The actual brightness of the sign color depends on the light source. Fluorescent yellow is approved by the U.S. Federal Highway Administration (FHWA) for use on all yellow warning signs. Fluorescent reflective signs are used for extra emphasis and conspicuity at dawn and in early evening times, considered the most dangerous times of the day. Fluorescent yellow-green has been officially required in a school zone or pedestrian/bicyclist area since 1992 when the FHWA experimented with this color on five pedestrian and bicycle crossings in Washington D.C. The study results indicated that the FYG signs caught the attention of drivers more effectively than just yellow signs.