Bumper Sticker Mystery

Ok, ever since I was a kid I've seen these little square bumper stickers placed on the driver's side of the rear bumper of cars around Chicagoland. It's a small green square with the letters EP in green and the background of the sticker is white. I know it's not "Enterprise" (like they bought the car form Enterprise rental car) and other than that I have no idea what it means. It never changes color so I doubt its a parking sticker for, say, the Elmwood Park train station or something. I still see them on cars today - the same as I saw them years ago - and I've never seen them anywhere else but Chicagoland. Any idea?