Why does the CDL manual recommend a driver leave at least one second of following distance for every 10 feet of vehicle length below 40 mph, adding one more second at higher speeds?

Explanation

A CDL driver should follow the one-second-per-10-feet rule (plus an extra second above 40 mph) because the vehicle a truck or bus most often rear-ends is the one directly in front of it, usually from following too closely, and a following distance scaled to the vehicle's own length and speed gives enough room to react and stop. Understanding how to manage space around the vehicle, including space ahead, is part of the required knowledge CDL applicants must have under 49 CFR 383.111(a)(10)(ii), but that paragraph only names "space cushions, e.g., controlling space ahead/to the rear" as a required knowledge topic; it does not itself state this seconds-based formula, which is standard driver-training guidance rather than a codified federal rule. The other options fail for a shared reason: each invents a source or mechanism for the rule that does not exist, whether a fixed-feet state law, an insurance documentation requirement, or an ELD-enforced violation threshold.

Source: 49 CFR 383.111(a)(10)(ii)