It depends on how motivated they are. My
chickens stay in our
yard, and don't go over the fence between us and our neighbors, normally. The fence is less than four feet high. But if they are badly scared, they can and will go over it in a heartbeat! Last year, another neighbor brought his
tractor over to till our garden, and I hadn't moved the chicken tractors far
enough away. When he got close to them with the tractor, they exploded out of the chicken tractors and several disappeared over the fence into the neighbor's yard. (I got them back later.)
As long as they are happy and well-fed where they are, and have enough room to run around, they usually won't go over a fence. If they want out, though, you'll have to put a roof on to keep them in! My grandmother had one one time that she'd trimmed its wing to keep it in, then looked out and saw the hen climbing up the chicken wire fence, like a mountain climber -- hook her beak in the wire and pull herself up a little farther each time!!
Kathleen