[tjohns]

Here in the bay area, they give you a mylar/antistatic bag with new FasTrak transponders for precisely this purpose. Put the transponder in the bag, nobody can read it.

(Also useful if you're using the new FasTrak-enabled carpool lanes in the east bay, when you actually have a carpool quorum and therefore are exempt from the toll.)

[mrb]

An on/off switch on the FasTrak transponder would be a lot more convenient than taking it out of or into a mylar bag... I have considered modifying mine with a switch.