The signal I usually give is that when I arrive at a line of (understaffed) counters, with all of them having long queues, I turn around with my basket or car

The signal I usually give is that when I arrive at a line of (understaffed) counters, with all of them having long queues, I turn around with my basket or cart, put everything back on the shelves, and leave. I do that if I it's clear that I have to wait more than a few minutes.