TSA security officers check boarding passes of passengers outside Terminal B at Ronald Reagan National Airport, Sept. 18, 2012. (Astrid Riecken/For The Washington Post)

A week before more than 3 million people overwhelm the nation’s airports during the busiest travel weekend of the year, a pair of reports raised a chilling question: What if the nightmare airport scenario of Thanksgiving were repeated every week?

Long lines, standing-room-only lounges, late planes, endless waits for luggage.

Failure to invest in airports and the aviation system will ­result in a steady increase in airport congestion as the demand for air travel is projected to increase by as much as 400 million passengers annually within 20 years, according to a report released Wednesday by the Eno Center for Transportation.

“The gap between the busiest day of the year and the average day is shrinking,” said Roger J. Dow, president of the U.S. Travel Association, citing another study put out by his trade group. “Many of our top airports already are experiencing a Thanksgiving-like day once a week. Within five years they will have two days like it each week. Within 15 years, they will have three days like it.”

Joshua L. Schank of the Eno Center outlined a series of policy changes necessary to avert airport congestion that could rival rush-hour traffic on urban expressways. Some were designed to provide revenue for airport modernization and expansion, including a change in the federal funding formula so that choke-point airports get more money and lifting the federal cap on the $4.50 passenger facility charge fee that airports are allowed to add to tickets after gaining FAA approval for construction projects.

The Travel Association report also pointed to an airline pricing tactic that has undermined airport revenue. By shifting some of the money passengers pay from ticket prices to baggage fees, the airlines have reduced the amount they pay under the 7.5 percent excise tax on tickets.

Without additional revenue, airports will grow increasingly congested, both reports suggested.

“It’s a coming crisis,” Schank said. “At several airports, demand already exceeds capacity for several hours of the day.”

Schank and his center’s report also recommended that the Federal Aviation Administration be stripped of responsibility for implementation of the $40 billion aviation control system known as NextGen.

“The FAA is moving very slowly with NextGen,” Schank said.

NextGen, which would allow the system to handle many more planes, should be expedited by putting it in the hands of another government agency or a nonprofit corporation, he said.

NextGen’s ability to deliver more planes to an airport through a variety of efficiencies is critical to the expansion of overcrowded airports such as New York’s John F. Kennedy, which has no room to grow.

“NextGen really is the answer for us in increasing capacity,” said Jennifer M. Hensley of the Association for a Better New York. “Our only option is to increase efficiency in the air. Obviously, it’s a big cost. It’s hard to get Congress moving on anything that involves spending.”

With the U.S. population expected to grow by 90 million this century, U.S. Transportation Secretary Anthony Foxx acknowledges, “We’ve got a problem.”

“When we say there’s a crush of people moving through our airports now, imagine when we have 90 million more people,” Fox said. “This could be a crisis if left unaddressed.”

Erik Hansen, a policy expert at the Travel Association, says the burden for action falls on Congress.

“While the rest of the world is speeding up [aviation improvements], the U.S. is slowing down,” Hansen said. “While there are solutions out there, Congress remains as a roadblock to any progress.”