A routing network turns a road map into answers. Given a set of roads, you can find the shortest path between two points, the nearest hospital to an incident, or the area reachable within ten minutes of a fire station. QGIS has the tools to do all of this, and OpenStreetMap provides the road data free.

Start by downloading road data for your area. The QuickOSM plugin can fetch roads directly into QGIS, or you can download an extract from Geofabrik and load the roads layer. Filter to the roads that matter for your analysis, usually all drivable roads, and check the attribute table for one-way tags, road types and speed limits, because these decide the quality of the result.

Build the network with the Processing Toolbox. The network analysis tools need a line layer with connected geometry, so check that the roads actually connect at intersections. OpenStreetMap data usually does, but bridges, tunnels and split roads can create gaps, and a gap in the network is a wall that no route can cross. The topology checker can find disconnected segments before you waste time on a broken analysis.

Run a shortest path analysis between two points, and QGIS will return the route as a line with a length. Change the cost from distance to time by dividing length by speed, and the same tool gives you the fastest route instead. The service area tool works the other way: give it a starting point and a cost limit, such as ten minutes, and it returns the area of the network reachable within that limit, which is the basis for coverage maps and isochrones.

Keep the limits in mind. OpenStreetMap road data is excellent in cities and patchy in rural areas, and one-way tags and turn restrictions are not always complete. Speeds are estimates unless you set them from local knowledge, so a ten-minute service area is an approximation, not a guarantee. Validate the result against a known route before you present it, and document the data date, because a routing analysis is only as current as the map it runs on.