I do queue watching at the larger market, up the street

They are Huffman coders, both customer and clerks. The one clerk is running the one open line, the other is doing stock work. When the line in the first counter lengths, past three, the second counter opens. Customers segregate, the one with few items jump over to the new line, other customers preferring not to drag a large load.

This is dual Huffman queuing structure. WalMart made a science of it. Walmart owes me enormous quantities.