Abstract
Optimum intensities for a red road- traffic signal light have been determined for observers with normal and protanopic colour vision. The term optimum intensity is used to mean the intensity necessary to minimize the chance of not seeing the signal and also the time of response. Protanopic drivers would require at least a 600 cd intensity. Surround screens were shown to improve the visibility of a red signal for normal observers only when the intensity of the signal was less than optimum.