The document addresses how to define and measure effective seat belt webbing length for buses of category M₂/M₃. It proposes defining effective length by pulling the lap belt at the geometrical midpoint under a 60-degree angle to form an equilateral triangle, where the leg on the buckle side defines 50 percent of pursued effective length for 2-point belts and 25 percent for 3-point belts. Additional webbing for attachment, handling, connection to spool, and spool length must be added based on seat design and geometry. The proposal suggests the definition may work using diagonal belt alone in the ghost position and questions where to include it in UN R173.