Why does driving significantly faster than the surrounding flow of traffic increase crash risk, even if the speed itself is within the legal limit?

Explanation

Driving noticeably faster than the flow of traffic increases risk because it forces a driver to repeatedly pass other vehicles, and each pass is itself a risky maneuver; the repeated passing is also more tiring, and fatigue further increases the chance of a crash. Adjusting speed relative to surrounding traffic is part of the required knowledge CDL applicants must have under 49 CFR 383.111(a)(9)(v), but that paragraph only names "speed and traffic flow" as a required knowledge topic; it does not itself state this reasoning, which is standard driver-training content rather than a codified federal rule. The other options fail for a shared reason: each invents a consequence, blanket illegality, lost insurance coverage, or a logging-device violation, that does not actually follow from driving faster than traffic while still within the legal speed limit.

Source: 49 CFR 383.111(a)(9)(v)