What do federal regulations and the CDL manual require for securing cargo with tie-downs on a flatbed trailer?

49 CFR 383.111(a)(16); 49 CFR 393.106(d); AAMVA CDL Manual (nhcdm.pdf) Section 3.3.2, p. 3-2

Under-secured cargo on a flatbed can shift or fall off during normal driving, a quick stop, or a crash, endangering both the driver and other road users. These specific numeric rules, one tie-down per 10 feet, at least two minimum, and half the cargo's weight in working load limit, exist because "a couple of straps" is often not actually enough to hold a load safely.