I don't know why you're getting downvoted. I think watching highway death numbers is an excellent way to judge the success of self-driving cars.

At a minimum a self-stopping car should be the basic requirement, such that any vehicle which senses lack of input from the driver, or crazy input which is differential to the experience of its sensors, should be able to safely turn on a hazard alarm for all other surrounding vehicles and safely stop itself on the nearest shoulder.

If all sensors indicate that there is no emergency obstacle requiring sudden swerving or braking or what not, the car should be able to safely decelerate and stay in a lane etc... but this might be to complex a problem. (I was thinking if a driver may have a seizure or some other episode - but if a driver is under duress this may be a bad thing)