How far from a city is safe in a doomsday scenario ? How far can a migrating crowd travel?

Posted by carrot1890@reddit | preppers | View on Reddit | 177 comments

Lets say if London or Paris got nuked, how far could survivors make it on foot? drinking water issues? Or assume 0.5-1 tank of fuel as an upper limit of range? Or would there be permament gridlock? Any ideas on a safe distance that wouldn't absord the crowds.

Just a hypothetical for example for farmers.