Quiet cars + alternate sounds
Apparently blind people don't like hybrids... they can't see them, of course, but they also often can't hear them coming.
The idea of adding artificial sounds opens up all sorts of possibilities. Galloping hooves, as Tom suggested, but how about a well tuned big block V-8, or a Harley, or a jet engine, F1 scream, cows mooing, water rushing....