For a long time, it has been part of air travel orthodoxy that it’s always a lot less expensive to buy a roundtrip ticket to your destination than two one-way tickets. But times are changing.

A new study from the Airlines Reporting Corporation – which serves as a clearinghouse for all travel agency ticket sales – determined that old belief “is simply no longer true” in many cases.

This was no small-time analysis: ARC said it evaluated three years of data covering more than 350 million tickets.

It found that since 2014, the so-called “one-way fare premium” – i.e. the increased cost of buying two one-way tickets vs. one roundtrip on the same itinerary – has shrunk to nearly zero in almost one-third of the 200 busiest U.S. air travel markets, whereas in years past it would have cost as much as 50 percent more for two one-ways vs. a roundtrip.

The one-way premium remains stubbornly in place in other markets. ARC said that was the case for selected markets including flights to and from San Francisco, Minneapolis, Philadelphia, and Washington Reagan National, among others. In those markets, “the one-way premium has been around $50 per direction and has remained largely unchanged over the study period,” ARC said.

But it dropped dramatically in other select markets, including flights to and from Newark, Dallas/Ft. Worth, LaGuardia, and Los Angeles, the study found. For some markets, like Denver and Boston, the one-way premium stayed in place for flights to or from some destinations but dropped off in others.

“The effect is based on market and not individual airports or airlines,” ARC noted.

As the cost differential withered away in many markets, ARC noted a corresponding shift in traveler purchasing practices: The percentage of one-way tickets sold jumped from 29 percent of all tickets in 2014 to 42 percent so far this year. But that increase was not equal among all market segments.

ARC said most of the increase in one-way ticket purchases came from leisure travelers and unmanaged business travelers (i.e., those who make their own purchase decisions without any company rules or guidelines). For “managed” business travelers, the shift to more one-way ticketing “has been almost non-existent,” ARC said.

But it also noted that the shift to a greater percentage of one-way ticket purchases held true no matter how far in advance the tickets were bought.

“While there are many reasons behind when and how travel is ticketed for corporate travelers, there may be situations where opportunities are missed for increased flexibility and even travel cost savings. By looking deeper into the options for one-way tickets cost savings and other benefits may be available to travelers,” ARC said.

The organization cautioned travelers that before they switch from roundtrip to one-way ticket purchases, they should consider how likely they are to require a change in their plans. “If the full itinerary is changed, the traveler may incur two change fees, and that may make roundtrip ticketing a better option in some cases,” ARC said. “However, ticketing an outbound non-refundable and a return refundable flight can in some cases only be done using the one-way ticket option because most restrictive fare rules are usually applied to an entire ticket when booked as a roundtrip.”

You can see the full report here.

Readers: Do you always check out both one-way and roundtrip pricing? Which do you usually buy? Do you have any tips or tricks to share on the issue?

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