There's a connected vehicle protocol (DSRC) which broadcasts all kinds of metadata about your vehicle. There was stuff like "transaxle temperature" and "steering angle" in there, if I recall, so I'd be surprised if there wasn't also "emergency flashers engaged".

Perhaps autonomous vehicles should be be sensitive to that field, signaling for manual control when they're too near a vehicle which has it enabled. That way any vehicle--not just the fire department--can create a "human control only" field by activating their emergency flashers. Perhaps emergency vehicles could be recognized such that they create a much larger field.