A stair-climbing wheelchair having a sensor at the front end thereof for sensing the presence of a stair step or an inclined ramp. The sensor emits signals which reflect from a surface and to a receptor in the sensor, the receptor being coupled to a microprocessor which determines not only the linear distance traveled by the wheelchair in a forward direction, but also the height of a stair step. Thus, when the tracks are down, a microprocessor associated with the sensor will shut off the drive motors to the tracks if the slope of the stairway is too great when the tracks are down. In such a case, the wheelchair backs away from the stairway and does not go down it. If the slope is less than a predetermined value, such as 30.degree.-35.degree., the wheelchair can move down the stairway in the normal fashion.