When I last had a dog, I spent quite a lot of time working with him to ensure he was reliable off-leash.
It’s not something that happens by accident.
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Horribly ignorant comment that far too many people believe. I have seen two dogs who were being walked off leash crushed by cars and I'm certain the dog's owners were ‘certain’ they were reliable off leash also. Until they weren't.
Those images have never left my head and I feel obliged to relay the story any time I hear something like you posted. They are dogs and their instinct will never completely be trained out of them. And their lives and the lives they may harm are not worth the risk simply because it's inconvenient to have them on a leash in any potential dangerous situation.
I will promise you this. No matter how well trained your pet is off leash if they see something which sets them off such as a cat,squirrel or another dog, at one point they will bolt.
The only exception I have seen to this are professionally trained k-9’s and those animals are actually constantly in training by their handlers so as to re enforce what they have previously learned.
My strong advice is to NEVER take your dogs out without a leash unless they are in a securely fenced area.