For whatever reason, I came about a map for the planned expansion of the Houston Metro light rail system (seen immediately below). What struck me was the geometric and emergent nature of the system in the sense of what the individual lines come together to create something altogether different. A line is but a single line, but multiple, interconnected lines become a network. And therein lies the value.
Viewed perhaps in abstract, when you take four lines linking housing to job centers you can form a pinwheel effect with a "loop" at the center. However, the loop isn't an actual loop, but a figurative loop created by four segments. In actuality, it becomes a micro-grid. If these were four streets, you would in effect create one loop. Or as we know it in common parlance, a block.
In practice this is a much more efficient system than if an actual loop was created with lines radiating from the inner-loop, as was initially proposed (and turned down by the feds) for the downtown Dallas streetcar system. When it was proposed, I argued against the inner loop as a tourist only system that didn't serve regular riders and because it didn't serve as housing to jobs link, it wouldn't be looked upon favorably by the TIGER grant (which ended up giving the grant to the Oak Cliff streetcar).
What is important in practice is that in a grid system (or micro-grid where only one "loop" or block is created), it allows people to get anywhere in the city (that is linked to the network) with no more than 1 transfer. However, if the streetcar loop had been funded and built in Dallas, you could be looking at many more transfers due to the inefficiency in the system (an inefficiency instilled by attempting to appease tourists and tourists alone).
For example, in a loop, hub and spoke, if you were on Lower Greenville and you wanted to get to Bishop Arts, you would take one streetcar to downtown, then transfer to the loop, then ride the loop around, then transfer to the Oak Cliff line (with delays at each transfer). In other words a terrible system. Instead, in a grid system created by segments to get anywhere beyond a singular line, you can transfer from an east-west line to a north-south line and generally have far greater "reach" within one transfer. In our example above, that would mean riding a line from Lowest Greenville to the West End (call it a hypothetical Ross line), catch a transfer to a "Market Street" Line which would run from Victory through West End across the Trinity to Oak Cliff.
There is a critical lesson that can be learned from Houston's metro plan with regards to capacity. And that is using a grid system to create the extra capacity for you where extra capacity is necessary, at hubs, or the core of a system, or in the cases referenced above, downtowns.
The opposite of creating capacity through a grid network is creating increased corridor capacity. We see this issue with DART's system. Because D2 has not yet been funded nor built, we have a backlog in downtown on the only DART corridor as new lines came online. The ripple effect was increased headways (time/distance between trains on single lines) further up each of the individual lines so that they could be spaced out enough when they all hit the single available corridor.
Absent D2, which in effect would create some measure of the grid system where it is necessary and effective, what are the other options? More corridor capacity. However, that isn't feasible nor desirable. To create more corridor capacity would mean to either build above, below, or create more parallel tracks where multiple trains come together (like at a typical european train station). It makes sense at station areas in European cities because the capacity has to be centralized as these are the ends of routes that stop unload, reload, and turn around. Above or below doesn't work because we're talking the same costs that sunk D2 in the first place. Wider doesn't work because then we're eating into private land and knocking down buildings.
However, the real lesson is to translate this concept from transit to cars, roads, and cities. If creating new corridor capacity doesn't work for transit, why do we insist on doing it for roads and cars? Mostly because we're stupid. Not stupid, as in the people are stupid, but rather smart people operating within pre-ordained standards and formulas and rules that don't recognize the impact transportation planning and design has on form and function of cities, much like an insidious corporation can be full of very nice, pleasant, and well meaning, individuals. It is the institution that is corrupt and continues to corrupt.
For example on how the capacity debate works in the real world, I like to point to Vancouver. Above, I'm showing a 1985 plan to add highways to the city of Vancouver which famously has been allergic to allowing freeways into their city since that became en vogue. And rightly so as history has proven them to be right in their steadfastness. This plan was rejected as "congestion relief" measure as was the initial 1960s era plan which would've criss-crossed the downtown peninsula with several freeways.
The reality is that these highways would've usurped all the traffic onto the grid because of the disconnective, dendritic, "funneling" effect that highways inherently have on the system. To create their singularity of corridor efficiency, they have to limit connections to them. By limiting connections to them (to increase speed) they also limit interconnectivity of the overall system. They end up rendering the rest of the grid impotent.
However, as we know, Vancouver never built any of these proposals that would've "cured" it. I guess they remain an institution uncorrupted by stupidity. The result, more importantly, is that their grids remain in tact. And in the case of downtown Vancouver grid, the total capacity of it is actually double what the proposed freeway capacity would've been.
In the quick sketch I drew above, I simulate a corridor entering two types of systems, city and anti-city. The corridor enters from the left into a core network, aka city. As it enters the city, built to create collisions of social and economic exchange, the anti-city system focuses on singular corridor capacity increases, widening and widening, thus disrupting the network and diffusing any value therein.
The true city corridor would actually scale down itself to fit within the city, but diffuse its own need for capacity onto the broader system. It is what we call a "reticulated" system or multiply interconnected. The multiple interconnections allow for route choice, and once again, a minimum amount of transfers from one corridor to another.
You can get anywhere in the system with just one turn. However, through the multiplicity of the grid, if there is a disturbance in one location, you can vary your route day to day based on conditions, needs, and desires. This makes for a smart system. User adaptability and choice. It is far smarter than the smartest of arterial and highway networks which rely on data and sequencing and everything else. Because there is a funneling effect to it that disrupts a smarter system (and city) from emerging.