This sounds like an excellent idea. There is a lingering issue about drivers not paying attention and even sleeping while their vehicle is in self-driving mode,

This sounds like an excellent idea. There is a lingering issue about drivers not paying attention and even sleeping while their vehicle is in self-driving mode, that's contrary to the requirement that the driver is actively supervising. I don't think it's a realistic expectation that drivers are always fully attentive and able to respond in time to a crash situation when the self-driving mode is active. Such a light would give me the heads up that I should stay on my toes.