I'm guessing it doesn't have anything to do with hearing, but rather with the inner ear's role in equilibrium and sense of balance. My grandmother has inner ear problems that make her prone to falling or briefly passing out, and there's been discussion in the family about getting her to stop driving because of this.
Thanks for the explanation... although it still strikes me as odd why balance problems would effect one's ability to drive. Deaf people have balance problems, too, they just have had longer to adjust to them.