Diverging Diamond Interchange at Bessemer/US 129 Bypass

Innovative interchange design improves traffic flow without major reconstruction

...very innovative design.

...great public outreach and complexity.

Facing an escalating traffic count at the US 129/Bessemer Street Interchange in Alcoa, Tennessee, city officials, in cooperation with the Tennessee DOT, turned to GS&P to create a custom-fit solution. Showcasing its expertise in creating innovative designs for challenging problems, GS&P selected a unique geometric design, the Diverging Diamond Interchange (DDI), at a time when no such interchange existed in Tennessee. Engineers successfully gleaned valuable best practice information from other DDI projects being planned and designed in the U.S., and incorporated those successes into the Alcoa design. The final configuration allowed the team to keep the existing bridge in place, and reduce traffic volumes and delays, all while saving millions of dollars in construction costs.

What was the driving force behind the project?

Jason Brady: In addition to the already growing congestion at the intersection, there were two main issues that were certainly going to impact future traffic flow. The first was that the connection to Middlesettlements Road did not operate efficiently and was a poor terminus to a State Industrial Access (SIA) project serving a nearby DENSO Manufacturing plant. TDOT was widening the road to tie back into the existing interchange, adding a larger amount of traffic into it, and efficient flow to the industrial complex was important.

Mark Holloran: And the second part was on the opposite side of the interchange. A huge commercial development was being built – sporting goods, restaurants, big box stores — also adding a lot of traffic. When we did the traffic analysis, the interchange was having trouble handling flow with only the existing two lanes going underneath the bridge, so the new development was putting added pressure on resolving the issues.

Why did you begin considering the DDI design for this project?

Michael Flatt: Not at first, but it was very early on when we began considering options after the client informed us of the limited construction budget. Because there was such a restriction on cost, and because we were quickly finding out that traditional solutions wouldn’t fit physically or financially, we knew we had to develop something innovative.

We created our list of critical issues that were the driving factors and proceeded to look for solutions that would adapt and apply within those restrictions. It quickly became obvious that the only affordable solution would be one that utilized the existing US 129 bridge in the new interchange. At that time the DDI concept was beginning to get some attention around the country. I had read some articles and was curious—even skeptical—about it, but never thought we would consider proposing one.

Mark: When we looked at putting in turn lanes, for example, we couldn’t get enough lanes under the bridge to get it to work. We would either have to replace the bridge, which wasn’t cost-effective, or do something like cut into the fill slopes behind the piers and try to put lanes between the abutment and the piers. Nobody liked that because it would start splitting traffic around the piers, and that’s not a safe thing to do.

Jason: We were brainstorming different designs, and one was a DDI interchange in France. It was massive, with multiple roads in each direction and a lot of very close side streets next to the ramps and so forth.

Mark: So there ended up being only two options that could be done with lanes just between the piers. One was the diverging diamond and the other was using roundabouts on each side of the bridge at the ramp intersections. It was determined that the roundabout design would send so many people under the bridge and around the roundabout to make a left-hand turn that there wouldn’t be any gaps for people to get in from Middlesettlement Road. The queue would back way up.

Lori Lange: So our focus became the DDIs that were being designed and implemented in the U.S. We went to visit one in Springfield, Missouri, and at the time it was the only one open.

How did you convey the unique attributes of the DDI to the owners?

Michael: It wasn’t easy, but this was such a unique case for us and everyone involved. The concept of a DDI wasn’t just new to the Alcoa or TDOT, it is new to the U.S., and when you are used to working with traditional solutions with years of statistical proof to back them up, it can be difficult to consider innovative solutions. It can be even harder to sell them.

Jason: When we were pitching this idea, we provided TDOT and the City of Alcoa with several different designs sketched out on paper, but they didn’t even want to talk about the DDI because the sketches make a simple, elegant design look complicated and confusing. That challenged us to take a second look, but also made us reconsider ways to sell what was quickly becoming the better option.

Michael: Explaining a new concept like this required additional simulations, interactive videos and explanations. Once the citizens and city understood the concept, the benefits and that it was a safe alternative, they were convinced that this solution was right for their community and they wanted to do this project. The city leaders really stepped up to help convince TDOT that it was the right thing to do.

Was the level of community involvement unusual?

Michael: It was surprising to me how much interest and support came from the community as the project was being discussed and as it evolved. It’s common for public meetings to occur, usually sparsely attended, but this project got a lot of press and attention—articles in the local newspaper and eventually a segment on local TV news. The city really began to own it and buy into it, and I credit our team for a lot of that.

In layman’s terms, how would you describe the geometry of the design?

Jason: I have the most success by describing it as two one-way streets that intersect twice in a very short distance. There are only two cycles at each signal and either you are waiting to go straight or you are going straight. All the other decisions happen outside of the signalized intersection.

Mark: Probably the new term that the Federal Highway Administration is using, double crossover, is a better way to think about it than diverging diamond, especially for the public to grasp the concept. Drivers approach the interchange and crossover to the left side of the road, and then after they pass through the interchange they cross back over to the right side of the road.

The beauty of it is once drivers cross over to the left, they can make a left turn onto an interstate, or in this case US 129, without crossing traffic.

In a traditional interchange, people who want to turn left onto an on-ramp would have to cross in front of traffic through a signal, what is called “conflicting vehicle movements.” This design eliminates that movement.

Jason: All turns onto and off of the ramps are with traffic–the equivalent of a right-hand turn on any street or a left-hand turn onto a one-way road. You never move across a lane of traffic to make a left like you do at a regular two-way street.

Another project objective was “traffic calming.” How is this accomplished?

Mark: The theory behind traffic calming is to manage traffic at a uniform, smooth speed. A mixture of somebody driving 60 mph and somebody driving 30 mph is often disastrous, so we incorporated things to slow down those who are speeding.

Also, the spirit of traffic calming is to deliver a clear and concise message to the driver about what you want them to do, or to influence driver expectancy.

What do you mean by “driver expectancy?”

Mark: It’s basically a feel for what you, as the driver, are supposed to be doing. This whole job is about driver expectancy. In everything, from the signing to the striping to the layout, it’s all about making sure that when drivers come into this, they can successfully get through it, even if they’ve never seen a DDI.

The hard part is helping people understand that they are doing the right thing by crossing over and being on the left side of the road. In addition to the signing and striping that was used, Lori designed a barrier wall between the lanes going under the bridge so people wouldn’t feel so uncomfortable seeing traffic on their right side.

Now that the DDI is complete, have there been noticeable improvements in traffic flow or safety?

Mark: N.C. State University is working on a report that focuses on driver expectancy, wrong-way moves, etc., and it’ll be interesting to see what they find. But even right now Alcoa hasn’t seen any backup at the intersection, even at rush hour, and that was a real issue before. And because the DDI is handling traffic so much better, it is helping relieve traffic in adjacent intersections.

And, as Jason pointed out, because the design calms traffic, drivers are going slow enough to navigate it. Some of the biggest problems with regular intersections is drivers speed up to beat a red light and someone pulls out and gets hit. In this case, drivers are going slow enough that they can react and it should help improve safety. In fact, in the six months since it opened, there’s hasn’t been one accident attributed to the design.

What about the DDI project makes you most proud?

Jason: For me it’s the fact that it’s in my hometown. My family uses the interchange, so I’m proud that it’s our design. I’ve managed to do something in my hometown that makes it better.

There aren’t any TDOT templates for what we’ve had to do here. It required a lot of thinking through to make this work, make it successful and make it safe.

So I’m proud of the fact that we were able to meet all those challenges. It was outside the box, both on paper and in how it was constructed.

Michael: To me, there’s really one story here. Anybody in the world can pick up some software or a book and design an interchange. The client came to us with a problem, and they didn’t have enough money to fix the problem by using traditional means.

Although TDOT was willing to perform improvements for the money they had, we were able to take that same amount of money and come up with a design that had overwhelming improvements and operated at a much higher capacity.

I also think we exhibited to our clients that we are not afraid to use best practices from other projects. We exhibited to them that we’re willing to not let the thought, “it’s not been done before,” be a reason to prohibit a good plan.

Ultimately, what do you think this project reveals about GS&P?

Mark: The thing that we want our clients to know is that we’re able to bring solutions to the table that can solve the challenges that they are facing. Most people would not have considered DDI as an option, but that’s what we do. We are a design firm that drives solutions because we’re not afraid to reach out and try something new.

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Gresham, Smith & Partners

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