Over the past decade, the auto industry enjoyed a boom unlike any other in modern history.

Sales of new cars and trucks rose steadily from 2009 to 2016, the longest growth streak since at least before the Great Depression. Millions of Americans traded up to bigger and more sophisticated vehicles decked out in leather and outfitted with gee-whiz electronics and safety features.

But sales to individual buyers are now falling. Even once popular sport utility vehicles and pickup trucks are sitting on dealer lots for longer stretches.

Car dealers around the country said that the upgrade cycle they rode to rich profits in recent years appears to be ending and that they are seeing fewer buyers despite offering discounts and other incentives. So, dealers say they are ordering fewer vehicles from manufacturers.

“We are turning down cars and are being more picky on the cars we stock,” said Brian Benstock, general manager of Paragon Honda in New York City. “We just can’t take more. We’re full.”