A big building. Over the Pike. For the first time in a long time.

The first Mass. Pike “air rights” development in decades has completed its deck over the highway.

Ironworkers attached a beam to the side of an office building going up along Mass. Ave. on a deck above the Massachusetts Turnpike, the first such "air rights" project to be built in the Back Bay in four decades.David L. Ryan/Globe Staff

The rumble of passing traffic becomes a roar when you step out onto Parcel 12 — a two-building complex of office, lab, and hotel space going up on a deck over the Massachusetts Turnpike at the entrance to the Back Bay.

And Tuesday, for the first time, a crowd stood close enough to hear it.

That’s when real estate development firm Samuels & Associates celebrated bridging the Massachusetts Turnpike at Parcel 12, which straddles not just eight lanes of highway, but active MBTA Green Line and railroad tracks as well. Construction firm Suffolk and its subcontractors have laid more than 520 tons of steel and precast concrete planks across the turnpike, crafting the base for a project that will hold the global headquarters of car-buying website CarGurus Inc., a CitizenM hotel, and a public plaza overlooking the highway. The project is scheduled to open next year.

“When we talk about infrastructure, I just want to say: You all realize that we are standing over a highway? This is an incredible feat,” said Segun Idowu, the city’s chief of economic opportunity and inclusion. “I’m standing over here going, ‘Wow! This is amazing!’ As we hear the cars going back and forth.”

Mayor Michelle Wu is “delighted about this particular project,” Idowu said, given both the partnership necessary between private developers and city and state transportation officials to build the first large-scale air rights project above active rail lines and eight lanes of the turnpike in decades. The project will include improved sidewalks, bike lanes, and public transportation on a crowded stretch of Massachusetts Avenue, including a new MBTA Green Line entrance and revamped bus shelter.

The Parcel 12 project is one of several so-called air rights projects underway in Boston right now, including nearby Parcel 7, where life-sciences development company IQHQ and local developer Meredith Management are building a $1 billion lab tower over the pike near Fenway Park, as well as WS Development’s 400 Summer St. in the Seaport, which rises over a mostly underground section of the highway.

The Parcel 12 construction is “an incredible feat of engineering” which “lays the groundwork for ... new economic opportunity, public transit improvements and infrastructure upgrades that will benefit the surrounding neighborhood,” Governor Charlie Baker said in a statement.

For CarGurus CEO Jason Trevisan, having the company’s future headquarters at Parcel 12 is a testament to its belief in Boston — and reflects its optimism about the city. The Cambridge-based car-buying website and auto research company first signed a lease for the Parcel 12 site in late 2019, just before the COVID-19 pandemic. Last year, CarGurus scaled back its footprint slightly, to 225,428 square feet, while Samuels sought city approval to convert nearly 120,000 remaining square feet at the office building to lab space.

CarGurus employs around 800 people locally and plans to grow that headcount, Trevisan said.

“In a hybrid environment where employees are working more frequently from home ... we need to create a draw for our office,” Trevisan said. “We think that that’s what this is, and that will be a key ingredient to us having a thriving hybrid environment.”

A plaza over the Massachusetts Turnpike is part of a large development underway on the west side of Massachusetts Ave. between Newbury and Boylston streets.Elkus Manfredi