Intensity of in service road traffic signal lights

This paper argues that on the basis of published experimental evidence that red road traffic signal lights should have a minimum intensity of 200 cd in those directions for which certain and rapid driver response is desirable. In considering this argument a knowledge of the intensities of signals in service is of interest since the signal lights installed do control traffic reasonably well. A method of measuring in service signal intensity is described and evaluated. The sample of signals measured by this method had an average on axis intensity much less than that argued to be optimum. However the average intensity of recently installed signals is considered on the basis of laboratory investigation to be adequate but affording little or no margin of safety under exacting conditions of observation. About one third of the signals measured had intensities which would lead to a high degree of uncertainty when the signal is seen against a bright background (a).