A driver who gets 6 hours of sleep or less before a shift faces what increased crash risk compared to a well-rested driver, and which additional factor further raises that risk?

AAMVA CDL Manual (nhcdm.pdf) Section 2.11.2, p. 2-25

Knowing the concrete numbers behind fatigue risk, not just that 'tired driving is bad,' helps drivers make real decisions, like choosing to take a scheduled break or nap rather than pushing through on too little sleep, especially during high-hour workweeks common in long-haul trucking.