In a psychophysical experiment the adequate luminous intensities for traffic lights were measured as function of time of day, colour, size and three perception criteria minimum, optimum, and maximum. By day the traffic lights were adjusted at average luminous intensities of 124 cd, 369 cd, and 616 cd for the perception criteria minimum, optimum, and maximum. By night these average luminous intensities are, respectively, a factor 35, 17 and 10 lower. By day small effect of colour and size was found. A performance level 2/1 of standard EN 12368, with a luminous intensity range from 200 cd to 800 cd fits the best on the experimental results. For dark environments a dimming factor of 10 is recommended for yellow and green lights and 4 for red lights.