Baseado (sob a licença da Creative Commons) no vídeo de Bill Hammack (Engineer Guy) - "Why the other line is likely to move faster" -http://www.youtube.com/watch?v=F5Ri_HhziI0

Bill Hammack - http://www.engineerguy.com/
Department of Chemical & Biomolecular Engineering - http://chbe.illinois.edu/
iFoundry - http://ifoundry.illinois.edu/

Outras fontes:
http://possiblywrong.wordpress.com/2010/12/26/why-multiple-checkout-lines-are...
http://www.nytimes.com/2007/06/23/business/23checkout.html
http://blog.mrmeyer.com/?p=4646
http://pt.wikipedia.org/wiki/Agner_Krarup_Erlang