Diverging minds


Engineering a way to make SPUIs look good

Dr Marohn over at Strong Towns has diagnosed an epidemic of insanity sweeping through the engineering profession.  Known as Diamantia, when stricken the victim will believe that down is up, black is white, and most commonly, that left is right.

Dr Marohn gives us a case study of the first known patient, in Springfield, Missouri, where the poor victim is so delusional that he thinks a narrow, dirty culvert as friendly for pedestrians.  Wikipedia lists 9 known cases in the US, with hot spots in Utah and near the source in Missouri.

Putting aside the over-the-top metaphor, diverging diamond interchanges really are insane – would a sane engineer design a street that encourages people to drive on the wrong side of the road?  Personally, I tend to favor seemingly-insane solutions, in part because I smoked too much weed in high school, but in part because I get the sense that motorists are more attentive when placed in unfamiliar situations.  If the diverging diamond really is safer than the traditional diamond (this is the claim; not sure if there’s evidence) then I’m in favor of giving it a shot.

Except that the majority of diverging diamond designs I’ve seen are truly terrible for pedestrians.  Chuck Marohn aptly analogizes the culvert into which pedestrians are herded in the Springfield interchange as resembling the grotesque bowels of the Death Star.  Worse, the design simultaneously

  • increases the time and distance required to traverse the interchange for pedestrians  (by requiring that they cross to the center of the road and back and by twisting and turning the sidewalk to accommodate turn ramps for cars); and
  • increases the dangers faced by pedestrians  (by requiring them to cross against the main stream of motor traffic, which is heavier and likely traveling faster than the cross streams, and by designing the turn ramps to maximize speed for motorized traffic).

This leads me to conclude that the diverging diamond is a symptom of something far more insidious than insanity.  It seems to be a systematic attempt to marginalize pedestrians.  Take a look at the 9 cases identified by wikipedia.  I’ve listed them along with the location of the pedestrian facilities, if they exist*:

Springfield, MO (I-44 and MO-13): Center

Springfield, MO (US-60 & National Ave): Center

St Louis Co., MO (I-270 and Dorsett Road): Side

American Fork, UT (Main & I-15): Side

Alcoa, TN (US 129 Bypass / Bessemer St / Middlesettlements Rd): None

Lexington, KY (Herrodsburg Rd & New Circle Rd): Side

Lehi, UT (Timpanogos Highway & Interstate 15): None

Salt Lake City, UT (UT-154 & UT-201): None

Pine Island, MN (US-52 & New Sprawl Rd): Center

Probably the best diverging diamond for pedestrians

The fact that a third of them allow the pedestrians to continue on the outside of the interchange shows that the center culvert is not a crucial design feature.  But the fact that a third of the diverging diamonds have no pedestrian facilities whatsoever indicates that the engineers that propose these things are not concerned with pedestrians.

MnDOT has adopted the Springfield design pretty much wholesale for a proposed interchange in St Cloud – included are the same crossings at obtuse angles to speeding motor vehicles, the same forced detours across the heavy main stream of through traffic, and the same creepy center culvert.  The two interchanges have a similar context: big box stores and ultra low-density housing with no effort on anyone’s part to accommodate pedestrians or cyclists (there is a bus stop about a block north of the Springfield interchange, but nowhere for the passengers to walk after disembarking).

Does a diverging diamond have to be so despotic and demeaning (Marohn’s words) to pedestrians?  I don’t think so, not if pedestrians are kept in mind in the project goals (as they should be when the interchange lies between a residential neighborhood and a hospital, as it does in St Cloud).  Simply add to the goals, somewhere between 1. getting as many cars through as quickly as possible and 4. saving money:

2. minimize pedestrian detours (no serpentine sidewalks or diversion to a center culvert)

3. maintain right angles at pedestrian crossings

Using only Visio and some chewing gum, I’ve redesigned the proposed St Cloud diverging diamond to keep these principles in mind.  Imagine what could be done if you were a Professional Engineer with whatever software Professional Engineers have.  The through or turn lanes are shown in a beige similar to the original (Warning!  The original layout is 32 mb and has crashed my browser more than once.) but I changed the sidewalks from pink to a more masculine dark blue.

Zig zag leg

This design maintains basically straight sidewalks on either side of the intersection and manages to achieve right angles at nearly every pedestrian crossing (the safety advantage of right angles, besides requiring the motorist to maintain awareness of his or her surroundings before mindlessly accelerating, is that neither stream of traffic is required to turn their head terribly far – if it’s more comfortable to look, people will take the time to see what’s coming).  I kind of cheated at the leg detailed at right, which might have to end up fairly oblique.

“What about the trucks?” you may ask.  “How can they turn on so tyrannical an angle?”  We don’t have to banish semis from the roads in order to make a walkable interchange (though maybe we should consider it).  Professional Engineers can use their knowledge of turning radii to design a softer angle ahead of the crossing, although that might result in the awful twists that sidewalks tend to go into around interchanges.  Alternately, the ornate stamped concrete islands that are always built on these things (because grass can’t survive in such hostile environments) could have a mountable curb, effectively softening the radius for that most American of occupations, the trucker.

Of course, putting sidewalks on the, um, side makes for a wider bridge, which is probably why the three diverging diamonds that are (sort of) walkable all are underpasses.  But I don’t see why the wide center median is needed – couldn’t you fit side sidewalks on the same width of bridge if you just shrink the culvert? Anyway, a right angle ramp design will save some money since it fits into a tighter footprint.

Making diverging diamonds walkable may seem like treating the symptom, not the disease.  But the insane part really isn’t forcing traffic to proceed on the left, it is the fact that people can design these things and think that it’s ok to send the pedestrians into a narrow concrete strip between two streams of speeding cars.  That shows a lack of contact with reality that is frightening, but all too common.  Good thing there are good doctors like Chuck Marohn to spread the diagnosis.


*Tennessee finished their walk-less interchange, connecting a residential neighborhood to a commercial strip, just 12 days before their complete streets policy went into effect.  Apparently they’re planning another diverging diamond now, but I can’t tell if it will accommodate pedestrians.

8 comments on “Diverging minds

  1. Transiteer says:

    This is bananas. Right up there with the SPUI at Hi-Lake and other terrible ideas of the past 50 years. Did you know there is/was a DD proposed for I-35E and Maryland in St. Paul? I’m unsure of the status.


    • Alex says:

      The madness spreads…. Thanks for the link – I love how in the presentation one slide shows a rendering of a massive, sprawling American DD with sidewalk on only one side, and the very next slide is the Versailles DD, compact and with two straight sidewalks and right angles. As though American drivers were uniquely incapable of turning sharply.

  2. MidiMagic says:

    Why can’t pedestrians make turns to cross the interchange? I think the National Avenue at US-60 interchange is quite pedestrian friendly, if the pedestrian has a brain. All of the crossings are under signal control except the free right turns onto the entrance ramps. There is no traffic turning across any of the crosswalks under signal control. When the pedestrian signal is Walk or flashing Don’t Walk, no traffic is allowed to cross the crosswalk. I would think that this advantage outweighs any disadvantage listed in the article.

    The high parapets on the median crossing over the bridge are necessary to block headlight glare, since the low beam headlights on the “wrong side” of the road would shine directly into the eyes of the drivers going the other way if the parapets weren’t there. But they have the additional result of protecting pedestrians.

    • Alex says:

      The problem with serpentining the sidewalks is that pedestrians don’t tend to follow the contours of the path too closely. There’s a SPUI in Minneapolis next to a light rail system where only like a quarter of the peds actually cross at the crosswalks, because they want to walk in a straight line. But I’d ask a similar question of vehicular traffic – why doesn’t every street look like Lombard?

      And why can’t the vehicular traffic make right turns? Why is a wide turning radius integral to the functioning of the interchange? Why can French drivers turn at right angles, but American drivers can’t?

      Integral to the definition of pedestrian friendly is the lack of doom. If all you can think about is survival, it’s not friendly. Beyond that, it may be more arguable, but I’d say that if you can feel the whizz of trucks passing, it’s not friendly. If cars screech to a stop, it’s not friendly. If you’re between two streams of traffic, it doesn’t matter if the divider prevents them from actually hitting you, because if you can smell and feel and hear the vehicles, it’s not friendly.

      • Midimagic says:

        Why? Because it is more efficient for large trucks to be on a large radius. Slowing down a large semitrailer truck to make a tight turn blocks everyone behind it, often in more than one lane.

        Note that most of the trucks in France are short, so they can make the tight turns that already existed there before the automobile. But that means that two truck trips are made in France that would be accomplished by one in the US, so France uses more fuel to move goods.

        Also note that NONE of the French diverging diamonds have the characteristics you want:

        – The Seclin interchange was kitbashed out of another interchange design (contraflow left, the reason it has 3 bridges). It has NO pedestrian facilities, because it is not in a city. Note that it is designed for semitrailer trucks, and three are using it in the Google Maps satellite view.

        – The Nogent-sur-Marne interchange has only one straight crossing, and it is a very long crosswalk across two different movements, with no intervening safety island. The other crossings are all at odd angles, not straight lines. And this interchange does not provide a crossing from the northwest quadrant to the northeast quadrant without crossing the bridge twice.

        – The Versailles interchange is even worse. It has only one pedestrian movement provided for, on the east side of the bridge to cross the freeway. The sidewalk is about two feet wide, and all crosswalks but one are at angles to the sidewalk. No pedestrian provisions at all are made for the northwest quadrant or the southwest quadrant, including crossing the minor street.

        Your drawing of how to make the pedestrian paths straight puts extra turns into the automobile paths, making it impossible for semitrailer trucks to use the interchange. It is pure idiocy to expect a huge compromise in the paths of automobiles to provide straight paths for pedestrians.

        Some of your premises are false too. If all traffic on the interchange (including right turns) is under signal control (with no turns on red), then the pedestrian needs not fear either a crossing oblique to traffic or a long crosswalk. And the path provided is the shortest path possible without waling diagonally across traffic lanes.

        • Alex says:

          Check out slide 6 in the presentation Transiteer linked to in her comment above. I was surprised to see it too, having already checked it out in google maps, but of course their imagery isn’t always current.

          And of course it still has flaws, and it certainly isn’t perfect as I’ve defined perfection. But it takes the pedestrian into account in a way that basically no American DD does. Even when you recognize all modes of transportation you can’t always design things perfectly for all of them; instead you do the best you can knowing you will have to compromise. That’s why I specifically highlighted that zig zag leg of my redesign, and acknowledged that it would probably have to be an oblique crossing in order for it to be usable for vehicles.

          Many diverging diamonds technically accommodate pedestrians (in that they don’t specifically ban them), but Versailles is the only one I’ve seen compromise vehicular convenience for pedestrian convenience or safety. Making people wait behind a truck as it makes a sharp right turn will not cause the world to end, but if it’s done to make it a better pedestrian environment, it may encourage more people to walk (whether they have any place to walk to is a different issue; in the Springfield DDs they certainly do).

          Can I ask you – do you ever walk for transportation?

  3. […] concept, it is with the way it has been executed, without exception in the USA, in an entirely and unnecessarily pedestrian-hostile fashion. Further, I have to conclude that this hostility arises from a fundamental misunderstanding […]

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