Commercial drivers are only allowed to work a certain amount of hours before they must take a break or have enough rest time to sleep. If a driver doesn't take this time off, it's possible that he may struggle to stay awake behind the wheel or that he will be distracted by long hours and repetitive driving.
When a driver is in charge of carrying passengers, the laws are very specific. The carrier is not allowed to work for more than 10 hours after 8 hours off duty. This means a driver can work a long day but should still have enough time to go home and sleep appropriately.
The worker doesn't have to stop working after 10 hours; he can continue to work loading vehicles or performing other non-driving duties. However, after 15 consecutive hours of work, he is not allowed to drive passengers at all, even if he had 8 hours off prior to that shift.
If the worker has been on the job for 60 hours across 7 consecutive days when the carrier doesn't operate every day of the week, then he is not allowed to continue to drive passengers after that limit. If the worker's employer operates every day, then the worker is limited to 70 hours across 8 consecutive days.
These regulations are in place to protect people like yourself who may be riding inside the vehicle or who may drive alongside a commercial vehicle. It's of utmost importance that the driver is in a healthy and alert state when he or she is behind the wheel, and these limits help to ensure exactly that.
Source: Federal Motor Carrier Safety Administration, "Hours of Service of Drivers," accessed May 13, 2016