RadiusMapperDefinition
Isochrone
Also known as: Travel-time polygon, Drive-time polygon, Reach polygon
An isochrone is a line connecting all points reachable within the same travel time from a single origin. The enclosed polygon shows the full reachable area.
An isochrone (from Greek isos, equal, and chronos, time) is a line on a map connecting all points reachable within the same travel time from a given origin. In practice, the term usually refers to the polygon enclosed by the line — everywhere you can get to within that time. Isochrones are the mathematical foundation of drive-time maps, service areas, delivery zones, and most trade-area analysis. Unlike a distance circle, an isochrone reflects real-world constraints (road networks, traffic, one-way streets, pedestrian infrastructure, elevation) and therefore produces an irregular shape.
History and context
The earliest known isochrone map is Francis Galton's 1881 'Isochronic Passage Chart' showing travel times from London to the rest of the world in days. Similar time-to-reach maps appeared in 19th-century railway planning and early 20th-century urban planning. The modern isochrone — computed on a road network graph rather than hand-drawn — emerged with Dijkstra's shortest-path algorithm in 1956 and became widely available once routable map data (OpenStreetMap, commercial road networks) could be processed by consumer hardware in the mid-2000s.
Key characteristics
- Calculated from a single origin point plus a time budget (e.g. 30 minutes).
- Shape reflects actual road and path networks, not straight-line distance.
- Can be computed for any transport mode: driving, walking, cycling, transit.
- Traffic-aware isochrones change throughout the day.
- Multi-threshold isochrones render concentric rings (5, 10, 15 minutes) to show decay of accessibility.
Common use cases
- Real estate commute radius searches
- Retail and restaurant site selection
- Delivery zone planning
- Emergency-service response analysis
- Urban accessibility and walkability studies
- Academic research on transportation equity
How it compares to related terms
Frequently asked about isochrone
What is the difference between an isochrone and a radius?
A radius is a straight-line distance — a circle. An isochrone is a travel-time polygon — it follows roads, paths, and transit networks. The same 15-minute isochrone will usually be smaller than a 15-mile radius circle, and its shape will be irregular, extending farther along highways and contracting in dense urban grids.
How are isochrones calculated?
Isochrones are calculated on a network graph where each road, path, or transit link has an attached travel time (factoring speed limit, traffic, grade, turn restrictions). Starting from the origin, a shortest-path algorithm expands outward until the time budget is exhausted, then a concave hull is drawn around every reachable node. Modern isochrone APIs do this in under a second for most urban areas.
Are isochrones accurate during rush hour?
Only if the isochrone engine uses time-of-day traffic data. Without traffic, a 30-minute isochrone assumes free-flow speeds and will over-estimate reach by 20-40 percent in dense metros during commute hours. Traffic-aware isochrones (RadiusMapper's default) apply historical or real-time speed profiles per road segment.
Can isochrones overlap when drawn from multiple origins?
Yes, and that overlap is analytically useful. Two overlapping 30-minute isochrones from competing restaurants show the shared catchment where both compete for the same customers. Multi-origin isochrone analysis is how retail strategists detect market saturation and expansion opportunities.