The Diverging Diamond Interchange: good for cars, but what about pedestrians?


About a year ago, shortly before I left Provo for Philadelphia, there was a big stir in the news about a revolutionary interchange being designed in American Fork, Utah. Horrocks Engineering was designing a diverging diamond interchange (DDI), only the third in the nation, to try to improve traffic flow over I-15.

What is revolutionary about this design is that it minimizes left turns, which slow down traffic both for the turners and those whose lanes they are turning across, by having all traffic cross onto the left side of the road for a short time. Although this may sound confusing, you can see how the various movements would work here.

I sort of forgot about this for the next year, until I read today’s article in Slate by Tom Vanderbilt. In it, Vanderbilt discusses this and other methods (such as the much-maligned Jersey Jug Handle) that have been tried to improve left-turn related highway problems. The DDI has gone through various tests (although there is not currently a standardized design) and drivers are able to overcome the strange feeling of driving on the left fairly quickly (although it should be noted that the DDI does depend on traffic islands to make sure that drivers don’t just continue going straight on into incoming traffic).

There are a number of issues related to the DDI and questions that, in its infancy, have yet to be answered. Vanderbilt begins addressing what I think it the most important issue, that of pedestrian access. “While the intersections are avowedly built with access for pedestrians and cyclists in mind, as this rather involved walk-through video of a DDI reveals, it doesn’t really feel like a human-scaled environment,” he says. The FHA argues that it is safer for pedestrians because they only have to cross traffic going one way each time they cross. While this is true, pedestrians have to cross four signals to get to the other side of the road, giving them plenty of opportunity to be hit by cars and also forcing them to wait in inhospitable environments at each crossing. On the plus side though, by breaking the lanes up into smaller segments, it makes it easier to jaywalk across (an inevitability that few planners or engineers consider).

There are two basic systems of pedestrian access in a DDI, as are detailed in this selection from the ACEC of Michigan’s report on DDIs:

In addition to those specified above, another disadvantage of the center crossing is the feeling of being surrounded by cars on either side. This can be mitigated with the high walls that are seen in the video above, but that just exchanges being surrounded by cars with a feeling of claustrophobia.

A number of sources say that the safety problems of crossing so many lanes of traffic can be addressed through signalization. However, doing so would interrupt the continuous traffic flow that is the main advantage of the DDI for motorists. You might as well do a traditional diamond.

You might be able to tell, but despite the professional assurances from highwaymen, I am skeptical about the advantages of the DDI for pedestrians. But I really want to know what you think. Is the DDI any better or worse for pedestrians than a traditional freeway interchange, and why?

Vanderbilt ends his post with a great point. He mentions that the assumptions about DDIs and their traffic improvements are based on the idea that traffic will continue to increase because we will continue suburban sprawl patterns of development that make it hard to be a pedestrian. The question is, if we continue to neglect the link between land use and transportation, and refuse to build balanced transportation systems, “Can you ever truly design your way out of congestion?”

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About Dave Munson
This blog is about architecture, cities, and myself.

5 Responses to The Diverging Diamond Interchange: good for cars, but what about pedestrians?

  1. dan reed! says:

    I FOUND YOUR BLOG ON THE INTERWEBS

    WHY DIDN’T YOU TELL ME THIS WAS HERE

    IT’S REALLY GOOD

  2. Generally, pedestrians aren’t going to WANT to walk through/near any highway interchange; like you said, it is not to human scale and people are not comfortable. The more basic question is, do we want to build interchanges that make accessing highways easier? Travel demand management theorizes that this leads to greater traffic volumes. So if you create greater access, it attracts more drivers until traffic reaches a new equilibrium, which doesn’t relieve congestion, but basically reverts to the level of congestion you started with. I think it does nothing to improve pedestrian access at the interchange and it perpetuates the suburban sprawl/vehicle dependent paradigm that typically neglects pedestrian access.

  3. DAN STOP YELLING AT DAVE!

  4. Troubleshooter says:

    Zoning is the reason we have long distance commutes. The elitists on the zoning boards don’t want any residences near the workplaces.

    The idea that making a new road causes more traffic is absurd. People have certain business they need to attend to. They go about that business by the most efficient and time-saving process available. They don’t go out of their way to create new trips because a new road is built.

    • Dave Munson says:

      I don’t often reply to comments, but I think this one warrants it. While I agree that zoning boards are at least partially responsible for commute problems, I would say it is the other way around; they want to protect residential property values by keeping workplaces far away. However, one need only to try and find a condo on Manhattan to realize that this theory is bunk.

      The idea behind induced traffic demand isn’t that existing residents will go, “Ooh look! A new road!” and go out of their way to drive on it. The idea is that by increasing road capacity, it gives an incentive for increased development in the area, so that new residents clog up the roads even faster. This is a huge problem when there is only one road that is simply widened, as is the case with highway development. I don’t know for sure, but my impression is that the issue isn’t the same when more surface roads are added.

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