Summer travel season — and the stress that comes with it — is upon us. 

When you’re crammed into one of those increasingly shrinking airline seats, a flight can seem like a hellish eternity. In fact, flights actually are longer than they were 50 years ago.

As Business Insider mentions, today a nonstop flight from New York City to Houston takes three hours and 21 minutes, while the same flight, according to a historic flight database, would have taken two hours and 37 minutes in 1973.

Given the myriad technological advancements made in the last half century, what’s with the lag time?

It comes down to one major factor: Fuel efficiency.

Gas prices have fluctuated considerably over the decades. In 1965, retail price for a gallon of gasoline was $1.83 (adjusted for inflation) compared to $2.45 in 2015, according to the U.S. Office of Energy Efficiency and Renewable Energy. Considering the laws of physics as well, it’s no wonder why airplanes are choosing to fly planes slower.

As Slate notes:

By the laws of physics, drag is approximately proportional to the square of the speed, so even a slightly faster flight requires a lot more fuel. Hiking a plane's velocity by 10 percent takes 21 percent more energy. Speeding up by 40 percent approximately doubles fuel consumption. Shorter flights can save airlines money on labor, but not enough to offset the loss in efficiency.

According to a 2008 report by the Associated Press, JetBlue saves approximately $13.6 million a year in jet fuel by merely adding an average of just under two minutes to each flight, and Northwest Airlines saves about $600,000 by adding just four minutes solely to its flights to and from Hawaii.

So the next time you’re lamenting your lengthy flight, consider that airlines, like many consumers, are trying to save a buck when it comes to gas.

[h/t Business Insider]