A spine is a really powerful network design idea that takes a moment to explain. This is how a spine works, in an example from the Dublin bus network redesign proposal.
[That diagram is by Dublin-based graphic designer Kevin Carter, and uses a style common in the UK. The National Transport Authority has hired Kevin to complete these diagrams for the other six spines. If you’re on Twitter, follow him at @yascaoimhin.]
A spine is several bus lines designed to share a common segment, with the buses evenly spaced on that segment to deliver a very high frequency. In this case, each spine branch runs every 15 minutes all day, so the common segment is every 3.75 minutes on average.
If you are in the inner city, where all the spines are running on their common segment, you just say “take any bus whose number starts with A”. The result is a high-frequency network map that’s easy to draw a map of, and to learn, remember, and explain.
(That image is ours, from the summary report.)
In the case of the A spine, all four branches are every 15 minutes all day so the common segment is a little better than every 4 minutes all day.
The National Transport Authority also did an animation, here.
Many, many cities have a geography where this structure makes sense. As you move out from the centre, the area to be covered gets wider but the frequency need gets lower, so you branch. But you make it legible. The inner city needs an extremely frequent line that’s easy to learn and remember, so we just explain that the A-spine is made of all the buses whose numbers start with A. Presto. You have a simple network of inner-city lines where the bus is always coming soon, exactly what people moving around in the core need.
Once you understand it, it’s simple. But it takes a moment to learn, and different people learn it differently.