Woodbridge_Heights
Senior Member
I.e., only one tower (or more correctly, decks over one or two of the parcels) was built at the same time. Other parcels have since been decked over and more will be in the next few years/decades, so plenty of communities are/will hover over existing roadways/railways.
Edit: For those interested, here is a report from 2000 on the history, economics and vision of developing the air rights over the Pike.
The report itself states that developing air rights over the turnpike is dependant on strong demand for real estate development. I doubt there is that kind of pressure to build over the Allen. For one the turnpike goes right through downtown boston.