Delta Air Lines has banned several travel websites from posting its airfares and routes as part of a bigger trend in the airline industry that has made it harder for travelers to comparison shop.

That is the conclusion of a study by a Yale professor who estimates that travelers are losing out on more than $6 billion in savings a year because of the limits. The restrictions have prompted up to 41 million passengers a year to forgo air travel, the study says.

“At a time when independent, transparent comparison shopping is most needed, some airlines are attempting to restrict access to their fare and schedule information,” said the report by Fiona Scott Morton, an economics professor at the Yale School of Management. She conducted the study for the Travel Technology Association.

The study said Delta removed its fares from about two dozen travel and metasearch sites since late 2010, including TripAdvisor.com, CheapAir.com, Vegas.com, AirGorilla.com and Fly.com. The study said American Airlines and United Airlines have also reduced distribution of their fares on travel websites in recent years.

Delta and several other airlines say travelers can still find many websites that let them compare. “Delta reserves the right to determine who it does business with and where and how its content is displayed,” the airline said in a statement.

FLIERS MAY FACE DELAYS AT AIRPORTS

If your summer vacation plans include flights in or out of New York or New Jersey airports, chances are pretty good that your flight will be delayed.

A study of on-time arrival rates over the last 10 summers found that the top three airports for summer delays are Newark-Liberty International, LaGuardia International and JFK International. Each has an on-time arrival rate lower than 68 percent, according to the study by MileCards.com.

Blame the most congested air space in the U.S. and afternoon thunderstorms for the delays, said Brian Karimzad, MileCards.com’s director.

The best airports for summer on-time rates are Honolulu, Salt Lake City International and Phoenix International, all of which have on-time rates above 80 percent, according to the study.

Hugo Martin, Los Angeles Times