Isochrone

An isochrone (from the Greek words for 'equal' and 'time') is a geographic boundary that encloses all locations reachable from a central point within a given travel time threshold. On a map, isochrones appear as irregularly shaped polygons that follow the road network, expanding along major highways and contracting where road connectivity is sparse. They provide a far more accurate representation of accessibility than simple distance buffers.

Isochrone calculation requires a routable network dataset (road centerlines with speed attributes) and a network analysisNetwork AnalysisNetwork analysis in GIS models and solves problems on linear networks such as roads, utilities, and waterways. It ena... algorithm. Starting from the origin point, the algorithm traverses the road graph, accumulating travel time along each segment based on speed limits, road class, and optionally real-time traffic conditions. It marks all network nodes reachable within the specified time budget and constructs a polygon boundary around the reachable area. Modern routing engines—including those from Esri, MapboxMapboxMapbox is a robust platform that equips developers with tools to create highly customizable, interactive maps for web..., HERE, and open-source projects like OSRM and Valhalla—can generate isochrones in milliseconds.

Isochrones can be calculated for multiple travel modes: driving, walking, cycling, and public transit. Each mode uses a different network and speed profile, producing dramatically different accessibility shapes. A 15-minute driving isochrone might cover a 10-mile radius along highways, while a 15-minute walking isochrone covers less than a mile. Analysts typically generate concentric isochrones at multiple intervals—5, 10, 15, and 20 minutes—to define primary, secondary, and tertiary accessibility zones.

In retail site selectionSite SelectionSite selection is the analytical process of evaluating and choosing optimal physical locations for new stores, facili..., isochrones define realistic trade areas that account for road network structure and traffic conditions. A site along a highway interchange will have a larger isochrone than one on a dead-end road, even if both are the same straight-line distance from population centers. Healthcare planners use isochrones to ensure that hospitals and clinics are within acceptable travel times for the populations they serve. Emergency services evaluate response time coverage using isochrones. Urban planners assess the walkability and transit accessibilityTransit AccessibilityTransit Accessibility measures how easily people can reach destinations using public transportation, considering fact... of neighborhoods by generating walk-time and transit-time isochrones.

Isochrones provide an intuitive, visual representation of how accessible a location truly is. They reveal asymmetries in connectivity—a location may be easily accessible from the north but poorly connected to the south due to a river or limited road crossings—that distance rings completely miss.

Isochrone accuracy depends on the quality of the road network data and speed profiles used. Traffic conditions vary by time of day and day of week, meaning a single isochrone may not represent typical accessibility. Multi-modal isochrones (combining driving and transit) are computationally more complex and require integrated network datasets. Isochrones are a cornerstone visualization and analysis tool in location intelligence. By translating travel time into geographic area, they provide the realistic accessibility context that underpins sound site selectionSite SelectionSite selection is the analytical process of evaluating and choosing optimal physical locations for new stores, facili..., service planning, and market analysis decisions.