Tuesday, November 1, 2011

Expressway Caps

Columbus has an interesting expressway cap on High Street. Check it out.
High Street cap from the sky
While the Tribune blog bloviated over its similarities to Columbus' former (lost) Union Station--alas, poor Union Station--for planners that's not the key takeaway: instead, the key takeaway is that it is very much possible (though not cheap) to build light freeway caps. (This one cost just under $10 million.) The other takeaway is that a freeway cap need not be just for recreational land use. Boston, I'm looking at you.
High Street crossing the cap. Look, ma! No expressway!
Instead of a fully underground, Big Dig-type scenario, the Columbus example gives us some counter-ideas. When it is necessary--for whatever reason*--to run a sunken expressway through an urban area (usually due to the inertia of one already existing), it would be far better--and cheaper--to mitigate the impact by deploying these cap structures--or more accurately, in most cases, salable cap slabs.
Such as these slabs in this unfinished development.
In either case, the interventions are different than the major interventions that usually involve acquiring an Interstate's air rights: in those cases, space for full cores needs to be found, and anchored on solid ground. In this case, however, the structures are diminutive enough--single story commercial, townhouses, walkups, or possibly retail with a townhouse or walkups atop--that they need only basic infrastructural hookups (electricity, plumbing, and fiber)** that can be routed through the cap--no need for such expenses as elevators. Additionally, well-known green roof installation techniques can be used to add grass to whatever open space local zoning laws require in the rear of the property, please; a low wall (about 10 ft.) separates the livable space from the transportation space. With caps another possibility may be to provide a cap base along an entire city block that e.g. garden apartments can be built on. Unlike the cap concept, however, this concept would become a full tunnel and necessitate ventilation equipment somewhere on site.
Click it to see it in all its glory!
This is the first of the two concepts I whipped up for this post. The Vine Street Expressway is a sunken highway dividing--despite the crossing frequency--Center City from North Philly. The sunken section runs from 10th St. to 22nd St., or 12 blocks (with 13 spans). It additionally passes by Logan Square, a pretty dismal City Beautiful mess anchored by an excessively autocentric traffic circle. This model proposes closing open caps currently situated between 18th and 21st Sts, and providing for small commercial or multipurpose salable slab space alongside each of the other bridges. As time goes on, cap additions between these caps can completely conceal the expressway's existence at street level.
Oooh shiny. Clicky.
This is the second. Where the Vine Street Expressway is ideal for more fly-by-night and urbanist interventions, Society Hill is ideal for something more closely approximating garden apartments, as the image explains.
* And usually it's not, from a technical perspective. Big Dig-type projects should usually be opposed on principle (pork-barrel automotive spending): what I am proposing here is a mitigation strategy when expressway closure has already wormed out of the picture, or for retrofitting of existing sunken expressways.
** Electricity for climate control and cooking as well as power; plumbing means fresh water and sewer hookups; fiber allows for phone, cable, and Internet.


  1. It seems obvious to think about "okay, so how much does land cost in this area per square foot? how much would a freeway cap in this area cost per square foot?" and yet I've never seen such figures bandied about. Whenever I've asked, I've been told that it's because the cost of such caps is very site-specific.

    I think that the Columbus cap was paid for out of Enhancements funds and was made easier by the fact that the freeway underneath was closed for reconstruction at the time (greatly facilitating construction staging). I'm not sure it'd be anywhere near as cheap with an active freeway still underneath. However, such projects seem like a natural fit for a TIF.

    The buildings do really make the freeway almost imperceptible from the sidewalk. It's funny, and tragic, that Eisenman's awful convention center is just south of the bridge--huge contrast in both architectural style and street life.

  2. Providence has a very wide pedestrian walkway over I-195, connecting the East Side with India Point Park. It's not a cap, but it's much more pleasant than a normal street that goes over a freeway. The problem is that the noise of the cars zooming by at 90 km/h is still there. Do you know if the caps are wide enough and the buildings big enough to block this noise on the street, and if they make sure to block the noise within the buildings on the sides facing the freeway?

  3. It's not cheap, but worth pursuing. The problem is that typically there isn't a lot of land produced from these projects, and the cost of the infrastructure is high, so you need to be in a strong real estate market to do it--if the project is driven by market forces rather than placemaking desires.

    Also look at the Reno Retrac project and the park they are building over a freeway in Dallas, not to mention the very old example of this exact thing at Forest Hills in Queens.

    - http://www.flickr.com/photos/rllayman/771428871/

    There are a few examples in DC, where we have built over the 395 "Center Leg" -- an apartment building, some judiciary buildings. Plus the big section of space between Mass Ave. and D Street will also be built over.

    - http://www.bizjournals.com/washington/stories/2010/06/14/story4.html
    - http://www.mvtriangleblog.com/?p=2948

  4. What do you imagine the red sections on your Vine St diagram looking like? You said they are just simple concrete caps? Not quite sure what you meant.

  5. Alon: According the the Trib article, yes. This makes sense to me--even some plantings have a very perceptible impact masking freeway noise.

    Richard: Agreed. One of the issues I'm thinking about is how much money you have to sink into a project when the density is lower (for example, most air-rights developments, e.g. Boston's Back Bay, still need to keep the elevator cores over solid ground). What I am proposing is nothing short of using (part of) our road construction budget to subsidize urban remediation--have the Interstate, in effect, "eat" the air rights cost of developing over it. You can use full commercial caps à la Columbus, or possibly infrastructurally prepped caps, able to take the added utilities, or some combination of the two.

    Liam: I hope that answers your question. The public agency would be prepping the cap and then selling it to a local developer. To make such a project break even in plain capital-cost terms, cap costs beyond the simple cost of local prepped land would be "eaten" by the public agency. To put it another way: the air rights value is deliberately not being taken into consideration.

  6. I moved to Columbus almost a year ago, and it was 8 months before I realized that this was a highway cap. I had NO idea, which is ideal!

    It is a great design, (tangent:) except for the fact that the building itself looks like a giant plastic toy. You can see the "plastic" seams on the columns from the street! SO TACKY! However, otherwise the design and the use of the thing is great, so I mostly forgive them...

    Too bad the other "caps" they've planned here are just going to be big grassy things that won't do anything to actually connect neighborhoods like this one does. And they're going to be SO MUCH more expensive! http://www.columbusunderground.com/chicago-studies-the-innovative-i-670-highway-cap-in-columbus It impresses me that we did it so well once, for so little money... yet we just... don't feel like doing it again? Bah.

  7. That makes sense, Steve. So the City (or whatever public entity) would advance construction on a few of the caps to jumpstart the concept, and sell off the rights to construction on the other caps.

    Even if the commercial aspect of the project never becomes profitable, capping would still be worth it, I think. It would go a long way to changing attitudes from "this is a highway with a street above" to "this is a street that has a highway underneath".