Arterials, Capillaries, Fractals and FClass
Connectivity and the functional classification question
I moderated a session last week on Functional Classification, which was an absolute blast. ITE is wrestling with the question of what FClass does for us and what it can or should look like when you’re thinking about more than just throughput. During the session, I had a curious thought:
How do veins get oxygen?
It certainly isn’t from what they’re carrying—the oxygen has already been used up. For that matter, how do blood vessels of any size get oxygen? The interior surface is exposed to red blood cells, but transfer takes time—at least 1/4 of a second of contact. The blood is going by too fast for that type of transfer to happen anywhere other than the capillaries. The walls of the vessels are too thick for the transfer too. The walls in the capillaries are quite thin, allowing the oxygen to flow through them (where they can use it too) and into the surrounding tissue. The walls on the other blood vessels are pressure vessels. They can’t be that thin if they’re going to support the pressure without breaking.
So, today, I’m doing a deep dive into human anatomy to see what applies to our transportation system—and functional classification.
Primary Circulation: Blood supply
Turns out that all blood vessels have their own capillary blood supply called the vasa vasorum. All vessels have some of this, but the larger ones get their own separate blood vessels that run parallel to it—kinda like service roads. They dump right back into the vessel itself, like a right out driveway.
Every scale of the body gets blood supply—like getting traffic to the place it needs to go. The scale of the vessels is linked to the access management that is needed. Through traffic gets large vessels. Destination traffic gets capillaries. Even the blood supply to the vessels themselves don’t get this mixed up.
So how does this relate to traffic?
The only way that money can flow from the people in the cars into the businesses along the side of the road is for the people to get out of the cars—or at least slow down long enough for the transfer of money and goods to occur. (Yes, this explains drive-through’s).
If you look at most organs, they typically have a blood supply that enters via larger vessels and branches off pretty quickly into very small vessels. The heart carries a lot of blood, but the blood it carries is useless to its own tissues. Clog up the blood supply for the heart tissue itself and you have a heart attack—the tissue dies. Block off maintenance access to a transit system or a highway and it will fall apart nearly as quickly. The network steps up and down in scale gradually depending on the size it’s feeding. There are few high speed off-ramps that directly feed specific tissues. It’s easy to idealize this as a dendritic system, but that’s not how the body works. Most vessels have redundancies to manage breakdowns. Your body will even make new vascular connections if needed—like detour routes.
The Vascular Bones of the City
Urban designers will often talk about roads as the “bones” of the city. That felt off to me as a metaphor. The truth is that the urban cores that they usually cared about function as the brain for the city. The brain has a bony structure that defines the space, like a wall around an ancient city, but inside that locked box, the tissue floats in cerebrospinal fluid. The internal structure is held in the right positions by the blood pressure in the vessels. The transportation systems in the brain work kinda like bones, but that’s about the only place in the body it works that way.
In all the other tissues, the organs are hung on or wrap around the bones. Each tissue within every organ all has a defined shape with its own circulatory mesh. In cities, different functions tend to cluster together based on the land forms themselves, but within each cluster there is a lot of freedom to connect. There also has to be higher speed access to other clusters, even if that’s only through a transportation hub, like the heart.
The segregation within the system works well because the size of each cluster is fixed relative to the system size. If anything grows too large, an internal differentiation happens again. A shopping district that grows too large begins to specialize internally—areas break off into specialties like the garment district, the publishing sector, produce distribution centers, and such. Still, there are limits on how much of one type of land use an area can support. Orlando is a prime example of what happens when there are too many malls built. They all die or have to reinvent themselves—cancer.
All life has scale limits before it fractures into fractals
I’ve talked about Marchetti’s constant before. Cities grow to roughly the size of a 35 minute commute using its fastest functional mode. Think of this as the distance to your toes. When Marchetti came up with the concept, he was thinking of the balance between adventure or acquisition and security.
Any mom will tell you that’s really not the limit. Once you get somewhere, it still takes time to do the stuff. There’s only so much margin in a day that you can allocate to travel and still do life. Travel time can be a useless, disconnected time. Life may be all about the journey, but the quality of the journey matters. When transportation does double or triple duty—getting us out, connecting to others, getting exercise—the feedback relationships are amazing. When it doesn’t, we wonder what is missing but can’t quite put our finger on the lack.
I think the place that we went wrong is that we forgot that human beings still have other limits, even if our transportation systems don’t. One key limit is that even extraverts can’t meaningfully relate to more than a few hundred people in a day. We start weeding them out of our minds as things instead of the eternal sacred beings they are.
We exchange emotional currency via the incidental contact with those in our extended social networks. We receive and give—sometimes good, sometimes bad, but never completely ignored, for that is a fate worse than death, and will ultimately have health consequences. When we have a bunch of non-transactional human interactions, it leaves us feeling like being stranded in the ocean dying of thirst: “Water, water, everywhere and not a drop to drink…” Lonely in a crowd.
In theory, people who regularly commute often see the same people and can build some acquaintance-level relationships that way, but it doesn’t seem to work that way. The anonymity of regular long distance, high person-volume transit can damage your soul because you begin to tune people out. Ever notice how a rural person interacts in public transit? They’re not overwhelmed yet. An occasional foray is exciting. A daily grind can be dehumanizing.
Before you suburbanites and townies break your arm patting yourself on the back, the car makes this a thousand times worse. The people in the other cars are literally and thoroughly de-humanized: all you see is the box. It doesn’t even look like a person. We talk about what the car does, not what the driver does. All the time you spend in the car is time you are incapable of those incidental connections. You’re not even sitting in an ocean. You’re in the middle of the Sahara.
The Fractal Nature of Life
The patterns we see in our cells and tissues are fractals. The pattern repeats at every scale. Each component has its own minimum and maximum domain. Above and below those scales, they aggregate or disaggregate, creating the same pattern. Cells differentiate into tissues. Tissues differentiate into organ systems. People don’t grow forever. They have children. Ideally, the parents remain in community with those children, but the children are their own distinct system. Children generate families; families create social groups; social groups collect to create communities; community clusters collect to create cities. Cities aggregate to create regional clusters. Each one has its own functional maximum and minimum in order to be self-sustaining with boundaries that delineate what is in or out. Each jump in order of magnitude in size drops an order of magnitude in quantity.
Here’s the catch:
Favoring any scale of the system shatters the whole.
You can’t make a body out of undifferentiated cells. Even Oregon’s Humongous Fungus has stems and roots and spores. Any super-sized organ causes the death of the organism. We call that cancer. Giants struggle for their hearts to keep pace with their mass and die far too early. A friend of mine struggled with a brain disease that created far too many small vessels in her brain—which caused strokes. Larger and smaller animals exist, but they have dramatically different operational systems to deal with the thermal constraints. We need more oraganelles than cells; more cells than tissues, more tissues than organs, more organs than systems, more systems than bodies, more bodies than families, more families than communities, and the list goes on. Destroy the functionality at any of these layers and it rapidly cascades to the other scales. You can’t have 5 kidneys, two hearts, 4 lungs, and 6 stomachs and expect the whole to work. A body dominated by large arteries and veins would starve of oxygen and every other resource—as do our cities.
An Eisenhower-Style Thumb on the Roadway Scale
For the last century, the federal government has had its thumb on the scale, tilting roadway construction toward larger functional classifications—like building a body with far more arteries than smaller blood vessels. Before that, the market dictated what transportation systems were needed and what got built. Now, it’s common for a jurisdiction to upgrade the classification on a roadway so it can be eligible for larger funding sources. For the last 20 years, the federal government has added another thumb to the other side of the scale—trying to support multimodal operations, largely by tacking amenities onto those same larger scale roads. Getting big money out of this is probably the best thing for all of us.
There’s a level of differentiation that we’ve left out. Who is going to fund the smaller vessels? The problem isn’t entirely in the functional classification system. The problem is in the imbalance we built in the parts. We had good reasons to build the Eisenhower system and it has benefitted us tremendously. We forgot how to fund and build the local network—and what we built was poorly connected. We have no problem building capillaries—we have plenty of driveways. What we don’t have are smaller network roads and streets and redundancy in those connections. When capillaries come right off an artery, the pressure doesn’t have time or space to drop before it either backs up the artery or blows out the capillary. The sketchy solution for that problem has been subdivision walls, which creates brand new problems.
Parallel networks
Blood is not the only thing that circulates in the body. We have two other circulatory systems—they lymphatic system and the cerebrospinal fluid (CSF) system. The lymphatic system acts much like the utilities, providing communications (hormones), police (immune functions), fire (inflammation management), and retention ponds, draining excess fluid from tissues.
The second system has only come fully to light recently. We’ve long known that the brain and spinal cord float in CSF and debated whether that fluid has its own rhythmic flow patterns. Anyone who has benefitted from a cranial osteopath recognizes they are onto something there. Turns out that circulatory system is far more extensive than we ever dreamed. Doctors at University of Florida (Go Gators!) have now discovered a tiny, primitive CSF system that travels along the outer layers of every single nerve.
These tiny fluid pathways have long since been ignored, but they also provide nutrients and remove wastes around nerves, much like pedestrian or bike pathways in a transportation system. The blood brain barrier applies here. The two circulatory systems do connect, but not completely. Blood and platelets are never permitted in the CSF circulation system and there are pretty intense gatekeepers. They have the equivalent of bollards that keep toxins and other factors that are in the bloodstream away from the more delicate nerve tissue. The bloodstream does most of the major oxygen circulation, but without the CSF circulation, the nerves die, starved of nutrition and drowning in their own waste. In the brain and spinal cord, this system is the primary circulation tool. Blood is important, but the CSF keeps everything else afloat, fed, and clean. Lose this, and the brain sags into the spinal column—which is excruciating (I’ve experienced it.) Even in the nervous system, redundant hierarchical networks are the rule, although in the brain itself, the pathways are far less defined. Should it be surprising that in cities, pedestrians don’t stick to their defined pathways as cleanly either?
What does this mean for our communities and functional classification?
If we look at our bodies as an ideal model of how transportation and land use can operate, is it any wonder that our society has lost the ability to feel? Without a small scale, pedestrian and bicycle network system to feed the nervous systems in our community, those communication systems die. How can you get a “feel” for a city without interacting with its people in a robust way?
Every system methodically breaks its scale down into hierarchical layers, stepping down gradually, but a strong system should also have ample redundancy. The body gives us some good examples of mode separation, with lower speed, lower volume specialized flows pulling resources from the high speed vessels using substantial protection. These parallel systems may go in similar locations, but the CSF system follows the nerves that it serves, rather than trying to follow every other blood vessel in the system—and they’re especially distinct from the larger blood vessels that would blow them apart. The land uses that provide communication and interaction functions will need these types of low speed flows—and just like nerves, those land uses need to be pretty common.
Takeaways:
Every part of the functional classification system is important. We can’t use our categories to favor one type of roadway funding over another—we’ve done that and it’s not all rosy. We need to backfill with what’s missing. I heard a jurisdiction talking about federal funding for a project this morning and they talked as if the money were free and didn’t come from their local citizens. It might not have come from their local budgets, but all government funding comes from someone’s taxes.
Many of our systems aren’t nearly redundant enough, which overloads the larger vessels and makes connections to the driveways dicey. It’s not just a protection for failure. It’s a pressure issue. Non-redundancy leads to high blood pressure, whether that’s in your body or on the road.
Pedestrians and cyclists need their own protection as they connect in with the important systems in the community. Without those protections, we’re essentially violating the blood brain barrier. That leaves the adjacent land uses open to attack and keeps good stuff (i.e. people with money to spend, neighbors who could interact with you) from getting where they need to be. Just because we didn’t recognize that those segregated systems needed to exist, doesn’t free us from the consequences of not having them.