This document, drafted in collaboration with the Permanent International Association of Road Congresses (PIARC) is a Guide concerned with photometric and colorimetric matters that are related to the design of road traffic lights. Its main purpose is to provide a basis for relevant national standards and codes of practice, but it should also be of direct use to road traffic authorities and manufacturers of the lights. It is derived from the Technical Report CIE 048-1980 (TC-1.6) Light Signals for Road Traffic Control, which contains the experimental evidence on which the Guide is based. The guide has taken account of more recent findings, but essentially it is supplement to CIE 048-1980. The visibility of a signal light depends on many factors, the most important of which are the colour, luminous intensity and luminous intensity distribution of the light. The Guide deals with these factors as well as with various symbols and "cut-out" figures that are used with traffic lights.