Why should you never leave a parked commercial motor vehicle unattended without applying the parking brakes or chocking the wheels?

49 CFR 383.111(b)(7)(iv) – Parking brake; AAMVA CDL Manual, Section 5, subsection 5.4.8 (p. 5-10)

An unsecured commercial vehicle rolling away, even a short distance, can injure bystanders or cause serious property damage before anyone can react. Making parking brake use or wheel chocking a non-negotiable habit every single time you step away from the vehicle is a basic, low-cost way to prevent that outcome.