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You can camp wherever you like. I mean I wouldn't want to be right next to someone else but I've camped with other tents in sight plenty of times. Usually because there's a very obvious, very good camping spot that lots of people know about.
Depends on access codes as well, here in Scotland and in places like Sweden you're free to camp wherever you like. In England you're not technically allowed to wild camp so I guess you'd want to hide yourself away somewhere remote so you don't get caught.
Edit: when I say busy route I meant more bike/walking route, not a motorway or something
Well there's a distinction between wild camping and camping in the wilderness. You could camp in a field outside your friend's house and it would still be wild camping. Or in a layby. Going on the definition of "is it a campsite? If not it's wild camping". If it's a busy route/place I'd have no qualms about solo wild camping.