Wow, are those transponders still that big or is that an older model? We have Good2Go here in the Seattle area, and the little RFID cards are about as big as a credit card (though a bit thinner I think).
They just started implementing them on some freeways, like I-10 (maybe 2 years ago?). They eliminated the (free) carpool lane and put this in place.
I know city workers who brag about getting their transponders subsidised and get to ride in the cool fast lane.
EDIT: The FasTrak lanes/"ExpressLanes" are actually combined with the free carpool lanes. Sorry for the confusion.
EDIT 2: This is how it works: https://www.metroexpresslanes.net/en/about/howit.shtml
The transponder and opening an accuont (both required) still costs money.
Further info: http://media.metro.net/projects_studies/expresslanes/images/...