A motorized device for transporting a human or inanimate payload. A platform is configured to accommodate the payload and a pair of wheel clusters are mounted to the platform at opposite ends thereof and are powered in both rotation and in translation relative to the platform, for example by one or more electric motors housed in or on the platform or wheel clusters. Each of the wheel clusters comprises an arm, and a first and a second wheel rotatably mounted to respective opposite ends of the arm. Each of the wheels is independently powered about its respective axis of rotation. An electronic controller commands the motors to enable stable rolling movement of the device over a surface with only one wheel of each cluster in contact with the surface, and to allow the device to steer and to ascend and descend steps.