Cheaper Air Fares Present More Confusion in Choices : Advertising: Consumers will need to read ads warily as more airlines compete on shuttle flights, especially in California.

It’s always been wise for travelers to read airline ads warily. But with at least two of the largest carriers emulating the low-cost strategy of smaller Southwest Airlines, the strange world of competitive air fares is getting more complicated than ever before. Those who read ads carefully and have a general sense of the marketplace could save hundreds of dollars.

Here are a few things to keep in mind:

When United follows Southwest’s lead, competition will be fierce and prices will fall.

Shuttle by United, as United’s new no-frills service will be known, starts Oct. 1 with 184 flights daily among eight cities, including LAX-San Francisco and LAX-Oakland routes, plus others from Burbank and Ontario. United plans to increase frequency of those flights over the following two months and add an LAX-Las Vegas route (in November) and an LAX-Phoenix route (in December). It appears that nine United shuttle routes on the West Coast will compete directly with existing Southwest flights.

The United shuttles will cost the airline less by flying often (at least five round-trips daily on each route) and using only 737 aircraft (as does Southwest), with seats assigned on the day of the flight. There will be no meal service, though there will be a first-class section. And United will try a new boarding procedure: holders of window seats first, then middle seats, then aisle seats. United officials also pledge stricter enforcement of carry-on luggage limits.

For United shuttle flights out of LAX, round-trip tickets for coach seats purchased at the last minute (also called walk-up fares, these are the highest-priced tickets) are tentatively set at $124 for LAX-Las Vegas and $178 for LAX-San Francisco. (As on Southwest, restricted, advance-purchase tickets will be available for less.)

Southwest, well-known for its low prices, its first-come, first-served seating, its lack of meal service, its all-coach seating and its profitability, has pledged to fight back, probably with new long-haul service in direct competition with existing United routes. Meanwhile, Southwest is maintaining its extensive and affordable California service. Round-trip walk-up fares for LAX-Las Vegas run $124; Los Angeles-Oakland, $138.

Continental is offering new Southern California no-frills flights too.

Continental started its Continental Lite service last October and now flies to dozens of cities under that name (the vast majority in Texas or east of the Mississippi). It also offers discounted “peanuts” fares to and from several more cities, including L.A. Unlike United, Continental has avoided direct competition with Southwest in most cases, instead targeting many eastern routes served by USAir.

Continental Lite tickets are unrestricted and don’t include meal service but do feature assigned seats. On some, but not all, routes Continental Lite makes deeper discounts available to passengers who are willing to buy tickets with restrictions.

Fares on all Continental Lite flights are known as “peanuts” fares, but there are also flights under the traditional Continental name that also offer “peanuts” prices. Since March, the carrier has been offering round-trip “peanuts” fares from Los Angeles to San Antonio ($388-$398 for walk-ups), Omaha ($398) and Denver (generally $398, but service there will be discontinued Oct. 31). Under an “Add a Penny, Add a Pal” program, some “peanuts” travelers can add one cent to their fare and bring an adult companion. (Other airlines have similar offers.)

Despite these new customer-conscious programs, many old rules for consumers still apply. For instance: Just because you see a price in an airline advertisement, that doesn’t mean you can buy anything for that figure.

This is a tradition now. When a fare war is declared, the major airlines take out full-page newspaper ads such as the ones American took out across the nation on July 26: For a New York-LAX flight, the larger type said, the discounted price was “$194.” But the smaller type above said: “Fares are each way based on round-trip purchase.” In other words, the product being sold in the ad cost $388, not $194, and there were no one-way tickets available for $194. (The offer expired Aug. 2.)

“We don’t think it’s misleading,” says American spokeswoman Teresa Hanson. “I think people are used to that now,” says Joe Hopkins of United Airlines. Certainly some are: Bill Mosley, spokesman for the U.S. Department of Transportation, says that “we would not consider that deceptive so long as the ad made it clear that (the fare) was only available for a round-trip.”

Another old rule: On a heavily promoted discount, airlines limit the number of cheap seats available, sometimes severely.

Late last year, the Department of Transportation fined Continental Airlines $20,000 and USAir $5,000 and warned them against future “deceptive advertising practices.” In the Continental case, federal officials alleged that the airline advertised discount fares between Newark and Europe, but “failed to make a reasonable number of seats available at those fares.” In the USAir case, federal officials found that the carrier had continued to run a recurring advertisement for a discounted new service between New York’s La Guardia Airport and Sarasota, Fla., despite the fact that by the time the last ad ran, only one discounted seat remained.