The positive mitzvah above requires us to erect a parapet on a flat roof in order to prevent falls. Our current mitzvah is its negative component: a prohibition against leaving hazards lying around exposed.
The reason for this is what we explained in the previous mitzvah. People will always have accidents; that’s just the way of the world. But determining such things is God’s job. Our job is not to be a “carrier” of such mishaps. Accordingly, we must secure potential hazards so that no misfortune occurs through our neglect.
This mitzvah applies in all times and places. It is discussed in the Talmud in Baba Kama (51a), Chullin (136a) and elsewhere. It is codified in the Shulchan Aruch in Choshen Mishpat 427. This mitzvah is #298 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #190 of the 194 negative mitzvos that can be observed today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.