Abstract: A method and apparatus for determining the presence of a burned out lamp of a traffic signal is disclosed. The current applied to one or more banks of traffic signals and the voltage applied across such traffic signal banks are measured simultaneously. An electrical characteristic, such as impedance, of such traffic signal banks, is measured for each pattern of switch settings which control lamp illumination for signalling as they occur. Such characteristic is stored for each of the unique patterns which are repeated over and over again to control traffic. Such previously stored characteristic provides a baseline characteristic for each successive pattern of illumination of the lamps. With each pattern change, such characteristic is measured and compared with the previous baseline characteristic for the pattern to determine if a lamp has burned out.