Not One But Two Air Rights Projects Rise Over Massachusetts Turnpike in Boston

Air rights development, which is the practice of building above an existing transportation or infrastructure corridor, is not a new idea. Cities have been doing it in various forms for over a century, from the commercial buildings above Grand Central Terminal in New York to the housing built over railway lines in London. What is notable about the situation in Boston is that two projects of this type are progressing simultaneously above the Massachusetts Turnpike, and that the scale and ambition of both represents a significant development in how the city thinks about its available urban land.

The Turnpike, which cuts through some of the most valuable urban real estate in Boston, was originally designed with the assumption that the highway was a fixed and permanent feature of the landscape. The deck-over projects challenge that assumption. They treat the space above the highway as developable land and the highway itself as a piece of infrastructure that can coexist with buildings above it, just as underground rail lines already coexist with the streets and buildings above them.

What air rights development involves

Air rights development is technically demanding in ways that go well beyond ordinary building. The first challenge is the foundation system. A building above a highway cannot use the highway carriageway as a foundation, because the highway must remain operational during construction and in perpetuity. The foundations of an air rights building have to be designed to span over the highway, carrying the building's loads to support points at the edges of the corridor or to specially designed columns that thread through the highway without disrupting traffic flow.

Those foundations are typically a platform deck, a horizontal structure that spans the highway and provides the base on which the building sits. The platform deck has to be designed to carry the full dead and live loads of the building above it, as well as any imposed loads from public space, landscape or other uses on the deck surface. It must also be designed to allow maintenance access to the highway and its structures below, which means that the columns and other structural elements supporting the deck have to be positioned to avoid blocking maintenance access routes.

The construction sequence for an air rights project is constrained by the operational requirements of the infrastructure below. Work on the platform deck structure typically has to be done in windows when the highway can be closed or restricted, often overnight or at weekends. Moving large structural elements into position, welding or bolting connections, and testing the completed structure all have to fit within those windows without compromising the safety of the work or the quality of the outcome.

The two projects and their locations

The two projects advancing above the Massachusetts Turnpike in Boston are at different stages and in different sections of the corridor. Both involve mixed-use programmes that combine residential units with commercial space and public realm at street level, and both are intended to address the way the Turnpike currently functions as a barrier, cutting off neighbourhoods from each other and creating a zone of urban discontinuity along its route.

The projects are proceeding under air rights licences negotiated with the Massachusetts Department of Transportation, which controls the Turnpike corridor. These licences define the conditions under which development above the highway is permitted, including the obligations on the developer to protect the highway, manage construction in a way that does not disrupt traffic, and maintain access to the highway infrastructure for the life of the development.

Negotiating and agreeing those licences is itself a substantial undertaking. The technical requirements are demanding, the legal documentation is extensive, and the process of satisfying the transportation authority that the proposed development will not compromise the safe operation of one of the most heavily used highways in New England takes time and considerable professional effort. The fact that two projects have advanced to active construction suggests that the process, while difficult, is workable.

The engineering of building over a live highway

Building above a live highway introduces constraints that most construction engineers do not normally encounter. The most obvious is the restriction on overhead work above open traffic lanes. Any operation that takes place above a lane that is open to traffic must meet stringent safety requirements, and in most cases, the safest approach is to close the lane entirely during the operation. Coordinating those closures with the highway authority, scheduling them to minimise traffic impact, and completing the necessary work within the closure window is a significant logistics challenge.

The vibration environment is also a consideration. A highway carries heavy vehicles whose passage generates vibration in the surrounding ground and structure. Those vibrations propagate through the foundations and into the structure above. The magnitude is generally manageable for residential and commercial buildings, but it has to be considered in the structural design, and in some cases, isolation measures are incorporated to reduce the transmission of vibration and noise from the highway into the occupied spaces above.

Noise is a related issue. A residential building above a highway has to achieve adequate acoustic separation from the carriageway below, which requires careful design of the platform deck, the building envelope and any openings. The acoustic performance requirements for dwellings are defined by building regulations and, in some jurisdictions, by specific environmental noise standards. Meeting them above a major highway requires a more thorough acoustic design exercise than would be needed for a building in a quieter location.

Air quality is another consideration that affects the design of the building, particularly its ventilation strategy. The air immediately above a highway carriageway is not the same as the air in the surrounding neighbourhood, and a building that sits directly above it has to take account of where its fresh air intakes are located, how they are filtered, and how the building's ventilation system manages the air quality it delivers to occupants.

Why Boston is pursuing this approach

The motivation for air rights development above the Massachusetts Turnpike is a combination of land scarcity and urban policy ambition. Boston is a dense city with limited available land for new development within the areas where housing and commercial space is most needed. The highway corridor represents a significant swath of underused urban space in locations that, if they could be built on, would be among the most accessible and valuable in the city.

There is also a broader urban repair argument. The Turnpike was built in an era when highway planners were willing to cut through established urban fabric in ways that would not be accepted today. The result is a corridor that separates neighbourhoods, generates noise and pollution, and creates a dead zone in areas that would otherwise be active urban streets. Decking over the highway and building above it allows new street-level connections to be made, new public spaces to be created, and some of the urban damage caused by the original construction to be partially addressed.

The public policy support for air rights development reflects a recognition that the cost of building above a highway, which is genuinely higher than the cost of building on ordinary urban land, is in part compensated by the urban benefits it delivers. Where those benefits are sufficiently clear, the public sector has been willing to work with developers on the licensing terms and, in some cases, to contribute to the cost of the platform infrastructure.

What air rights development does for urban form

The most immediate effect of building above the Turnpike is the creation of new street-level connections across what was previously a barrier. A pedestrian or cyclist who currently has to divert to a bridge or underpass to cross from one side of the highway to the other can, once the deck is in place, walk across at grade, through what will feel like an ordinary urban street. That change in accessibility has effects that spread through the surrounding neighbourhood.

The new buildings that sit on the deck also contribute to the surrounding urban fabric in ways that empty highway corridor does not. Active ground-floor uses, public space, housing and offices all generate the kind of street-level activity that makes an urban area feel connected and well-used. The buildings become part of the city rather than a monument to the infrastructure below them.

What these projects show, in combination with similar initiatives in other cities, is that the urban fabric is not as fixed as the highway engineers of the mid-twentieth century assumed. The infrastructure they built can be worked around, built over, and in some cases progressively replaced or reduced, as cities reclaim the space their predecessors gave over to the private car. The Massachusetts Turnpike deck-overs are one chapter in that longer story, and a technically and financially significant one.