Too often transportation planning externalizes far too much information. In fact, it always does. Furthermore, the focus is always far too narrow, thinking about singular corridors rather than the network, only cars rather than all forms of transportation, a static land use form and density rather than a dynamic market that reacts to any changes to the transportation network, and rarely does it account for trip length, which is largely a by-product of the previous point.
Transportation is currently reactive rather than proactive when we should be treating it proactively in order to nudge the real estate market to respond favorably to a better city form. Developers will always be profitable. They're just playing by the rules of the game we establish. Build transportation to instill and incentive to sprawl, they'll respond that way because the market seeks the cheaper, freer good. Even though in this case, it is actually quite expensive...all because we have the transportation / land use equation backwards. Helluva price to pay for an entire generation of transportation planners that were trained with no concept of how their work negatively impacts city form and human behavior.
Today (and often), I lament how much easier it is to make much longer trips in DFW than short trips. This is subsidization of bad behavior and puts more cost on both the private sector (longer trips) as well as the public sector (bigger, more, ever new infrastructure) because the real estate market responds to this perceived advantage by stretching out along the hierarchical dendritic network that funnels you to ever bigger roads, eliminating route choice and adaptability by disconnecting the grid...because intersections are friction that slows flow.
Unfortunately for this logic, friction is also social and economic exchange occurring. City, in action. Precisely why this logic is fundamentally anti-urban, anti-social, anti-economic and is failing the world over. It just takes 2 to 3 generations to play out as it takes a generation to build the system, a generation to test the system, and the third generation to say, "hey, wait a minute!"
Here's a critical point. In the arterial/highway network, the hierarchy is imposed by central planners. Therefore, it's more often than not going to fail. On the other hand, through a highly interconnected gridded or grid-like system, hierarchy of place emerges based on accessibility. The market responds to the most attractive, most accessible, most compelling places. It HAS to be attractive and pedestrian friendly.
This is why the great public spaces in the world are at convergence points in the network. Look no further than Campus Martius in Detroit how a convergence point can be redesigned for people rather than flow, a place to go to rather than thru (though you can still go through), to drive exponential value.
There has been no shortage of research about the advantages of reticulated (multiply interconnected) grids vs dendritic hierarchical networks. You can glean a bit of that from these graphics:
Before even getting into the impact on real estate, the instilled demand for more parking, the widening throughout the hierarchy which then makes places pedestrian unfriendly and repulsive where commercial areas need to be attractive, in the physical sense of the world (compelling), this graphic illustrates how each trip is lengthened by the funneling and minimization of route choice.
I have no idea why the above image skipped so much space. Deal.
Transportation planners love to talk about mobility. However, you can see how much more mobile a pedestrian is in the grid network vs. the disconnected, hierarchical system. A 1-mile walk on the loopy doopy streets barely gets you a quarter of a mile. Incredibly inefficient. Similar for a car. And even worse, because of the inherent, imposed hierarchy, all trips must funnel to bigger and bigger roads, which means the traffic will inherently be worse and more congested on the higher ordered road, thus compelling transpo dinosaurs to further widen. And on it goes. Until it dies.
I'm going to take the remainder of this post to compare generic versions of the two systems to show how notions of capacity and mobility from the conventional, 20th century perspective fail. And fail even to their own standards and priorities.
Before we compare the two road networks, a couple of assumptions. First, we're going to approach it statically rather than dynamically. In other words, we won't factor that a more highly interconnected network will generate more accessibility thus more demand thus higher density and greater commercial activity. That's space syntax in action and I've covered that ad nauseum. For this post, we'll approach it in the narrow-sighted, static approach that nothing changes after road widenings, but instead road widenings are a response to what we may conceive as demand. Wrongly, because it is shaped by the network. Tally ho...
First, we're going to assume these two squares are each exactly a square mile. I drew them in CAD at 5280 x 5280. The internal streets have roughly the same linear distance, ie roughly the same amount of total infrastructure. For now, until some of the behavioral dynamics start to compel changes to the system, depending on the logic you use to approach the design.
As for further assumptions, let's use Dallas average population density of 3,645 people per square mile. Let's also use FHWA data suggesting each person makes 3.79 average trips per day. So below, we have two square miles each with 3,645 people living, for a combined total of 7,290. Each square produces 13,814 trips. Our starting point is that every street is two lanes, and the central spine is four lanes.
Moving North-South for the purposes of this analysis you see there are two totally different capacities. First, the gridded side would have 12 total lanes getting you north-south. The less connected, hierarchical system (with less intersections so you can go faster, yay flow! Kids, out of the street! Pedestrians, dive for cover!) only has 6 lanes. So more of the traffic has to funnel over to the central spine.
However, no system exists in such a bubble. We're dealing with 7,000 people total which is enough for a few neighborhood services like a small convenience store and a dry cleaners. It can't support a grocery and all of the other daily needs we have, so we have to expand the system. Let's say we extrude these two blocks into eight:
On the left, we remain connected. On the right, because the logic is one of isolation and minimized "friction"/maximized flow we even further disconnect. So the middle north-south road is no longer connected between neighborhoods. All traffic must funnel to the central spine road. Let's see what that looks like.
If we're just adding up the right side of the spine road, we now have 14,580 people. Enough to start supporting far more commercial and recreational amenities, particularly if we add the two sides for 29,000 people. From just the right side, that's 55,200 trips that MUST funnel to the spine road. That means the formulas and standards will suggest we need to widen the east-west connectors to 4 to 6 lanes and the spine road to 8 lanes plus however many protected left turns. The intersections get huge and we have to condemn all sorts of private property all because of the terrible road network (another disinvestment and displacement dynamic that we're not focused on here).
Total capacity is only 8 lanes and it is always congested with primarily only cars, travel speeds are likely much higher due to widened roads, reduced intersection density, and in all likelihood density/tax base diminishes and thus the area can no longer support it's own infrastructure.
On the flipside, because the entire left side remains connected north south, we still have 12 lanes linking north-south, more capacity than the 8 lane road has because we have multiple streets.
Also, similar to the analyses at the beginning, if you pick any random point on one side, mirror it on the other, the distance to get to the commercial spine would always be much longer, less convenient, and less adaptable on the right than on the more interconnected left. Thus, there will likely be more pedestrian activity on the left.
Remember, the daily 3.79 trips doesn't necessarily mean vehicular trips. That's how many times we're leaving the house. If you don't need or want a car to make a specific trip, and you don't have to, that won't count against the vehicular capacity of the road network.
That allows us to reduce the 4 lane spine road to 2 lanes with more sidewalk space for the increased pedestrian activity, increased space for programming and activities like cafes, public art, bike lanes, etc etc. All of which is even higher capacity because people obviously take up far less space than does a car. Speaking of, since these trips can be made conveniently by foot or bike, that's less necessary parking space which maintains higher quality of space, less cost on the developer/business to provide the parking, and less cost on the consumer/residents since those costs will get past through to them.
Transportation planners from the modernist 20th century like to city "their training" says this or their training dictates that. What if their training was wrong all along? Should they be making the most important decisions that dictate urban form and market behavior? That is precisely why the require increased oversight and communities and their elected officials must stand up to them in order to achieve the goals of better neighborhoods, better communities, and a better city.