Abstract
Since September 1969, a freeway in Arizona has been equipped with a directional sign powered by Krypton 85, a radioisotope, that excites a modular light source (phosphor). This isotope, has a half life of 10.6 years, and it is expected that the sign will have a service life at least this long. The equipment is described. As Krypton 85 is expected to become a plentiful isotope in the near future, its price will make it adaptable for highway uses.