Hours of service regulations limit how much time a driver can spend behind the wheel. As an example, a trucker cannot drive more than 60/70 hours in a 7/8 day period. There are other rules that limit the amount of driving in a single day and mandate a half-hour break after eight hours.
Federal law also prevents truckers from using a hand-held cell phone while driving. Hands-free devices are allowed, though some truckers push the limits or use illegal devices.
Other laws regulate the trucks themselves. For example, Georgia sets height, width, and length limits for commercial vehicles. These rules apply to both the tractor and the trailer.