The surface level sensor continuously scans the surface of road ahead and reacts if there a considerable change in the level height.
For example usually the sidewalks are made higher than the street level. The surface level sensor will detect the difference in the surface level and will provide the incoming information to the main computer to react accordingly in selecting the optimal path of travel to avoid colliding with the side walk, or to keep staying on the sidewalk, and also to prevent falling from steps.
