I would say print Ecomodder Signature Image (like this one:
) ten to twenty inches in size and put it on window or door of your car. it even does not need to be a foil if it is from inside (glued by white glue or even egg white)
iirc using any light device with moving picture is banned (at least here is) as is considered hazard. And average consumption usually settles down so you will be giving accurate info, to some extent, anyway