Photo by Ken Lund on Flickr.

Washington, DC is a lucky city. Its downtown has been filled up with new construction over the past few decades to such an extent that it has virtually no space for new office buildings.

Some, like Matt Yglesias, have suggested that one way to resolve this problem would be to increase densities by ridding the city of its height limit, which in essence makes it impossible to build structures in the city that are over about 10 stories.

Lydia DePillis has argued that the municipality still has plenty of developable sites which, though they may not be directly downtown, still offer opportunities for more office space.

What would be the manifestations of these different approaches? How can we weigh the advantages and disadvantages of upzoning the center city for more office space? Is our goal to produce vital, walkable, and dense downtown districts, or simply to expand new construction there, no matter the use?

The missing ingredient in this discussion is transportation. When we discuss the demand in downtowns like Washington’s for more office space, we sometimes make an assumption that the transport network will be able to handle whatever is thrown at it.

In fact, there is a direct relationship between a downtown’s growth and the transportation provided to it. In general, businesses want to locate their offices in places that are accessible and that provide the benefits of agglomeration, and this sometimes means downtown, but not always.

If the trip to and from the center — by whatever mode — becomes too arduous, there are significant reasons to locate outside of it. How does this fact apply to a place like Washington?

Once a downtown — which I will define as a traditional single-use American CBD — reaches a certain size, once it provides employment for a certain number of people, it has three basic options:

One, it can do nothing to its transportation network, in which case the downtown has no capacity to absorb increasing growth. In these cases, residential uses become more important since the relative land values demanded for office space decrease (as it is harder for more people to enter into the downtown from elsewhere and there is more interest in walking to and from work).

This is arguably what has happened to places like Chicago’s West and South Loop, where almost all recent development there has been in the form of residential towers despite the close proximity to the downtown core.

Two, it can expand or improve transportation through the highway network, in which case parking lots become increasingly valuable and may displace existing buildings. This was the choice cities like Houston took since 1950, sacrificing what had once been walkable neighborhoods for an automobile-dominated core.

Three, it can expand or improve transportation through the transit network (bus and/or rail), in which case higher densities become increasingly valuable, and taller buildings may replace shorter ones or parking lots. This has happened in DC since the construction of Metro, beginning in the 1970s.

The discussion in Washington has hinged around the opposite side of the conversation, focusing on land use instead of transportation. The argument, asserted by people like Stephen Smith, suggests that the problem is that the government is exerting inappropriate control over densities by limiting heights and the result is that rents in the office core are increasing far higher than they would were there to be skyscrapers.

The problem is compounded by the fact that downtown Washington’s growth is limited, notes Ryan Avent, by the fact that outlying neighborhoods are stuck to one- or two-story buildings (and there is little push to challenge that condition), so the Paris approach, in which the entire city is made up of 6 to 10 story buildings, is not much of an alternative, either.

These arguments are compelling: mini-downtowns in the suburbs, such as along Arlington’s Rosslyn-Ballston corridor, can absorb some of the growth, but there is clearly strong demand for continued concentration in the center city.

Whether this is a long-term phenomenon, however, depends on the transportation provided into the downtown. Imagine that the height limits in Washington were lifted — or, at least, buildings twice as high could be built. In the short-term, this would surely produce the desired effects, allowing downtown to absorb more of the region’s job growth, reduce office rents, and aiding in the continued gentrification of the city as a whole.

In the longer-term, however, as the city’s downtown building stock is gradually replaced, the worker density in the center of the city would roughly double. Would this be sustainable?

If the city’s transportation network remains as it is, mostly relying on the existing Metro network and a functioning, if not great, bus system, this would cause significant problems.

Here’s why: Much of the Metro system is already at capacity during peak hours. In essence, today’s transportation network is designed with a capacity roughly equivalent to what is generated under the current height limit.

Moreover, road expansion is simply not an option, not only because there is no room for new highways into downtown but also because, as already stated, a focus on roads-based transportation encourages downtowns to be transformed into automobile-based neighborhoods.

As the transit system becomes more congested, because of job expansion and a lack of transportation improvements, the cost of transportation into the core — in terms of time and money — will increase. This will reduce the appeal of locating offices downtown and encourage new construction to be residential rather than office-based. Is this desirable for Washington? Does the city want a mixed-use core or a office-based one?

The alternative is allowing an increase in zoning along with an improvement in the transportation network. This may seem obvious, but Washington has not yet committed the funds to an expansion of the Metro network or serious improvements to the bus corridors, putting in question the viability of a lifting of the height limits. The downtown’s growth must be approached by considering transportation and land use in complement with one another.

Cross-posted on The Transport Politic.