Dunkin’ long ago discovered the business-building value of drive-thru service; 6,400 units, about two-thirds of U.S. locations, include one, says the Allegra World Coffee Report. Going forward, Starbucks, too, is betting big on the draw of window-to-window service. About 3,900 of its units have one, a number expected to grow as greater focus is placed on smaller locations with no inside seating. And why not? Customers love the convenience.