When barriers are required, such as for the uneven surfaces described above, they should meet the following criteria. Please note that even though Whiz is equipped with cliff detection sensors, the sensors may not detect downward spaces less than 6 cm (2.5 inches) in height. The barrier should be more than 8 inches tall. The barrier must be heavy and stable enough to not fall or move when hit by the machine’s front bumper. The barrier must be wide enough so that Whiz cannot enter through any gaps. The surface of the barrier cannot be reflective (e.g. mirror) or light-transmitting (e.g. glass, net, fence).