When the American Automobile Association ordered 32 million tour books for its members this year -- 9 percent more than last year -- it had no idea it would need to reorder even before the heaviest travel season arrived.

But that was before anyone realized that more Americans than ever before plan to take summer vacations this year, a spokesman for the U.S. Travel Data Center said yesterday.

Spurred in part by their confidence in the economy and travel costs that rose only 1 percent last year, a record 109 million Americans will take vacations between June and September, according to the center's annual summer travel forecast.

Vacationing Americans are expected to spend more than $100 billion on those trips, said Douglas C. Frectling, director of the nonprofit travel and tourism research center.

AAA already is experiencing the effects of what Frectling described as record numbers of Americans choosing to travel around the country this summer.

While only 3 percent of Americans usually travel overseas each year, factors encouraging them not to visit foreign shores this year are the decline of the dollar and recently increasing fears of international terrorism, said Frectling, who pointed out that 1.8 million Americans have changed or canceled their plans to travel abroad because of terrorist incidents.

"Terrorism is a more potent overseas depressant than the dollar's decline," he said. "This is not going to be a good year for Western European travel. Evidently, it's not going to be a good year for Eastern European travel either because of Chernobyl."

He said a much greater factor in increased domestic travel this summer is Americans' perception that the United States and Canada are travel bargains.

Domestic travel will have a strong year "because the fundamentals are sound, not because Americans are shifting from overseas travel," he said.

Among the factors encouraging Americans to travel at home are gasoline prices that are 15 percent below those of last summer, according to the forecast based on interviews with more than 4,000 adults.

Eighty-two percent of American tourists are expected to travel by car, truck or recreational vehicle, the center said. Air travel will decrease slightly compared with last year, when many Americans took advantage of a wide range of discount air fares.

Airline representatives have indicated that, after a $500 million loss by the airlines in the first quarter of this year, some of them may raise their weekend rates and fares to popular vacation spots such as Los Angeles and some Florida cities this summer.

The hottest U.S. vacation region to travel to by any mode of transportation is expected to be the Southeast, which the center projects will host 7 million more vacationers than it produces, according to the survey.

That area will benefit from its theme and amusement parks, such as Disney World and Epcot Center, in addition to its beaches.

The one foreign destination that is expected to receive an increased number of American tourists this year is Canada, which was selected as a travel bargain by 26 percent of those surveyed, up from 16 percent last year.

In addition to being a relatively cheap destination, tourism experts say that Canada is perceived as being safe, and Expo '86 in Vancouver has been widely promoted in this country.

The world's fair already has surpassed its goal of selling 11 million tickets, according to Frectling, who said it could sell as many as 15 million.

Asked to describe a typical American vacation party, Frectling offered up the following scenario: two adults and a child with a family income of $30,000 or more will travel by car to the southeastern part of the United States, probably to a beach. They'll spend 6 to 7 percent more on a vacation than they spent last year.

And if you live near a beach or amusement park, watch out. In a switch from last year, more travelers are expected to stay with friends and relatives than in hotels and motels, perhaps because of what the center reports as a 5 percent increase in hotel room rates over last year.