Bava Basra Chapter 2, Mishnah 14 - the final mishnah of the chapter, and it too deals with matters of trees. This time we are not discussing a tree leaning over a neighbor's private property, but rather a tree leaning over the public domain. The question before us is: Is it permitted to cut the protruding branches?
The language of the Mishnah:
"Ilan shehu noteh lirshus harabbim - kotzetz kedei sheyehei gamal over berochevo" - A tree that leans into the public domain, one cuts it so that a camel may pass with its rider. If a person has a tree in his private property, and its branches protrude over the public domain, they may be cut when they interfere with the movement of passersby. And how much height must be left clear? The measure is that of the tallest thing that might pass through the public domain in the days of the Mishnah - a camel and a rider sitting on its back. Any branch found within this height range is cut, so that passersby can pass with their camels without interference.
The opinion of Rabbi Yehudah:
Rabbi Yehudah disagrees and says: "Gamal ta'un pishtan o chavilei zemoros" - A camel laden with flax or bundles of vine branches. The clearance is not determined by a person sitting on the camel's back, but rather by a camel laden with a load: flax or bundles of vine branches. This load is lower than the height of a riding person - about the height of one foot above the camel's back, as opposed to a sitting person, whose height is about two feet above its back.
The basis of the dispute:
The approach of Rabbi Yehudah: A person can bend down, as he sees where he is going. Therefore, only inanimate objects loaded onto the camel need clearance, while people can move themselves out of the way - and there is no permission to cut the tree in a place where a person can bend down.
The approach of the Tanna Kamma: Indeed a person can bend down, but who guarantees us that he will actually bend down? Perhaps he will not notice the branch, perhaps he will not be paying attention - and a moment later he will strike it. Therefore, clearance must be left according to the measure of a person riding on a camel.
A third approach - Rabbi Shimon:
Rabbi Shimon raises a completely different issue: "Kol ha'ilan keneged hamishkoles" - The entire tree corresponding to the plummet. Any tree leaning over the public domain must be completely cut, along the boundary line, corresponding to the plumb line literally. And his reason is not due to traffic, but rather because of tumah: if a corpse is lying in the public domain, or even an olive-sized piece of a corpse, and the branch of the tree leans over it - the branch serves as a tent, and all passersby in the street become tamei. And if they are kohanim, and do not know that they became tamei, they might enter the Temple or eat Terumah in a state of tumah.
From here Rabbi Shimon determines that trees leaning over the public domain must not be left at all, lest they form a stumbling block for the public and cause tumah. The Tanna Kamma, by contrast, holds that this scenario is extremely unlikely - how often does it happen that an olive-sized piece of a corpse or a whole corpse lies under the tree without being noticed? And because of this, there is no place to enact this decree.
In conclusion: The halachah follows the Tanna Kamma - when branches interfere with traffic, and even when it comes to a person who can bend down, it is permitted to cut them up to the measure of a camel and its rider, in order to ensure free and unimpeded passage in the public domain.