By CNN Interactive Writer Marla Edwards

See also: Travel forecast for Thanksgiving '98

(CNN) -- How do the pundits -- the people who forecast the size of crowds in airports and on highways every year -- know how many millions of people will be traveling for the holidays?

They call around and ask.

The Travel Industry Association collects data each month in telephone interviews, said Catherine Shaw, the organization's director of marketing research. As the holidays approach, TIA gears up to forecast travel by including questions geared to the season in its monthly phone surveys.

The TIA's National Travel Study uses a nationally representative random sample -- 1,200 to 1,500 U.S. adults are called on the phone and asked about their travel plans. They must be planning a trip at least 100 miles (160 kilometers) from home to be included in the survey.

The survey covers planes, trains and automobiles, asking the adult in the household with the soonest upcoming birthday (which ensures a random sample) how they will travel and where they will stay when they get there -- whether with a relative, at a hotel or in several other types of lodging including campers, rental cabins and vacation homes.

The study has had an accurate track record in recent years. For example, it predicted 30.4 million Americans would travel over Thanksgiving in 1994, and subsequent research revealed 30 million actually did travel over the holiday.

"It's usually very accurate as far as what is predicted and what's actually done," said Cindy Sharpe, managing director of public and government relations at AAA.

AAA uses the TIA research to makes its annual holiday travel predictions.

The research for Thanksgiving reveals an increase in the number of holiday travelers every year since 1994. In 1997, 33 million Americans traveled for the holiday, more than the 32.4 million predicted.

"There has been a gradual increase," Sharpe said. "People are a lot more confident in the economy and where they are financially. Because of their consumer confidence, they're willing to spend a little bit more on leisure travel."