It may be one of the nation's favorite airports, but Portland International wants to spend more than $1 billion on an upgrade to tackle problems that have been creeping up over a number of years.

Ever since 9/11, authorities in Portland have found facilities becoming increasingly stretched as passenger figures have grown. This is particularly evident in the security screening areas, which now require greater space to cope with passenger lines, security operatives and their ever-stringent procedures.

Last year, around 18 million passengers used Portland International, up 1.5 million from the previous year. The airport is based around a single-terminal concept, with five concourses of airline gates. The largest airline users are Alaska Airlines, Horizon Air, Delta and Southwest Airlines.

With the proposed developments, the renovated terminal would improve passenger flow significantly by reducing the areas where mixing of arriving and departing passengers takes place, causing congestion.

If agreed, work could begin in 2020 and would see the central terminal area reinvented to improve the somewhat-modular approach that has been taken to upgrading the 1950s-era building so far. Improvements to passenger flow and security, as well as airline gate allocations, would take place as part of this, along with an upgrade of the plumbing, seismic capabilities and electrical system to expand on the solar panels already installed.

Elsewhere, the parking lots will be expanded to deal with congestion and increased demand from road users.

The work, which would cost an anticipated $1.3 billion, would need to be approved by airline operators before contractors move in, with funds being raised from operating fees that are likely to be increased and potentially passed on to passengers. A plan for reducing disruption for passengers and flights would also need to be developed and approved.

"We have made do with what we could until now," said Curtis Robinhold, the newly minted executive director of the Port of Portland. "We're simply running out of capacity to manage the passenger flow we're getting today, and that we'll be getting in the days to come."

This month, Portland was voted Best Domestic Airport for the fourth year running by Travel + Leisure readers. It received this based on passengers' experience with security and screening, airport amenities such as shops, restaurants and bars, as well as the overall design of the terminal, being dubbed a "foodie heaven." In the past, Portland was also identified as a top business traveler airport by Condé Nast magazine.

A vote is set to take place this fall to agree on the works and the plan for funding them.