Abstract: The performance of an interactive virtual crowd system for entertainment purposes can be greatly improved by setting a level-of-details (LOD) strategy: in distant areas, collision avoidance can even be stealthy disabled to drastically speed-up simulation and to handle huge crowds. The greatest difficulty is then to select LODs to progressively simplify simulation in an imperceptible but efficient manner. The main objective of this work is to experimentally evaluate spectators' ability to detect the presence of collisions in simulations. Factors related to the conditions of observation and simulation are studied, such as the camera angles, distance to camera, level of interpenetration or crowd density. Our main contribution is to provide a LOD selection function resulting from two perceptual studies allowing crowd system designers to scale a simulation by relaxing the collision avoidance constraint in a least perceptible manner. The relaxation of this constraint is an important source for computational resources savings. Our results reveal several misconceptions in previously used LOD selection functions and suggest yet unexplored variables to be considered. We demonstrate our function efficiency over several evaluation scenarios.