Richard is correct.. some years ago I prepared a set of Avoidance Polygons for a trucking application in the NY-CT-NJ tri-state area... somehow just using intersections worked properly because of the way the on/off ramps were coded in the street network database, when a major road needed to be avoided, there was always an unconnected crossroad that could pass freely underneath. I still attribute it partly to luck, I'm not sure it would work in other states such as my home state IL where trucks are not allowed on many streets.

Eric

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