Many older adults who use assistive walking devices to improve stability and locomotion also report falls while using their device. The present study investigated how walking devices alter the perception-action system of the user. Specifically, the study assessed how walker users perceive their ability to pass through a doorway. One's ability to pass through an aperture is constrained by their widest frontal dimension (body-scaling) and the dynamic properties of the individual in motion (action-scaling). In order to compare the unique impacts of body-scaling and action-scaling, novice users of a standard walker, wheeled walker, cane, or no device (control) made static and dynamic judgments of aperture passability while their lateral motion variability was recorded. Hierarchical Linear Modeling revealed that novice users successfully scaled their passability judgments to the width of the walker, and that the introduction of movement for the dynamic judgments resulted in more conservative perceptions of passability. Unexpectedly, motion variability was not a significant predictor of passability judgments, which suggests that the self-motion produced during dynamic judgments revealed additional environmental information (rather than intrinsic dynamic information) and allowed for the application of a margin of safety. Results of this study suggest that experience using the walking device is an important factor in ensuring new users understand their action capabilities and avoid injurious collisions and falls.