DEUT1155 ... a man is required to make certain investments in his property so as to prevent it from causing physical harm to other individuals when they are within its confines. The Shulhan Arukh is quite clear and emphatic on this obligation. “It is a positive commandment for a man to make a fence around his roof, as it is written in the Torah: ‘And you shall make a fence on your roof [in order to prevent somebody from falling from the roof]” [Deuteronomy 22:8]. He who does not make such a fence disregards this positive commandment and is also guilty of violating the negative commandment ‘And you shall not spill blood in thy house’ [Deuteronomy 22:8]. And so it is with every obstacle [even in a public thoroughfare] that is liable to cause damage to the human body: it is a positive precept to remove it [stones, garbage, even seemingly harmless items] and to do so diligently. Is written in the Torah, [and you shall surely pay heed and beware.’” Choshen Mishpat, Hilkhot Shmirat Hanefesh, section 427.