What is the primary function of a logbook for interstate drivers?

The primary function of a logbook for interstate drivers is to serve as a daily record of driving hours, off duty time, and sleep time. This is crucial for ensuring compliance with federal regulations that govern how long drivers can operate their vehicles without rest, ultimately promoting safety on the roads. By accurately logging hours of service, drivers can avoid issues related to fatigue, reduce the risk of accidents, and maintain legal limits to ensure they do not exceed the allowed driving time within designated periods.

Furthermore, this log serves as a vital tool for enforcement agencies and can be audited to ensure compliance with regulations such as the Federal Motor Carrier Safety Administration's hours-of-service regulations. Keeping a thorough and accurate log is essential not only for the driver's well-being but also for the overall safety and legality of transportation operations in the interstate commerce environment. In contrast, the other choices focus on different aspects of transportation management—like inventory, maintenance, and financial records—none of which align with the core regulatory purpose of the logbook itself.