Anything other than vision from cameras for self-driving is the wrong way to approach it. The vision capabilities of these cars has to be advanced enough to recognize and respond to the unknown and unexpected, to work even where roads have changed overnight, to work in adverse conditions where it's hard to see, etc. Recognizing emergency vehicles is very far down on the list of difficult things to do, and is not difficult at all compared to many other challenges.

Having transponders is just another thing to do fail, or to forget to turn on, or to be impacted by other things, or to give false positives, or to be abused by drivers, etc. Really nothing can be relied upon other than what you see.