Sea-Tac Airport is testing the new Concourse D Annex which opens in the fall. D Annex customers will not board their plane through the terminal, but instead take a bus to a remotely parked plane.

Sea-Tac Airport remains one of the fastest growing airports in the country. Now, they're taking dramatic steps and expanding to try and keep up with the number of travelers.

On Wednesday, Port of Seattle and airline employees, along with friends and family, volunteered to act as passengers to evaluate a new suite of gates not yet open to the public. But the gates don’t connect to planes.

It’s called the Concourse D Annex, and you get there from a short skybridge from the main D concourse.

You won't cross a jet bridge to get onto your airplane when you arrive. Instead, you'll get on a big low rider bus and meet your flight out away from the terminal. The busses, which are specially designed for airports, are extra wide and have multiple doors on either side to hold lots of passengers.

The meetup spots away from the terminals are called “hardstands." The concept has been around for decades at some airports around the world, but it's relatively new in Seattle. Sea-Tac began using them in 2016, and their use has exploded ever since.

In February of 2018, Sea-Tac saw 121 loading or unloading operations at hardstands. Five months later in July, hardstands operations ballooned to 761 trips. One day in August saw 36 operations alone.

Airlines are increasingly buying into the idea.

European international discount carrier Norwegian only uses hardstands at Sea-Tac, where a large vehicle consisting of ramps connects passengers to the airline’s fleet of Boeing 787s.

Also see | App helps visually impaired navigate Sea-Tac airport

Sea-Tac still claims the title as the fastest growing large U.S. airport. The airport says passenger traffic over the past five years increased 43 percent, from 33.2 million in 2012 reaching 46.9 million in 2017.

Before the D Annex opens, the airport has been using specially created gates mostly on the lower level of other concourses and connected to the hardstands by the special busses.

One of the reasons to build the D Annex is that the airport is well into a construction boom that is temporarily costing it traditional gates.

These projects included extending the North Satellite and creating a new International Arrivals Facility along the street side of the A concourse, complete with a skybridge connecting it to the South Satellite.

Right now six traditional gates are closed off due to construction. Eventually, that new construction will end, and the gates will open, increasing the number of gates around Sea-Tac overall.