Hang on, if she is so deaf to not be able to hear you talking right besides her, how would she hear an approaching siren?
My Nan is a little deaf and once (a while ago) I was in the car with her. We were travelling down a main road with no traffic infront or behind us. Suddenly, I heard a siren behind and saw an ambulance was approaching. When it got right up behind us, I was waiting for my Nan to pull over and allow it to pass. But for some reason she didnt.
So I said to my Nan, "what are you doing?" To which she replied "Nothing, why?", "Well there is an ambulance behind you with the sirens on and beeping and you havent pulled over".
She then looked in her mirror and pulled over (slowly).
As the ambulance passed, we were met with some "WTF are you doing?" from the passenger paremedic. What followed was a long conversation between me, my Nan, and my mum (who was also in the car) about my Nan's hearing and the fact that my Nan isnt that deaf to not be able to hear an ambulance right behind her.
She now has a hearing aid (thank god) but complains about it being too loud (doesnt know where the volume control is).