Which of the following is NOT an indicator for en-route inspections?

An important aspect of driving a tractor-trailer is conducting en-route inspections, which help ensure the vehicle remains in a safe operational condition throughout the journey. Each of the scenarios listed is a common practice for when drivers typically perform these inspections to check for issues like tire pressure, load security, and overall vehicle condition.

At nightfall is not specifically an indicator for conducting an en-route inspection. While it may be a good practice to be more vigilant when visibility decreases, there is no established guideline that mandates an inspection strictly based on the time of day. In contrast, inspecting the vehicle after the first 100 miles is a common recommendation to check for any issues that may have arisen during that period of driving. Similarly, conducting inspections every couple of hours allows for ongoing monitoring of the vehicle's status to ensure safety, and changing drivers often prompts an inspection to familiarize the new driver with the vehicle's condition and ensure it is still roadworthy.

By focusing on specific conditions related to vehicle operation and the need for safety checks during driving rather than time periods that do not directly relate to vehicle health, the practice of en-route inspections can be effectively understood and applied.