Expressway Caps

Columbus has an interesting expressway cap on High Street. Check it out.

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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.
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* 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.