Travel flow aggregation: Nationally scalable methods for interactive and online visualisation of transport behaviour at the road network level

Origin–destination datasets representing millions of travel desire lines and routes are common in transport planning, but visualising such datasets is challenging. Existing methods often produce illegible results, low spatial resolution, or only a relative indication of the variation of flow on each road. This paper presents a new open-source algorithm called overline along with an accompanying method, to efficiently convert disparate geographical transport data into a policy-relevant summary form. Specifically, overline aggregates many individual routes into a route network map. These vector and raster maps provide total flow counts for each road and junction and are scalable to regional or national datasets. The method is demonstrated by visualising four million routes for a publicly accessible web mapping application, the Propensity to Cycle Tool, across the whole of England and Wales.