In one aspect, a method of determining a risk of conflict between a movable device and potential obstacles is provided. The method includes dividing a space into a plurality of positions. At each of a plurality of successive times, the method further includes: determining a conflict function for the movable device in a first position at a respective time, the first position having one or more neighbouring positions, wherein the conflict function is determined based on whether or not an obstacle is present in any of the first position and the one or more neighbouring positions; and determining a respective risk value for at least one of the first position and the one or more neighbouring positions using the conflict function and a risk value associated with a second position, wherein the movable device is planned to move from the first position to the second position at a subsequent time.