A successful air-rights project has not been built over the Massachusetts Turnpike since Copley Place opened in downtown Boston in 1983. Now, the first phase of a $600-million air-rights project in the city is close to starting construction. But the second phase, which includes building a deck to support a 27-story tower over the eight-lane toll road, is still up in the air.

The Massachusetts Dept. of Transportation board on July 17 voted to grant the 4.5-acre Fenway Center project a 99-year lease to build two towers on firm ground alongside the turnpike. Agreeing to pay the state $21 million for the lease, developers John Rosenthal of Meredith Management and partner Gerding Edlen, a real estate investment firm, hope the project’s first phase will help secure financing for the second phase. The deal stipulates that Rosenthal and Gerding Edlen will have to pay a $3-million penalty if they don’t begin construction on the second phase before the end of 2020. Phase 1 calls for 313 residential units and 67,000 sq ft of commercial space; the Phase 2 tower includes 335 residential units and 160,000 sq ft of commercial space. Rosenthal has been trying to build the project for nearly 15 years, but has been stymied by litigation and recession.

Real-estate attorney Bob Ruzzo, who did air-rights work with the turnpike authority in the 1990s, says building Phase 1 first will make Phase 2 more difficult to complete because there will be less laydown area. But, he says, Phase 2 could pave the way for other proposed air-rights projects, such as 100 Boylston St., for which the Boston Planning and Development Agency recently issued a “scoping determination” detailing which areas of the proposal require further study.

“We need a success on air rights,” Ruzzo says, noting that if Rosenthal doesn’t build  Phase 2 it will send a signal that such projects “are probably a few more decades away. If they elect to pay $3 million to do nothing, that makes a statement.”