FREILASSING, Germany — Traffic along one of Europe’s busiest highways, which used to flow unimpeded, now often backs up for miles at a newly installed checkpoint, where a phalanx of German police officers screens trucks and cars for hidden migrants.

At this border crossing, as a result, Austrians who work in Germany have trouble getting to their jobs. Many companies in Germany must wait days longer for deliveries of food, machine parts and other goods. Shoppers who made quick weekend jaunts to Freilassing’s stores now mostly stay away.

“It’s really bad,” said Karl Pichler, the owner of a large gardening center here in Freilassing, whose sales of tulips, rose bushes and other plants has slumped as longtime customers from Austria have stopped coming.

More than two decades after much of Europe began abolishing border controls under the so-called Schengen Agreement, the free movement of people and products between countries has helped transform the European Union into the world’s largest economy.