A record 38.7 million U.S. residents were expected to travel 50 miles (80 kilometers) or more for the holiday. Some were already en route by the early morning, hoping to beat the evening rush on what is often called the busiest travel day of the year.
"Wednesday ends up getting hairy," AAA spokeswoman Christine Brown said. "Many people have to wait until after work to leave, and they're competing with commuters as well."
About 31.2 million travelers were expected to drive to holiday celebrations in spite of gas prices that were nearly 85 cents more per gallon (3.8 liters) than a year earlier, according to the AAA. The national average for regular gasoline on Nov. 16 was $3.09 ( EUR 2.09) a gallon, up from $2.23 ( EUR 1.51) on Nov. 17, 2006 .
At New York's Pennsylvania Station, hundreds of travelers were already heading out of town Tuesday night, wrangling their bags and sprawling on the floor as they waited for their trains to arrive.
Robert Kaldenboch, 18, dressed in his uniform from the U.S. Merchant Marine Academy on Long Island , looked around wide-eyed at the crowd.
"There are more cows than people where I'm from," the Texas native said as he waited for his train. "So this is quite a change."
Some, like Kate Funke, were forced to get an early start on their holiday because of sold-out trains.
When the 27-year-old online advertising saleswoman discovered that all of Wednesday's Amtrak passenger train seats to Boston were full, she started her visit to her boyfriend's family a day early - and brought her work with her.
"It is what it is," she said, looking around at the crowd. "It's really busy."
Amtrak expected more than 115,000 riders on Wednesday, about a 70 percent increase over a usual Wednesday, spokesman Cliff Cole said. An electrical breakdown had snarled train traffic on the Northeast rail corridor over the weekend, but everything was running smoothly for the holiday, Cole said.
It was not just the rails and roads that were expected to be crowded. Holiday delays at U.S. airports have become such a fixture that President George W. Bush last week called it "a season of dread."
Bush announced steps to reduce air traffic congestion, saying the Pentagon would open two military air corridors to commercial airliners from Wednesday afternoon through Sunday, creating a "Thanksgiving express lane."
Travelers heading to New York City area airports had special cause for concern, with a crush of 3,492 takeoffs and landings planned for Wednesday at John F. Kennedy International, Newark Liberty and LaGuardia airports. Delays at those airports have been getting steadily worse, and almost three of every four flight delays in the country can now be traced back to a problem in the greater New York area.
In all, about 4.7 million U.S. residents were expected to fly for the holiday, according to the AAA.
At least the weather seemed unlikely to cause any significant delays. Michael Musher, a National Weather Service meteorologist, said light snow in the Midwest and light rain elsewhere around the country could cause only minor problems.
The AAA's predictions for holiday travel are based in part on an online survey of U.S. residents, whose answers are weighted based on factors including education, income and geography. Participants are contacted via e-mail and elect to answer a questionnaire online.