This is why having one line for multiple checkouts (like tellers at a bank) is more efficient than multiple lines for multiple checkouts (grocery store). People are inherently bad at making queuing choices. Do I pick the line with three customers waiting, but few items per customer, or the line with one customer who has many items? There's no good way to know, because too many factors are at play. Creating one line, and having people go to whichever register is open based on their initial queue creates efficiency and gets rid of that bad feeling you experience when someone enter a line after you, but is checked out earlier.

I read about all this in a study, but can't for the life of me remember where. Probably Reddit.