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.