Sirens are primarily a means to get you to look in a direction. 360° cameras can notice the emergency vehicle as soon as it's visually relevant. And if they decide they need an audio siren detector, that's like, practically intern level signals detection at this point. Hardly a dealbreaker.
100% hands-down would pick autonomous car vs airplane. Flying a plane isn't just moving the aluminum bird through 3 space and periodically taking off or landing.
Autonomous aircraft don't exist because a huge amount of the ritual of flight is before and after the captain is even on the plane, let alone flying. There is a tremendous amount that the pilot and copilot go through, on the ground, before taxi, after liftoff. It's way, way more involved and way more generally intelligent. We can design AI to take off, path to a destination, and land. Those 3 things are the easiest parts of flying, yet do not comprise the act of flying a 2-seater, let alone an airliner.