We'll talk some more about this idea, this - what is the name of it?

VANDERBILT: Diverging diamond interchange.

COX: The diverging diamond interchange. Before we do that, let's talk to Amber ,who's calling us from Detroit, Michigan. Hello, Amber.

AMBER: Hi. Thanks for taking my call. Here in Detroit, the Motor City, we have what's called the Michigan left. And it's, essentially, you turn right onto a one-way street and do a U-turn so you're going the opposite direction. And there's no left-hand turn.

COX: That's because you have a U-turn in the middle of it. It's a right and a U that end up becoming a left.

KEN: Exactly. And they're very effective. People from other states say that they don't use those stupid things where they come from, but we - we're all about moving cars where they need to be efficiently, and it seems to work pretty well here.

COX: Well, thank you for that. What about that. You know, you do - actually, in the article that you wrote, one of them mentioned this. I don't know that it's always called a Michigan left, but that's something that is used in other parts of the country as well. Right, Tom?

VANDERBILT: Exactly. That sort of pinpoints sort of what both callers were saying there is that, you know, drivers are basically resistant to change. And, you know, with good reason, engineers, you know, put out new solutions like this. They don't do it lightly. They put a lot of research into it because, you know, people get used to driving in a certain way. You build up a muscle memory of how to drive. You're suddenly asking someone to go on what they think of as the wrong side of traffic.

So before this first interchange was rolled out, it was tested in driving simulators for a number of years from the Federal Highway Administration. And, you know, they find that with proper signage, you know, people can get it. Just sort of show them what to do. And people always will say, you know, well, this looks so complicated. Why can't we just go with the regular intersection? And a regular sort of four-way intersection may, you know, look simple.

But that simplicity, you know, sort of masks something which is going on there, which is that drivers often sort of let their guard down when they're going through that. And this is why roundabouts are sort of a safer solution because - precisely because it feels a little bit more risky. You have to do a little bit more negotiating on your own. You're a little bit more vigilant. And, in a four-way intersection, if you have a green light, your mind sort of shuts off and you're not really looking for hazards from the side or someone who might have run a red light. So just this one cautionary note there.

COX: That's an interesting point. If you're just tuning in, this is not actually traffic school for people who have been cited for violating the traffic laws. We are talking about the left turn. And it is one of those maneuvers in driving that creates a great deal of problems. And the statistics show that a number of accidents - maybe not the majority of accidents but a large number of accidents - are the result of left turns. And our guest is Tom Vanderbilt, author of "Traffic: Why We Drive the Way Do and What It Says About Us."

And we're talking about a new way to make a left turn. Now, in describing it for you over the radio, it's really hard to grasp the concept, which is why we have a website where you can go and see it. Just go to npr.org. Take a look at this and see what you think, and give us some feedback on it. Before we take another call, let me read a couple of emails that we have gotten. This one comes from Julia in Grand Rapids. It says, my strategy is simple: gun it and pray. Yikes.

Here's another one from Paul This says, how does the diverging diamond differ from the single-point urban interchange? What about - now, I don't know what the single-point urban interchange is. Do you, Tom?

VANDERBILT: Yes. But I fear that to try to describe the difference between those two would take us into probably another show and a level of complexity. But one difference is, as I understand it, this diverging diamond interchange is what's known as a two-phase system - seen, again, we're already getting into a complexity here. Let's just say that the diverging diamond has been shown in studies to be more efficient and has a better safety record, also a bit cheaper to install than a single-point.

This is an important consideration when designing these large interchanges, especially over bridges. You're looking to save as much money as possible and the divergent - diverging diamond has been shown to be about half the cost of traditional diamond interchanges and other types of intersections. So just another thing to keep in mind. But I fear we will lose even more listeners if we go into a...

COX: If we go into that. I'm going to ask you in a moment about the design that we were talking about, because it seems to me that it would work better in rural or suburban areas than it would in downtown Detroit or Manhattan or Los Angeles or even here in the District.

The single, biggest part of my left turn strategy is using my blinker. This comes from Jeff Arnold. The next part of my strategy involves watching the traffic flow and looking for turn alternatives. Ideally, I'll make my left at a light, even if that means turning left a couple of blocks before or after my actual intended turn. If no light is available, then I'll look for a break in the traffic which will let me turn across multiple lanes, a block or two before or after where I actually want to turn.

So it gets kind of confusing even for him. Let's take another call. This is Peggy from Bentonville - is that Alaska or Arkansas?

PEGGY: Arkansas.

COX: Arkansas, OK. Welcome to the show.

PEGGY: Thank you. We have a traffic circle up in the house. It's a new one and they're putting in the Crystal Bridges Art Museum here in Bentonville. And so we're going to have large volumes of people probably coming off bypass(ph), and that's just one of the things they've included. In one access to them is then they have the traffic circle, and I really love using it. It's actually fun to use, I think. I don't know - I think that people proceed with caution when they approach it because they realize it's something different. But - in any case, it works very well. It goes very smoothly.

COX: Well, thank you for the call. You're listening to TALK OF THE NATION from NPR News. Let's take another caller. This is Ian(ph) in - well, it just says San. I'm not sure what city you're in California. Ian, where are you located?

IAN: That will be San Francisco.

COX: San Francisco, welcome.

IAN: Hey, thanks for taking my call. A couple of things. I ride a motorcycle, a car and a bicycle, so I'm kind of aware of the trickiness of intersections. The big thing for me is being direct and clear with your intentions. You know, it was mentioned earlier, making a left turn with a blinker makes a huge difference, also looking for eye contact from people coming towards you. You can see where about they're looking if their windows aren't tinted out, and that's another thing that makes it a lot safer for me. I have some experience with the roundabouts in my old town and I love them. They're fantastic. They work really, really well, and they're really not that complicated. But the safety of them just makes a whole lot of sense, especially being on a motorcycle.

COX: Thank you, Ian, for that call. Let's go to Gary, who is in Lawrence, Kansas. Gary, you are - oh, hold on. Hold on. There you are. Gary, are you there?

GARY: I'm here (unintelligible)...

COX: OK. Gary, you - this is Gary, right, from Lawrence?

GARY: No.

COX: No. OK. Well, tell us your name and where you're calling from.

GARY: Lawrence, Kentucky.

COX: Oh, OK, Gary. Go ahead.

OK. I just have a couple of comments. The left-handed turn as opposed to the right-handed turn, it makes a lot of sense, you know, stopping left-handed turns. You run the risk of cutting in front of somebody. Also, you're using more gas making left-handed turns than you are right-handed turns.

And why is that...

GARY: (Unintellligible) the intersection (unintelligible) turning lane, your engine is using gas and you're not moving.

COX: Oh, I was going to ask you why...

GARY: (Unintelligible) turns.

COX: ...why is that the case. You mean, when you're idling, waiting to make a left turn?

GARY: Yeah, you're idling, your car is not moving, but you're still using gas.

COX: OK.

GARY: If you're making a right-handed turn, you keep that flow going and, you know, it's easier on the engine if you keep it going instead of just setting still, eating your own gas. I'm sorry.

COX: Gary, thanks for that call. Philip(ph) in Statesville, North Carolina, welcome to TALK OF THE NATION. You're on the air.

PHILIP: Yeah, I appreciate the opportunity. I'm a motorcycle rider myself and left-hand turns cause a vast majority of conflicting traffic accidents between cars and motorcycles. And it's the people that are turning left in front of you, obviously, become a pretty hard obstacle, so we don't like them very much. I'm familiar with the various forms of intersections that you're talking about, and I like every one of them. And if there was a way we can eliminate left turns, I'd happy for it - even people pull in from side streets onto the main thoroughfare, if they're turning left, the last place they look before they turn left is to the left. And if a motorcycle is coming from the right, they won't see that motorcycle and it winds up being a collision for the motorcycle and a whoops for the driver.

COX: Thank you very much for that call. This has been kind of interesting, Tom, hearing people talk about these left turns. Again, I want to remind people that they can go to see the idea that you and I were talking about at our website, npr.org. Really quickly, because we're coming to end - the end of the program, what chance does this new idea have of becoming reality?

VANDERBILT: It's quite popular. I mean, there have only been about a half dozen built to date, but there are basically many more sort of either in the works on the planning boards. And there's about several dozen other sort of innovative intersection treatments that are on, you know, people's sort of wish lists or in various stages of being drafted. But, you know, this is an issue that, you know, every place deals with.

And this diverging diamond isn't right for every intersection treatment, just as a roundabout isn't right. I mean, engineers will tell you that each specific location needs its own particular solution, but, you know, what they all share in common is this kind of age-old desire to eliminate this left turn or, you know, in some ways make it different...

COX: Got to stop it there, Tom, the clock says we got to go. Tomorrow, we'll talk about calorie counts, new research showing the numbers on menus can be off. Tom Vanderbilt, thank you very much. We'll talk about all of these things again tomorrow. This is TALK OF THE NATION from NPR News. I'm Tony Cox in Washington.

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