[NyxWulf]

Queueing theory is a prominent topic in Operations Research. If you want to read a detailed book on it, Fundamentals of Queueing theory by Thompson et al is the bible.

A couple of additional thoughts, pooling queues like this only works if the changeover time to process different types of things is low. So in the case of a grocery store where there is no separate changeover or batch setup time it works well. If you are pooling work to people though it breaks down if only certain people can do certain types of work or the change over time to switch to a different task exceeds a certain threshold.

The other thing is the title of this post is almost certainly incorrect. Switching to a single queue reduces average queue time, but it does not reduce work time. In order to be 3x faster you would have to reduce both. The other factor at play is that people continuously rebalance from slow lines to fast lines, which also helps mitigate some of the problems.

* some grammatical edits

[swalberg]

I'd also recommend some of the books by Neil Gunther at http://www.perfdynamics.com/, notably the PDQ one and Guerrilla capacity planning.

He takes queuing theory down to a slightly non-academic level and applies it to modelling computer systems such as web applications. From that you can fit observed data to your model and predict capacity on shorter cycles.