Driving Deaf

I saw an interesting sight on the highway. I could see a driver in the carpool lane gesturing wildly with both hands. He was using American Sign Language, and communicating with a passenger in the back seat, and she was signing back. I assume the driver saw her responses in the rearview mirror. I doubt it's legal to drive if you're deaf, since I know it's illegal to wear headphones while driving, because you need to hear when people are angry at you and honk. I'll assume it was the passenger that was deaf. However, I would appreciate it if the driver would keep at least one hand on the wheel when I'm passing him by.