A driver is operating a 60-foot rig at 50 mph. Per the CDL manual's following-distance rule, how many seconds of following distance should be maintained?

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

Highway speeds are exactly when stopping distances are longest and following-distance mistakes are most dangerous, so the manual builds in an extra margin above 40 mph on top of the length-based formula. Applicants need to apply both parts of the rule together, vehicle length and current speed, not just one or the other, which is why this is a frequently tested combination on the real exam.