PLANNING A TRIP somewhere in the United States this year? According to predictions at the recent "1980 Travel Outlook Forum" in Washington, here is what you may expect:

An overall rise in travel costs averaging about 13 1/2 percent over 1979.

A price tag of between $1.40 and $1.50 per gallon of gasoline by the end of this year, with additional increases probable thereafter.

An increase of about 60 percent in air fares -- no surprise, considering that a rise of one cent per gallon of jet fuel translates into a $100-million annual cost to the airline industry.

An increase of nearly 10 percent in food costs, coupled with a reduction in food and beverage services offered by the lodging industry because of higher labor costs, and a greater number of vending machine services to take their place.

An average price of 10 to 12 percent in the cost of hotel and motel accommodations, sometimes along with a reduction by management of hitherto traditional services and conveniences.

The forum was sponsored by The U.S. Travel Data Center and the Travel Research Association.

Some 225 participants representing major sectors of the travel industry -- air, rail, bus, automobile, hotel/motel and restaurant -- came from as far away as Europe and Australia to match wits with the three key forces shaping the coming decade of domestic travel in the United States: double-digit inflation, the rising cost of energy, and the question of continuing availability of gasoline for all modes of transportation.

Dr. Douglas Frechtling, director of The U.S. Travel Data Center, pointed to the inescapable fact that the increasing cost of gasoline -- at present about 17 1/2 percent of consumer travel expenditures -- affects more than the traveler himself, since recreational, lodging and eating facilities also depend on transportation fuel for their maintenance and operation.

For the giant $115-billion-a-year travel industry in this country and the 6.6 million jobs dependent on its good health, the next few years will be critical, and officials predict major changes.

Automobile travel will continue to be the most prevalent and least costly mode of travel in the '80s, say the travel experts, but the possibility of recurring gas lines will make more popular the combination of air-travel-plus-rental-car vacation trips, which presently amount to 15 percent of the travel market.

Travel value will take on new importance -- depending, naturally, on vacationer interests. A concentration of major attractions will make such travel destinations as Florida and California increasingly desirable.

According to Patricia Pinto of the National Restaurant Association, innovations, many brought about by energy pressures and rising labor costs, are on the way. The association foresees the development of "specialized markets," which call for modified eating facilities and menus for specific types of travelers representing special food interests and restaurant seating needs. Among these travelers with special interests are the growing number of singles, the "over-50s," and senior citizens.

The increased awareness of healthful eating habits will be reflected in menus featuring smaller portions, low-fat and/or sugar-free diets, and including nutrition information.

Rising labor costs will lead to a greater number of self-service and cafeteria facilities, replacing more costly table service and item-heavy food offerings.

Travel industry experts anticipate greater reliance on sophisticated multi-access computerization.

Thus, the potential traveler of the 1980s may be able to obtain important travel information through the courtesy of his home television set merely by the switch of a button. He may learn about specific trips, and be able to reserve a hotel bed, a berth on a cruise liner, or a seat on a plane without a special trip to his travel agent.

(Many agents are seriouly concerned about the impact of computerization on their business, both in the present sense of processing accounts and handling reservations, and the future sense of information banks that may be available directly to potential clients in their homes for a fee. However, many experts believe that there will always be an important role for the agent who provides personalized, face-to-face service, though computers may radically change and curtail operations of small travel agencies, which are often only marginally profitable.)

There was a general consensus among Travel Outlook Forum participants that while the nation's energy future does not presently lood very bright, sufficient energy for most travel needs can be realized through across-the-board energy conservation, and changes in travel patterns through such devices as energy-efficient vacation packages of the fly/drive variety, the use of smaller cars, and more short-haul trips as opposed to multimile ventures.

This, they believe, is the best hope Americans have of retaining their cherished freedom to travel.