Much of the air space ("air rights") over the Boston Extension has been leased to third parties for commercial development. This concept was originally designed to "knit together communities" that were divided by the new highway, since the turnpike had been described as "wider and more divisive to the city" than the original Central Artery. More recently, the income received from the leased air rights have been used for paying off the Big Dig. There are 23 parcels of air space over the highway, the majority of which have not been developed. Among other objectives, guidelines established by the "Civic Vision for Turnpike Air Rights in Boston" in 2000 recommend that the proposed use of the parcels "increased use and capacity of public transportation" and "[reinforce the vitality and quality of life in adjacent neighborhoods".