Abstract
In a previous communication we reported data supporting the recommendation that a red road traffic signal should have an intensity of 200 cd for optimum from 100 m. when the signal is seen against a very bright sky (10cd/m). This confirmed the earlier result of Boisson and Pages. The present paper extends the data to include a-the effect of signal size on optimum signal intensity for a practical range of angular diameters (4,1 to 16,5 min of arc), and b- the effect of background luminance for a range of luminance's of 1.5 ft-L to 2250 ft-L.