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In QGIS I have a point layer of approx. 50 points around a city that represent the on ramps to- and exits off the highway. I would like to calculate the isochrones / service areas of each individual point to see where most car drivers would potentially go if they let's say take 'exit 15' or originate from when taking 'on-ramp 15'.

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Using the Valhalla QGIS plugin I'm able to create these isochrones based on travel time. However, these isochrones overlap and I would like to end up with a map that shows the distinct areas that are best served by each individual on ramp and exit. Therefore, ideally I would like to use some sort of method or algorithm that creates these isochrones gradually and stops as soon they touch other isochrones, a bit like a watershed model. Or process these overlapping isochrones afterwards to make them not overlapping. How can I achieve this?

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You can use Voronoi polygons tool under Vector geometry (or a GRASS version of the tool) in qgis to create a diagram of areas closer to each given point. It will not reflect areas that are faster to reach, though, but it will give you a rough approximation of service areas. There is a solution that calculates voronoi diagram based on travel time on roads instead of based on distance in Maptitude GIS (https://www.caliper.com/learning/media/areas-of-influence-and-network-partitions/), but it's not free. I think it might be possible to use ORS routing or QGIS service area calculation to create a custom Python script that calculates an accurate travel time based voronoi isochrones, but it's above my paygrade. If you came to know a better solution, please do let me know.

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