Washington ? Beginning today, the federal government will open parking lots at the country’s biggest airports that have been off-limits since Sept. 11 because of worries about car bombs.

Federal officials also will change the way air travelers are screened after they pass through security checkpoints over the next few weeks, checking them only at randomly selected gates, according to Robert Johnson, Transportation Security Administration spokesman.

“We’re trying to avoid screening Grandma two or three times as she makes her way home for the holidays,” Johnson said.

Johnson said the prohibition on unattended vehicles parking within 300 feet of a terminal will be dropped today as long as the terrorist threat level is at code yellow, or “elevated,” the middle of a five-point scale of risk developed after the terror attacks.

TSA chief James Loy was scheduled to announce the change at an airport security conference co-sponsored by the Airports Council International-North America and the American Association of Airport Executives.

The so-called 300-foot rule will be reimposed if the threat level rises to orange or red, Johnson said.

Airports, though, will have to draw up plans outlining how they would deal with the threat of an explosion. They won’t have to close the parking areas during an elevated threat if their plans don’t call for that, Johnson said.

New layers of airport security allow the rules to be eased, Johnson said, listing a better-trained screener work force, federal air marshals, background checks of people who work beyond airport security checkpoints and screening of checked baggage at 252 airports.

Todd Hauptli, an airport lobbyist, said the airports have pushed the TSA hard to let them reopen their close-in parking lots.

“This has been a thorn in the side of many airports,” Hauptli said, and the rule brought no significant improvement to security.

Not only do some airports lose money because they’ve closed parking, but also they have to bus people from remote parking lots, he said.

“It’s definitely a passenger convenience, customer convenience issue,” Hauptli said.

Like the parking restrictions, screening passengers just before they board their plane was on a “stupid rule” list that Loy drew up. He promised to look at the list and try more common-sense approaches.

As part of that effort, the TSA will change boarding pass procedures at all or part of eight airports, adding to the nine that are in a pilot program to eliminate gate screening.

Johnson said the boarding pass procedures will be changed at Milwaukee’s Gen. Mitchell International Airport, Cincinnati/Northern Kentucky International Airport, Houston’s Bush Intercontinental Airport, Boston’s Logan International Airport, Memphis International Airport in Tennessee, Charlotte/Douglas International Airport in North Carolina, Seattle-Tacoma International Airport and Pittsburgh International Airport.

The places where some or all passengers must get their boarding passes before going through security checkpoints are Los Angeles; Long Beach, Calif.; Newark, N.J.; Detroit; Minneapolis/St. Paul; Miami; St. Louis; and LaGuardia and Kennedy in New York.

At the 17 airports, passengers will have to get their boarding passes before they go through security. Air travelers selected for the secondary screening previously conducted at the gate will be more thoroughly screened at the security checkpoints. Johnson said there’s a very slim chance they might be screened a third time at the gates.

At all other U.S. airports, mobile teams of screeners will check passengers a second time at randomly selected gates, Johnson said.

“It will be random and unpredictable to the public,” Johnson said. The effect, he said, will be that the numbers who get screened “will go up and down.”

Hauptli said the practical outcome of the new policy is that fewer people will get screened a second time.