What a circuity factor tells you about a route
The ratio between how far a route actually runs and the straight line between its ends is one of the more useful numbers in transport, and almost nobody publishes it.
Circuity is the ratio of route distance to straight-line distance. A perfectly direct link has a circuity of 1.0. Nothing has a circuity of 1.0.
Typical values
Rail on our network runs between 1.12 and 1.31. Road is higher — 1.26 to 1.34 — because road alignments follow settlement patterns rather than gradients. The coastal short-sea links are the worst at 1.45 and 1.55, because a vessel has to round a headland that a train goes under.
Why it is worth publishing
Because it tells you where the flexibility is. A link with a circuity of 1.12 is close to the best that geography allows; there is no clever alternative alignment waiting to be found and no point asking for one. A link at 1.34 is a link where a different route might genuinely exist.
It also tells you something about resilience. High-circuity links usually got that way by going around an obstacle, which means the obstacle is still there and there is often no second way past it. When a 1.5 link fails, it fails completely.
Every leg on this site carries one
Open any service page and the leg table lists the circuity of each individual link alongside its distance. We would rather show the ratio than quote a round distance that hides it.