Tag Archives: geometry

I can’t believe I’m writing a post on Personal Rapid Transit!

Morgantown WV PRT System, as seen from Google Streetview

Morgantown WV PRT System, as seen from Google Streetview

Reading through the history of the personal rapid transit (PRT) on the Verge by Adi Robertson, I couldn’t help but think of the similarities with many familiar projects. Cost overruns, scope creep, politics, government red tape, all conspiring to erode the value of an otherwise promising concept.

First, you can’t write about PRT without acknowledging the inherent geometric flaw of the concept: it can’t scale. Jarrett Walker frequently talks about the fundamental geometry of transit, and succinctly explains the geometric flaw of PRT:

Bottom line:  When “personal rapid transit” succeeds, it succeeds by turning into a conventional fixed route transit system.  The fantasy of “personal” transit is that a vehicle will be there just for our party and take us directly to our destination, but in constrained infrastructure this only works if demand is low.  But PRT was meant to the the primary transport system in a car-free city, so demand would be high.  It was never going to work.

This is also true of the Morgantown, WV PRT system, which makes use of different operating modes. During times of high demand, it operates as a fixed route transit system between the busiest stations; during low demand periods, cars stop at every station, regardless of demand.

Mass transit might be an out of fashion descriptor, but it helps illustrate transit’s scalability. Good transit doesn’t just move large masses of people, it requires mass to succeed. ‘Personal’ transit rejects the masses; it also requires expensive infrastructure to inefficiently move people.

Robertson skirts around the geometric limitations of PRT as a concept, but never appropriately douses the concept with cold water. Any history of PRT must focus on the Morgantown, WV system. Any article about PRT will inevitably draw comparisons to current research on driverless cars. Comparing the two exposes the conceptual flaw:

Self-driving vehicles, he points out, wouldn’t have taken cars off Morgantown’s crowded roads — at least, not in the same volume. As long as they’re intermingled with human-driven cars, they can’t run with the same centralized efficiency. And once you start thinking about the obvious solution — a dedicated lane for self-driving cars — you might start running into the same problems as PRT.

Leaving aside PRT’s conceptual flaws, Robertson’s history of the concept echoes common challenges in the American history of infrastructure projects: shifting government mandates, political interference, procurement regulations, and so on. Some highlights:

Goals for transit: Robertson documents the history of federal funding for PRT, with the Urban Mass Transit Administration providing research grants to explore the concept.

The focus on new technology in transit often meant unnecessarily reinventing the wheel (see BART’s broad gauge track), but also exploring new concepts like PRT. New concepts are sexy, even attracting the direct interests of President Nixon:

His mantra, as Alden puts it, was that if “Kennedy can get a man on the Moon, I can get a man across Manhattan.”

Lack of clarity about the UMTA’s goals for the program help add to the confusion. Is the goal to provide effective transit, or to prove a new technology/concept? Crosstown transit is a practical goal, but it doesn’t require big technological innovations. Landing on the Moon is an impractical goal that wasn’t possible without new technologies – and the moonshot analogy makes it easy to conflate two different goals.

From the start, there’s tension between researching new technologies and practical, proven, cost-effective projects. Many PRT boosters in West Virginia were approaching this a big experiment; the government bureaucrats wanted a functioning system. Once the system proved more conventional than revolutionary, Robertson notes, “the age of experimentation was over.”

Politics: Robertson also shows the competing interests of the various parties involved in funding and executing the Morgantown project. West Virginia University approached PRT as an experiment, while UMTA wanted a more practical proof of concept – something that could be built elsewhere if successful. On top of these turf battles, President Nixon wanted a completed project to include in his re-election campaign materials, pressuring the team to complete things before they were ready.

Procurement and red tape: As WVU championed the PRT project, they looked for federal funds to offset the cost. Then, as now, those dollars had strings attached. UMTA required a NASA JPL redesign of the vehicles; one of the independent engineers took patents to established defense contractor Boeing in order to better compete in project bidding.

Right of way: The single most important element of the Morgantown PRT system is the elevated guideway. Complete grade separation from the traffic at street level and the interference from cars, bikes, and pedestrians not only speeds travel, but made PRT’s automated operation possible (note: this remains true, it should be far easier to automate a subway system than to create a fleet of driverless cars).

Despite the inherent geometric challenges of personal transit as a service, the system nevertheless demonstrated the value of guideways; and also the reasons why we don’t have more of them: local opposition and cost. One PRT booster:

To Kornhauser, the issue is less that the technology was inherently inadequate than that it was expensive and inconvenient. “You didn’t need that much intelligence in the vehicle to be able to do all this stuff,” he says. “The problem was that nobody really wanted to invest the money to build the exclusive guideway. That’s the short and the long of it.”

And Robertson on the local opposition to erecting concrete guideways all over the city:

Even the most time-tested (and desperately needed) public transit systems have trouble securing space and laying track; New York City’s history is littered with unbuilt subway lines that were killed by local protests and a lack of money. PRT guideways had some advantages over trains, like their near-silence, but they would still require cities to build miles of concrete chutes. And unlike a subway line extension, there would be no guarantee that people would accept the new system. Or, as one former transportation commissioner told NPR when asked about personal rapid transit last year: “The last thing you want to do is put up some track all over the place and have it just there.”

Also, unlike a more traditional elevated line (something I’ve defended here previously), the ideal of PRT means offering door to door transit, which in turn requires a guideway of some kind from door to door.

More on the geometry of transportation: “Transport is mostly a real estate problem”

In June, the Urbanization Project at NYU’s Stern Center posted several graphics looking at the space devoted to transportation in our cities. As the author, Alain Bertaud, frames it, “transport is mostly a real estate problem.” That is, different transportation modes require different amounts of space to accomplish the same task.

Comparison of population/employee density and street area per person. Image from NYU Urbanization Project.

Comparison of population/employee density and street area per person. Image from NYU Urbanization Project.

Each of the selected examples cluster around the diagonal blue line, representing an average of 25% of a city’s land devoted to streets.

Percent of land use devoted to buildings, streets, etc. Image from NYU Urbanization Project.

Percent of land use devoted to buildings, streets, etc. Image from NYU Urbanization Project.

Two observations: the 25% pattern is remarkably consistent; as is the geometric relationship between modes of transport and the intensity of land use.  The green horizontal lines show how much space a car uses at various speeds – the faster the car goes, the more space it requires. A parked car occupies 14 square meters, while one moving at 30 kph takes up 65 square meters.

The obvious corellation is between a city’s density and its type of transportation network. Cars take up a large amount of space relative to their capacity, and a transport system based on cars alone cannot support a great deal of density.

Alex Tabarrok frames this in terms of “the opportunity cost of streets.” While there is certianly an opportunity cost to various street uses, it’s worth noting that some space must be devoted for streets in order to access property. Charlie Gardner at Old Urbanist takes note that the role of streets is not solely about transportation:

In addition to their transportation function, streets can also be understood as a means of extracting value from underserved parcels of land.  The street removes a certain amount of property from tax rolls in exchange for plugging the adjacent land in to the citywide transportation network.  Access to the network, in turn, increases the value of the land for almost all uses.  For the process to satisfy a cost/benefit analysis, the value added should exceed that lost to the area of the streets plus the cost of maintenance. (This implies rapidly diminishing returns for increasingly wide streets, and helps explain why, in the absence of mandated minimum widths, most streets are made to be fairly narrow.)  For many of the gridded American cities of the 19th century, as I’ve written about before, planners failed to meet these objectives, although these decisions have long since been overshadowed by those of their 20th century successors.

Charlie also notes that many great, dense, walkable cities around the world devote about 25% of their land to streets, yet many American downtowns use a much higher percentage of their land to streets.

Some of those numbers might depend on the exact method of accounting. While Charlie’s estimate for downtown DC shows 43% of the land used for streets, DC’s comprehensive plan shows approximately 26% for the city as a whole:

Land Use Distribution in DC, from DC's 2006 Comprehensive Plan.

Land Use Distribution in DC, from DC’s 2006 Comprehensive Plan.

The graphic doesn’t specify if the street figure refers to street right of way, or just the carriageway portion of the street, but not the ‘parking area.‘ Seattle’s planning documents also showa similar pattern: 26% of land city-wide used for streets, but also a higher percentage of downtown land devoted to streets.

Seattle land use distribution by neighborhood. Image from Seattle's 2005 Comprehensive Plan.

Seattle land use distribution by neighborhood. Image from Seattle’s 2005 Comprehensive Plan.

The Seattle calculation looks at land devoted to right of way for streets, rather than just impervious surface.

Making better or different use of existing right of way is one thing; however, once that right of way is set, it is very difficult to change. Transportation networks awfully path dependent. Chris Bradford looks at Austin’s post-war planning and the abandonment of the street grid – path dependence in action:

Back then, “planning” chiefly meant “planning streets.” It’s a shame that planning lost that focus. The street grid that permeated Austin in 1940  is of course still with us, and forms the backbone for a number of quite livable neighborhoods.

So what happened? Developers building large, planned subdivisions (Allandale, Barton Hills) continued to add decent street networks after 1940. But the City itself appears to have gotten out of the grid-planning business not long after this map was made…

Collectively, these could and should have been platted into 40 or so city blocks. Instead, they remain two big blobs of land. The lack of connectivity funnels traffic onto South Lamar and Manchaca; impedes east-west mobility, dividing eastern and western neighborhoods; forces people to make circuitous trips to run even simple errands; and forecloses any sort of low-intensity, mixed-use development in the area. Then there’s the sheer loss of public space: South Austin should have a few more miles more of public, connected streets than it has today.

Once the street grid is set, it is very difficult to change.

Cities and the constructal law

CC image from Other Think

Several months ago, I picked up a copy of Design in Nature as an impulse buy at the bookstore. I was purchasing a gift and the cover caught my eye. A quick perusal of the jacket and a few pages of the introduction was enough for me to fork over the cash.  I didn’t get around to reading it until I had several airline flights this summer (with the accompanying missed connections) to dig into the book.

The basic premise of the book is that the similarities we see in nature (why trees and lightning bolts and river deltas share the same branch-like architecture) isn’t a coincidence, and it certainly isn’t the result of divine inspiration.  Rather, these similarities are explained via thermodynamics and the ‘constructal law‘ as coined by the author, Adrian Bejan.  The law states: “For a finite-size system to persist in time (to live), it must evolve in such a way that it provides easier access to the imposed currents that flow through it.”

In short, the flow systems and the laws of physics that govern them influence those similarities in design. From the wiki summary:

The constructal law represents three steps toward making “design in nature” a concept and law-based domain in science:

  1. Life is flow: all flow systems are live systems, the animate and the inanimate.
  2. Design generation and evolution is a phenomenon of physics.
  3. Designs have the universal tendency to evolve in a certain direction in time.[2]

The constructal law is a first principle of physics that accounts for all design and evolution in nature. It holds that shape and structure arises to facilitate flow. The designs that arise spontaneously in nature reflect this tendency: they allow entities to flow more easily – to measurably move more current farther and faster for less unit of useful energy consumed.[3] Rain drops, for example, coalesce and move together, generating rivulets, streams and the mighty river basins of the world because this design allows them to move more easily. The constructal law asks the question: Why does this design arise at all? Why can’t the water just seep through the ground? The constructal law provides this answer: Because the water flows better with design. The constructal law covers the tendency of nature to generate designs to facilitate flow.

Reading the book, I thought back to previous examples of similar observations of cities:

  • The similarities of subway networks across multiple cities (linked previously here)
  • The work of Geoffrey West on a universal theory of cities (also here), economies of scale and the benefits of agglomeration
  • Jarrett Walker’s analogies of transit systems as rivers (both here and here), particularly with the usefulness of drawing out key principles (e.g. ‘branching divides frequency’).
  • Any number of urban economic studies of agglomeration, innovation, and human capital – studying the flows of information in cities (examples here, here, and here, among many others)

Jarrett Walker’s recent Email of the Month post sparked me to write this.  Walker’s emailer, Kenny Easwaran, notes:

At the time, I was thinking of the various transportation systems we know of that aren’t designed by humans.  The main examples I could think of were things within the human body, and I noticed that things like the circulatory systems of animals and plants, and the digestive system of animals, seem to follow somewhat different trajectories from grids.  In particular, they either have a branching tree structure, or something more like an extended linear structure.

Having recently finished Adrian Bejan’s book on constructal theory, the analogy to tree-like systems immediately caught my eye. For me, Bejan’s description of all of these phenomena as flow systems ruled by common principles of physics helps shape my thinking, even if it is a bit vague.  Walker’s analogies of transit networks to rivers is a similar case.

 

Driverless cars don’t change geometry

Via the Streetsblog Network, I came across this Salon piece from Michael Lind praising our future driverless car overlords.  Angie Schmidt at Streetsblog did a nice job to take down some of Lind’s loaded language, particularly the bits about “rigging markets” (which rings just as hollow as the cries about “social engineering” – as Timothy Lee notes, there’s no such thing as an intervention-free infrastructure policy).

Those issues aside, the biggest thing that Lind misses isn’t about technology at all – but rather about geometry, land use, and the relationship between transportation and the built environment. Lind writes:

As the white windmills fade from the picture of the future, so do the bullet trains speeding past them.  Even before the end of President Obama’s first four years, unrealistic fantasies about high-speed passenger rail had collapsed.  Federal funding for high-speed rail demonstration projects has been minuscule and symbolic.  State and local governments continue to conclude that the costs of high-speed passenger rail outweigh the alleged benefits.

In the longer run, robocars may be fatal for fixed-rail transportation, at least for passengers rather than freight.  Google has been test driving self-driving cars in California and Nevada has become the first state to legalize driverless vehicles.  No doubt it will take several decades for safety issues and legal arrangements to be worked out.  But high-speed trains might find competition in high-speed convoys of robot cars on smart highways, allowed higher speeds once human error has been eliminated.  And the price advantage of subway tickets over taxi fares in cities may vanish, when the taxis drive themselves.  Point-to-point travel, within cities or between them, is inherently more convenient than train or subway journeys which require changing modes of transit in the course of a journey.  Thanks to robocars, much cheaper point-to-point travel everywhere may eventually be cheap enough to relegate light rail and inter-city rail to the museum, along with the horse-drawn omnibus and the trans-atlantic blimp.

Paraphrasing Jarrett Walker (aside: his recently published book is an excellent read), technology does not change geometry.  A driverless car is still a car, the geometry that governs the car is the same regardless of who (or what) is at the controls.  Despite predictions about how this technology could change everything (see a whole series of GGW posts), I find the possibility for change to be marginal.  Driverless Johnny Cabs, Total Recall-style might decrease the cost of providing taxi service, but that won’t fundamentally change the inherent capacity limitations of taxis compared against a subway system.

The choice of the taxi as a demonstration for the technology is interesting. Most taxis operate in big cities, and big cities tend to be dense.  Density helps support high levels of transit service and ensures that lots of potential trip destinations are easily reached by foot or by transit, thereby diminishing the market for these automated taxis.  Cars, regardless of who’s driving, don’t have an advantage in point to point travel over pedestrians, transit, or other modes in cities.

The other point Lind makes is in investment priorities for government-funded infrastructure (hence the earlier comment about “rigging markets”).  Lind seems to view the built environment as static, rather than an evolving system that changes in concordance with the changes to the transportation infrastructure.  New York’s subways fueled its dense development, and that density in turn provides the market for high capacity rapid transit.  Given growing populations and constantly changing cityscapes, these infrastructure investments in transit are step along the process of letting out cities continue to grow.

(semi-related sidebar on growth patterns: check out this article in Scientific American on the patterns of growth among subway networks around the world.  The authors concluded ” that the geometries of large subway networks are guided by simple, universal rules.” – reminiscent of Geoffrey West et al)