December 16, 2023: Pedestrian Skyways

Good evening. Today we will look at pedestrian skyways, or skybridges, how they can make transportation in a high-value downtown area more efficient, and some of the issues surrounding them.

Drone Delivery Redux

Before we get to the main topic, I have to note that the energy figures I calculated in last week’s review of drone delivery were in error. In particular, I mistakenly used the energy per kilometer of flight rather than the energy per parcel delivery figures of Rodrigues et al. Had I used the correct figures, and otherwise done all the calculations the same, the results would be slightly less than half of what I reported. In other words, world energy savings potential looks more like 700-800 petajoules rather than 1.6 exajoules.

On the same subject, a reader with work experience in the commercial drone industry suggested that I glossed over the treatment of safety. He reports that the crash rates of drones due to mechanical failures, as well as the inability to fly during bad weather conditions, are problems that make drones too unreliable to be used for parcel delivery in a widespread manner, and that these problems cannot be solved without a significant breakthrough. I have not made any additional attempt to evaluate these claims, so I pass them along without further comment.

Pedestrian Skyways

The idea behind pedestrian skyways (or skybridges; I will use the terms interchangeably) is simple. In a downtown area, real estate is constrained by transportation costs, since a downtown needs to function with easy and fast transportation between all points. Therefore, an innovation that streamlines transportation will enhance a downtown’s potential. I talked about drone delivery, another such idea, last week. Skyways are another such concept.

Skyways, both for pedestrians and otherwise, are one of those ideas that have been part of urbanist thought, and urbanist futurism, for a long time and may now appear dated. Fritz Lang’s 1927 film Metropolis, for instance, features elevated monorail connections between skyscrapers. The movie has been described as a dystopian depiction of the conflict between managers and workers, setting a negative connotation that has persisted for a century.

In real life, skyways owe much to Victor Gruen, who led in the development of Minneapolis’ skyway system. Gruen is best known for pioneering the design of enclosed shopping malls in the United States, including the Southdale Center in Edina, Minnesota, the first such mall in the country. By the end of his life, though, Gruen had denounced shopping malls as a bastardization of his vision of pedestrian-oriented cities.

For a much more detailed treatment of the history behind pedestrian skyways, I highly recommend this article by Yoos and James. They go into great detail about the origins of the concept, about Victor Gruen’s thinking in particular, and various implementations around the world.

This paper examines some more implementations of skyways and related concepts. Related is The Continuous Monument, a world-spanning megastructure that was meant as a demonstratio per absurdum of the modernist impulses in architecture. New York City’s High Line is a combination of pedestrian transportation system and urban park and a very pleasant place to be in my opinion. The paper also considers specifically the idea of constructing a skyway network within and between urban superblocks, particularly the Champs-Élysées superblock in Paris.

I discussed skyways a few months ago in the context of arcologies, as a network of enclosed passageways between buildings can, in effect, turn a cluster of downtown buildings into a single megastructure. Some notable skyway examples include Minneapolis’ aforementioned system, the +15 network in Calgary, the network in Hong Kong, the Mumbai Skywalk Project, and the Dallas Pedestrian Network (a series of tunnels rather than skybridges, but otherwise a similar concept), among many others.

This could go on for a long time, but I’ll mention just two more projects with which I have some personal experience. The first is the network in Spokane, Washington, which spans 16 blocks. It is a small system compared to the systems listed above, but Spokane is a smaller city. I found it to be a very fun system to explore, but once we get past the coolness factor, I’ll admit that I found its utility limited. The network’s small size played a role; once I was in, I didn’t find that there were many places I wanted to go. The non-uniform hours of operation were a problem too; sometimes I found myself getting stuck and forced to backtrack become some critical passageways would be closed.

I was favorably impressed with Singapore’s pedestrian underpass system, which for reasons that are unclear to me does not get as much attention as some other systems. These underpasses are convenient ways around busy roads and a welcome escape to cooler temperatures amidst the hot and humid climate. Often the junctures at these underpasses would open into large food courts or shopping areas. Singapore does many things well from an urban planning standpoint, and the underpass system is one of them.

All right, now let’s get into the economic aspects. I’ll keep this as simple as I can. The construction costs of a pedestrian skybridge have been estimated at $150-250 per square foot in 2016, which is about $190-320 when CPI-adjusted to 2023. The cost of a pedestrian underpass has been estimated at $120/sqft, or about $150 when CPI-adjusted to present. I don’t have any good estimates of the maintenance costs for these facilities, but a general rule of thumb, when lacking data, is that an annual maintenance cost of 4% of construction costs is reasonable. Another suggestion is that a 3.5% discount rate is good for public sector construction projects, which seems a bit low to me, but OK. All that said, the net present cost (2023 dollars) of a pedestrian underpass would be about $330/sqft, while for a skybridge, $410-680/sqft. This includes maintenance, as discussed, but not operations costs such as air conditioning, security, etc.

Let’s compare these figures to my city of Portland, Oregon. In 2022, the average price for real estate sales was $225/sqft. This is a big hit from the pre-COVID days, during which prices peaked at $562/sqft. The average for the last five years was $323/sqft. If the value of a skybridge per square foot is the same as the value of commercial real estate in general, then on this cost comparison alone, it would be hard to justify a skybridge in any but the most expensive parts of town. And indeed, we see few skybridges. Some of them the the OHSU (Oregon Health & Science University) bridge, which connects the OHSU hospital with the ritzy West Hills neighborhood; the skybridge terrace at the World Trade Center; and a new skybridge at Terwilliger Plaza, a senior apartment home building, to allow safe passage and nice views.

Now compare this to Hong Kong and Singapore, which in 2020 had, respectively, average prices of $1987/sqft and $1214/sqft (US Dollars). The economic case for a skybridge network in Hong Kong and a tunnel network in Singapore is much clearer.

Beyond the financial aspects, skybridges are controversial. Here’s a negative treatment from Strong Towns. Here’s one from Streets.mn. And here’s one from the Architect’s Newspaper. And one from Greater Greater Washington. Governing has a more even-handed take but also highlights some of the problems. Most of these critiques come from urbanist-leaning sources, which seems strange at first glance; why would urbanists be the ones to find fault with an innovation to help pedestrians?

These articles raise several issues, but their biggest complaint that by bifurcating pedestrian traffic, with one segment (primarily downtown office workers) directed into largely private spaces, a skybridge network cuts into the level of foot traffic at grade-level retail and thus hurts the vibrancy of downtown. This concern is even raised in a Minneapolis master plan. I don’t know if there is much evidence behind this assertion, though.

Another complaint, such as in the Greater Greater Washington piece, is that skybridges are meant to facilitate easy travel for pedestrians and automobiles alike by separating the two. But if you believe, as many urbanists do, that car usage is a destructive factor in cities, then any tool that aids the flow of traffic is harmful.

The whole thing illustrates two pathologies that are common in the urbanist movement. First, that the movement is rooted too heavily in aesthetics and lifestyle politics, and not at all in the first principles of city functioning. Second, the movement has really gotten hung up on this “war on cars” thing. It’s almost as though making life difficult for motorists is the end goal.

Anyway, for all the debates we can have, and as much as I like the idea of traversing a skybridge network, it does not seem likely to me that the idea will expand much farther than a few expensive megacities, for which there is a strong economic case, and niche applications in other cities. Still, for those megacities where there is a good economic case, I find it puzzling how little the concept has expanded. Where is the Manhattan Skyway Network?