Drivers are prohibited from driving, and motor carriers may not allow them to drive, when the driver’s ability or alertness is so impaired (or so likely to become impaired) that driving would be unsafe. This impairment could come from illness, fatigue, or any other cause. The only exception to this rule is when there is a “grave emergency” and continuing to drive is less hazardous to human life than not driving, in which case the driver can continue to drive to the nearest place of safety.

Note that this prohibition is not found in the Department of Transportation’s hours-of-service (HOS) rules, so even drivers who are exempt from the HOS rules are prohibited from driving while ill or fatigued.