There's interesting work on queuing theory. In practice, each shopper picks the queue that she thinks is least loaded. How many queue lengths do you need to sample? It turns out that if you sample just two queue lengths, you get a huge benefit (in terms of reducing the avg queue length), but not so much if you sample more.
See "Power of two random choices": http://www.eecs.harvard.edu/~michaelm/postscripts/handbook20...
EDIT: link