CHICAGO — One after another, planes roared down a new runway Thursday at O'Hare International Airport, where years of crippling delays stalled the nation's entire aviation system and earned the busy hub a reputation as a kind of traveler's curse.

Chicago aviation officials promised the new 10,800-foot airstrip would reduce delays at O'Hare — one of the nation's most important crossroads — by up to 50 percent while allowing for nearly 90,000 additional flights per year. It is part of an $8 billion overhaul that began in 2003 and is reconfiguring O'Hare's outdated layout of crisscrossing runways into a modern and more efficient parallel system.

"It will improve the efficiency of the national aviation system from coast to coast," Chicago Aviation Commissioner Rosemarie Andolino said of the new runway at a ceremony to mark its opening.

O'Hare still ranks at or near the bottom in on-time departures. Opening in 1955, it became a victim of its own success in building itself up as bustling air hub and one of the busiest airports in the world. It was so overwhelmed by the 1990s that a delay taskforce had to be formed.

"O'Hare's been bottled up for so long. This could lead to some exciting things, some new services," said Joseph Schwieterman, Chicago-based transportation researcher at DePaul University, adding that more capacity might even draw in a low-cost carrier.

O'Hare's old lattice network of runways was conceived to allow pilots to take off and land under different crosswind patterns; aircraft technology has largely eliminated that need. When the project is complete, O'Hare will have six parallel and two crosswind runways.

The major expansion pieces yet to be completed are two more parallel runways, a control tower and an extension to an existing runway. One of those new runways and the control tower are under construction, but the city's airline partners in the mega project have yet to agree on how to divvy up the funding of $2.3 billion worth of work still needed to build the final runway and extension.

The airlines already have invested $2.2 billion in the expansion project, said Jim Compton, vice chairman and chief revenue officer at United.

With the new runway, most planes are now taking off and landing on east-west flight paths at O'Hare, and more flights will be able to do so in poor weather and with reduced visibility. During good weather, up to 150 planes an hour will be able to take off from the runway, according to the Federal Aviation Administration.

The impact will be felt far way, especially in smaller airports around the country whose regional jets to Chicago were the first flights to get delayed, hurting those airports' attempts to offer reliable one-stop service to the other side of the country.

Aviation consultant Greg Principato said that while he was president of Airports Council International - North America, from 2005 until earlier this year, he heard repeatedly from small airport directors that the construction project at O'Hare was one of the most important issues for them.

"For a lot of those small communities it's going to be huge," he said of the new runway.

Chronic flight delays at O'Hare, where around half of travelers are just transiting, sent paralyzing shockwaves around the nation's air system in the 1990s. It was a sign that you were an experienced traveler if you said you were trying to avoid O'Hare, Schwieterman said.

"You could hardly mention O'Hare without somebody pulling out a horror story," he said. "And that was well deserved. Everybody had seen trips go up in smoke due to intolerable delays."

Recent data indicates the airport still has some catching up to do.

Of the nation's 29 busiest airports, O'Hare ranked dead last in on-time departures throughout the first seven months of this year, with only about 67 percent of flights taking off on schedule, according to data from the U.S. Department of Transportation.

That represents a slip of three places in the rankings over the same period a year earlier, when O'Hare's on-time rate was 77 percent.

City aviation officials had to overcome numerous challenges to get to this midway point in the project.

For the new runway alone, they had to relocate a cemetery and its 1,500 graves, a section of railroad line, a waterway that had to be moved twice, two large cargo facilities and an aircraft fueling station.

The city has also had to negotiate hard with American and United, which are helping bankroll the expansion project. The airlines sued Chicago in 2011 to stop it from issuing bonds to finance the project, arguing that the city was violating a lease the gives them authority to review and approve expenditures for capital projects. Then-U.S. Transportation Secretary Ray LaHood had to intervene to break that dispute.