What is one of the main responsibilities of employees regarding vehicle usage?

The primary responsibility of employees regarding vehicle usage includes monitoring passenger conduct. This duty is essential to ensure the safety and security of everyone involved, including passengers and crew members. By overseeing passenger behavior, employees can help prevent potential issues that could arise during transit, such as disruptive actions or unsafe behavior that could affect the vehicle's operation or the safety of individuals on board.

The other options focus on aspects that, while important, do not encompass the broader responsibility that employees have in ensuring a safe environment during vehicle usage. For instance, reporting vehicle issues is vital but is a reactive measure rather than a proactive one. Driving at all times is not practical as it does not account for the need to manage passenger conduct actively. Filling out maintenance logs is part of vehicle upkeep but doesn't directly relate to the overall safety and management of passenger behavior during vehicle operation.