The idea behind honking at a drive-thru is not that they are honking at the drivers, they are honking at the nim-wits inside who are busy chatting about last night's date, rather than working busily to prepare the meals to go.
From what I've experienced, when one honks, everybody starts honking and that typically gets the manager's attention and he puts a fire under his employee's feet.
Begging for spitburgers.
Sadly, food workers need very little encouragement to defile your food these days.