Routing

Routing is the calculation of a path between two or more points across a transport network, along with how long that path takes. It is the engine underneath most accessibility work: catchment areas, isochrones and travel times are all routing results, presented differently.

A routing calculation needs a network with costs attached. The network is the set of streets, paths and transport links. The cost is what you are minimising, usually time, sometimes distance, sometimes something composite like a comfort or safety weighting. Change the cost and the best route changes, which is why a cycling route optimised for speed and one optimised for safety are different routes.

Each mode brings its own rules. Walking can use steps and cut through a park. Cycling depends on gradient and on infrastructure. Cars follow one way streets and turn restrictions. Public transport is not really a network problem at all but a timetable problem, because the link only exists when a service runs on it.