Federal regulations limit an 18-wheeler driver to 11 hours of driving per day and 77 hours per week. However, surveys have found that many 18-wheeler drivers violate these regulations.

McCartt, A.T.; Hellinga, L.A.; and Solomon, M.G. 2008. Work schedules of long-distance truck drivers before and after 2004 hours-of-service rule change. Traffic Injury Prevention 9:201-10.

So when you are traveling on the highway, you should not assume that the truck driver in the 18-wheeler or other large truck is rested and alert. 11 hours of driving is a long time in a good luxury vehicle. Imagine driving an 18-wheeler that long.