A realtime path modification algorithm based on user walking ability and intent is discussed in this paper. This path planning method is being tested by a passive-type mobility aid device called Omni RT Walker-II (ORTW-H) which its motion is exclusively generated by its user, while its controller constrains undesired motion by control of four servo brakes at each wheel assembly. The Potential Canal is an accessible area of the environment in which user-walker motion is constrained. Fall prevention is the first factor which is considered in realtime modification of the Potential Canal. User intent in modifying the accessible are is being interpreted by a force/moment sensor and is used for realtime modification of the Potential Canal. Experimental results by ORTW-II are provided and discussed.