Wouldn't it be cheaper to write those words on the sign with a Sharpie?
Also how many signs are there to mark or sticker? Don't want anyone wasting their gas money to travel to mark or sticker far away signs.
Most signs have dark-colored backgrounds to discourage such vandalism. But they do make white stickers stand out nicely!
Remember that you'll need two stickers for most signs, as most signs are two-sided.