(CNN) -- Planes crisscrossing the East Coast were given access to military airspace Wednesday afternoon, a handy accommodation as fog and winter weather threaten to make one of the year's busiest travel days even more tedious.
Family members greet each other at the Denver airport on Wednesday.
Thousands of square miles of airspace usually reserved for the military became available to commercial airliners at 4 p.m. ET. It was up to the Federal Aviation Administration to decide whether to use it.
President Bush last week ordered the FAA and Defense Department to open the airspace, which the White House says will create "Thanksgiving express lanes."
The FAA will have the option of using the airspace through Sunday night.
David Castelveter, spokesman for the Air Transport Association, the interest group representing major carriers in Washington, said the airspace will be available for planes flying between Florida and Maine and could help flights avoid bad weather.
"If we have weather coming in from the west, for example, it allows us to route planes to the east in order to get them on a good path," he said. Check the FAA's real-time airport status map
Wednesday evening, the National Weather Service had severe thunderstorm warnings in effect for a few locations in Texas, Louisiana, and Arkansas, and a winter storm warning in effect for parts of Maine.
The FAA reported scattered delays at the nation's airports on Wednesday evening.
At 7 p.m. ET, thunderstorms delayed flights out of Houston, Texas, by more than an hour.
At New York's LaGuardia Airport, low-lying clouds pushed arrival delays to an hour or longer for many flights for a while on Wednesday afternoon.
Katie Bates and her family faced an hour delay on the departure of their United flight to Chicago, Illinois, meaning they would probably miss their connecting flight to Traverse City, Michigan.
"I think it's horrible," Bates said. "We're going to sleep in Chicago."
Barbara and Mike Richter drove to LaGuardia from their home outside Scranton, Pennsylvania, a 2½-hour trip, only to learn their son Kyle was still stuck in Chicago waiting for an AirTran Airways flight to take off.
"Part of the holiday, part of the airline situation," Barbara Richter said as she and her husband settled down to wait. Watch what to expect at airports for the holiday »
New York is a particularly important holiday travel hub. Nearly one-third of all air traffic passes through the New York area, and three of four chronic delays around the country can be traced back to those in the Big Apple, according to a White House news release about the military airspace.
Transportation Security Administration screeners at LaGuardia began X-raying luggage at 4:30 a.m., a half-hour earlier than usual, and there were 25 percent more security personnel working the holiday shifts, according to the TSA.
"We were prepared as far as staffing," said Warren Kroeppel, LaGuardia's general manager, adding that he is also thankful for the better-than-expected weather. "We've been praying, praying, and the weather stayed good today."
Smooth travel conditions could change at any moment, as the Air Transport Association is expecting 27 million passengers to fly during the 12 days that began November 16.
All FAA personnel will be focused on keeping flights running on time, and air transportation experts said the military airspace made available by Bush may help mitigate holiday delays.
"It's like chicken soup -- it can't hurt, but it certainly can help," said David Stempler, president of the Air Travelers Association, an advocacy group for airline passengers.
The Air Transport Association's Castelveter said the military airspace has been made available during extreme weather but never for holidays. If the airspace proves helpful, "then we will have a similar opportunity over Christmas," he said.
However, Patrick Forrey of the National Air Traffic Controllers Association said last week that the extra airspace won't matter because the number of runways and air traffic controllers is the same.
"The bottom line is you can only land a number of aircraft at one time on any given runway, so the capacity at the endgame and at the beginning game needs to double or increase before you start killing delays," he said. "This is just another gimmick to try and appease the flying public, and it's not going to work."
The key for air travelers is to arrive at the airport with plenty of time and get to the gate early, Stempler said.
"They're going to board these planes extra early. Don't saunter up there 15 minutes before. We're recommending being at the gate at least 45 minutes in advance," he said. Try these tips to travel like a pro »
Gather information about your flight, security regulations, conditions at the airport and suggested arrival times in advance of your trip, Stempler said. Bring snacks, reading material and the 800 number for your airline in case of delays or cancellations.
Travelers checking bags should put a copy of their itinerary and their name, address and phone number inside their bags in case they're misplaced.
Meanwhile, Americans hitting the road or rails can expect crowded highways and trains as well.
Amtrak was expecting a crush of holiday travelers on its busiest day of the year -- perhaps as many as 116,000, a 70 percent increase over an average Wednesday, said spokesman Cliff Cole.
The train company expects it will carry 600,000 passengers between the Tuesday before Thanksgiving and the Monday after the holiday, Cole said.
Americans planning to drive during the holiday will have to consider the cost of gasoline, which jumped an average of 13 cents in the last two weeks, according to a survey published Sunday.
The average price for a gallon of self-serve regular is $3.09, the Lundberg Survey found. That's 9 cents below the record set in May.
The travel organization AAA estimates 38.7 million Americans nationwide will travel 50 miles or more from home this Thanksgiving. That represents a 1.5 percent increase over last year. E-mail to a friend
CNN's Allan Chernoff, Bob Constantini, Ed Lavandera and Tracy Sabo contributed to this report.