While vehicles have proven to be capable of safely maintaining their speed unassisted, steering themselves is a different matter. While not as common in North American vehicles, Nissan, Honda, and Toyota all offer some sort of active lane keeping function in the Japanese market. The systems actively work (through steering or bias braking) to keep a cruising vehicle in its lane.
Toyota's Lane Keep Assist is a camera-based system that watches the lane divider lines on the road and the driver's face. If the system sees that the vehicle is drifting over a line without a turn signal, it will buzz the steering wheel. If the system sees that the driver is also not paying attention (perhaps looking into the back seat), it will gently nudge the electric power steering rack, pulling the vehicle back into its lane and alerting the driver.
Of course, the driver's hands must be on the wheel for the system to work and the driver is still able to push through the system's resistance if the lane change is intentional.
Lane Keep Assist is currently available in the Toyota Prius
and the Lexus HS 250h