I've always been interested in how to make mass transit (air
transport, railways, subways, buses and trams, etc.) more friendly --
not sure if it can be classified as IxD or usability, but information
visualization and industrial design are definitely playing a major role
here.
Everyone knows the ubiquitous London Underground Tube map example,
which I've read a lot about, I also have some books about other
underground maps and signage designs for airports, etc., but it's far
too little to keep me satisfied.