Relevelling is the automatic correction of car position when it drifts away from floor level while the doors are open, typically as passengers load or unload. Hydraulic lifts relevel most often because oil temperature changes cause the car to creep, while traction lifts relevel to compensate for rope stretch under changing load. EN 81-20 permits relevelling with doors open only within the door zone and at a reduced speed of no more than 0.3 metres per second. Accurate relevelling prevents trip hazards at the threshold.