This paper presents a new vision-based navigation assistant for power wheelchairs, called SpheriCol. Inspired by the parking assist system in cars, an omnidirectional image captured by a twin-fisheye camera is overlaid with colored distance markers from range sensors mounted on the wheelchair, to generate an augmented view of the surrounding environment. Such an image is displayed in real time to the user on a screen. To evaluate the effectiveness of the proposed system, able-bodied subjects and older adults with motor impairments have been asked to drive a consumer-grade power wheelchair equipped with SpheriCol, via a standard joystick. Our clinical trials with different obstacle courses, consistently indicate that SpheriCol is effective in improving safety and comfort, and in supporting a driver's decision during challenging but prevalent maneuvers, such as reversing out of an elevator, corridor centering, and turning on the spot.