Why is it necessary to block the wheels of a trailer that will be left for any length of time?

Blocking the wheels of a trailer left unattended is essential to prevent runaway situations that can occur when air drains from the braking system. Most tractor-trailers are equipped with air brakes, which rely on compressed air to operate. If the system loses pressure, the brakes may release, allowing the trailer to roll away. By blocking the wheels, you create a physical barrier that prevents any movement, thus ensuring safety and stability until the trailer is ready to be moved again.

While ensuring stability, preventing theft, and complying with regulations are important considerations in their own right, the primary concern in this scenario is directly related to the integrity of the air brake system and the inherent risks associated with leaving a vehicle unattended.