I like the idea of keeping a copy of your doctor/audiologist's diagnosis/letter/note/whatever along with everything else. I've never considered that, and it sure is a great idea! I recall a couple stories/jokes about hearing people trying to evade cops by pointing to their ears and shaking their head. Providing the cop with proof of deafness is could be a good way to stop wary cops from doubting your deafness!
 

The Kentucky Commission on the Deaf and Hard of Hearing has a great sunvisor card for the Deaf/HOH/Oral Deaf that they developed in conjunction with the Kentucky State Police. I would highly recommend that you obtain a copy of it, it is a fantastic alternative that has picture-type signs on it that both the deaf citizen and the Police officer can point to on one side, and a very detailed message much like the one contained on this one on the other side.