I worked for an old 3M chemist who happened to be the inventor of reflective paint which utilized the process of using microscopic glass bubbles in paint to reflect light, or in the case of the Apollo missions, to create the world's blackest paint by using tiny black glass bubbles to absorb sunlight. He also invented scratch n' sniff stickers which used the same concept, glass bubbles filled with scent.  This dude ultimately created the demise of all these old signs.