The report describes the factors hindering signal effectiveness, such as driver preoccupation with roadway congestion and automobile barriers against audible signals. Problems of signal variability are also discussed, including signal variability among emergency vehicles and a lack of uniformity among warning systems within and between communities. An analysis of warning systems outlines factors contributing to a good system, such as optimum conspicuity, environmental acceptability, and ease of use; describes the five different responses that are expected of a driver who is exposed to a warning signal; and gives the results of a limited number of tests conducted by the National Bureau of Standards on incandescent lamps and gaseous-discharge flash tubes, using the effective intensity as the important parameter. The report also describes the voluntary performance standard for emergency vehicle warning lights that is currently in preparation at the National Bureau of Standards. The standard's intent is to specify performance levels and test methods for warning light systems, irrespective of the color coding embodied in the systems. Finally, the report recommends steps that users can take to improve their emergency vehicle's visual and audible effectiveness where applicable laws and regulations permit. These include alternating color with white, using the largest bulbs possible, painting vehicles to be more conspicuous, and making certain that multiple sirens are in phase. Figures and tabular data are given; 11 references are appended.