Abstract: A riding vehicle, such as a lawn mower, has a flexible apron positioned behind the rear wheels with a lower edge of the apron above the ground. The mower can back up without impediment from the apron, unless an object is encountered. In that case, the object contacting the apron pushes it against the rear wheels and the apron is pulled under the tires where it abruptly snubs the tires and prevents further backing motion. To achieve this, the apron is supported by the mower through yieldable means which holds the apron above the ground until it contacts the wheels. The apron is also anchored to the mower to limit the extent of downward movement of the apron. In a preferred form, the yieldable supporting means for the apron also is resilient so that the apron can be returned to the original position after it is released from the rear wheels, as by driving the mower forwardly again. The apron is capable of stopping the mower securely and abruptly and reliably does so in a fixed, relatively short distance.