Using the CDL manual's following-distance rule, how many seconds of following distance should a driver keep behind the vehicle ahead when driving a 40-foot truck at 35 mph?

49 CFR 383.111(a)(10); AAMVA CDL Manual (nhcdm.pdf) Section 2.7.1, p. 2-16, Figure 2.12

Following too closely is the single most common cause of the crashes large vehicles have with the vehicle ahead, according to the manual's own accident-report discussion. The length-based seconds formula gives drivers an exact, repeatable way to judge safe following distance for their specific vehicle, rather than relying on a vague 'few car lengths' habit that doesn't scale to an 80,000-pound rig.