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Thread: Diverging diamond interchange: no left turns

  1. #1
    Cyburbian LTKS's avatar
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    Diverging diamond interchange: no left turns

    I saw this article this morning and thought it might be interesting to get people's take on the concept. Apparently there are few locales that have implemented this, but not anywhere near me. Seems to make sense, but having seen so many people not able to figure out roundabouts, I'd be interested to see how this goes.

    Thoughts, experiences, etc?

    http://www.slate.com/id/2300425/?GT1=38001

  2. #2
    Cyburbian Queen B's avatar
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    We drove through the Springfield on on vacation this year. You dont really realize that what it is until you are through it. They spoke about how unusual that is for drivers. Not really they divert you to the wrong side of the road for construction. We liked it.
    It is all a matter of perspective!!!

  3. #3
    OH....IO Hink's avatar
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    It looks crazy from above, but lots of intersections do. The only argument would be the amount of impervious surface and repaving required.

    Otherwise I like it.
    A common mistake people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. -Douglas Adams

  4. #4
    Cyburbian wahday's avatar
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    Intriguing! And I will say this about traffic circles: Everyone complains about them - about how drivers don't understand them, etc. But my observation here with our ONE (yes, only one) traffic circle is that a) it keeps traffic moving and b) there has yet to be a single accident there, despite the inability of many to really understand what is going on. The key, I think, is that all of the traffic is moving slowly through the circle such that any confusion or miscalculation still leaves time for others to react. So, confusing?, perhaps, dangerous? not really. So far, it is a lot safer than the controlled intersection that used to be at the same locaiton.

    Keeping traffic moving, even at a slow pace, reduces the stress of drivers and also has a significant impact on air pollution by reducing idling, which contributes significantly to poor air quality.

    I agree with Hink_Planner in the concerns about hardscape and simply the ROW required to implement these. But overall, I think the benfits outweigh the costs.
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  5. #5
    Cyburbian Tarf's avatar
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    From the pictures/examples, it seems this concept works mostly at very heavy intersections (freeway/arterial), so I'm not surprised with the amount of pavement.

    However, I do wonder if this concept has also been tried at smaller intersections (e.g., two arterials, rather than freeway off-ramp/arterial). And does it always require grade separation along one of the segments (again, as in the examples shown)?

  6. #6
    Cyburbian HomerJ's avatar
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    Looks pretty cool. It seems one criticism is that it would be too confusing for drivers. It doesn't seem that bad really but I wonder how it would work in higher density areas. Is there any way for pedestrians to get across without risking their life?

    Also, I wonder how it would handle something like construction/repairs and emergency vehicles going through (I imagine an ambulance would find this a little more tricky than just stopping a traditional intersection).
    Insanity in individuals is something rare - but in groups, parties, nations and epochs, it is the rule.

  7. #7
    Cyburbian mgk920's avatar
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    These 'diverging diamond' interchanges, although counter-intuitive on their surfaces, appear to be working well where they are being tried, not just at that I-44 interchange in Springfield, MO, but elsewhere, too. One is also under construction in Lexington, KY. They allow all turns to be made without having to directly cross lanes of oncoming traffic, the same with modern roundabouts.

    An interesting design item with the one in Springfield, MO - the sidewalks cross to the center of the road, behind the barriers between the traffic lanes, where its mainline street lanes are reversed.

    According to traffic safety statistics, making left turns across traffic is the most dangerous normal traffic maneuver of all and from what I can tell, is also the main basis for the 'Welcome to New Jersey - all turns from right lane' thing.

    One more thing about roundabouts, they keep working when the power is off.

    Mike

  8. #8
    Cyburbian Joe Iliff's avatar
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    I have driven through the diverging diamond intersection in Springfield, MO several times. I like it.

    I have also been through a continuous flow intersection in Ciudad Juarez, Mexico, where left turning vehicles are already on the left side of the road, and don't conflict with ongoing traffic.

    I was surprised recently to find that Texas DOT has installed a Michigan left turn at the intersection of Preston Road (SH 289) and Legacy Road in Plano, Texas. I has been a bad intersection for many years, but TxDOT has done a great job of improving it.
    JOE ILIFF
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  9. #9
    Cyburbian LTKS's avatar
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    It seems like a more efficient design, and I'm curious to see how quickly it might catch on throughout the country. I had the same questions about pedestrian activity though. Would love to see one in person one of these days.

    I'm a huge fan of roundabouts. People need to get the image of Clark Griswold from European Vacation out of their minds when driving through or looking at conceptual designs for their community - they work wonders for flow and general movement, with fewer accidents.

  10. #10
    Cyburbian WSU MUP Student's avatar
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    I saw that Slate article the other day and after looking at the diagrams, this looks to be about the most pedestrian-unfriendly crossing I can recall. Granted, I'm not a transportation engineer but from my count, a pedestrian trying to cross in a straight path would have to deal with 4 separate points of oncoming traffic as opposed to 1 or 2 spots at a normal intersection.

    This DDI type of intersection also looks like it takes up an inordinate amount of physical space which would result in a much larger dead-zone for commercial or industrial uses (or residential if they happen to be in that area).

    Lastly, the designer stipulates that these types of intersections are really only appropriate at intersections/interchanges that have at least one direction of traffic much less heavily traveled than the other directions. What happens when all directions become more heavily traveled? I could easily foresee much longer backups developing as traffic gets stuck in the path of opposing traffic between red lights. What sort of cost would be associated with re-configuring a DDI when that point arrives?
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    Cyburbia Administrator Dan's avatar
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    Moderator note:
    Changed the title from "No Left Turns???".

    Carry on.
    Growth for growth's sake is the ideology of the cancer cell. -- Edward Abbey

  12. #12
    Cyburbian LTKS's avatar
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    This is quite the hot talk topic of the moment. Just heard an NPR Talk of the Nation segment on it during lunch. Loved the calls in - people have very creative ways of either avoiding left turns or risking their lives to complete a left turn.

  13. #13
    The biggest issue I see is if you have heavy through movements. The design has one direction in the through movement stopped at all times. It seems to work well with a large number of left turns, and if that's the issue, then it's a good solution. We have too many retirees to try to implement something that requires too much thinking!

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