This report is concerned with matters that have a direct influence on the road user in his perception of road traffic signals. It does not cover other important matters concerning traffic signals: - the location of signal lights; - the coupling of them within and between road intersection; - the regulations which include the number and sequence of colours in the system legal obligations of local authorities and the road user; - and electrical and mechanical engineering, including standardization and interchangeability. The visibility of light signals depends on many factors, the most important of which in the context of this report are the colour, luminous intensity, and luminous intensity distribution of the lights. The correct recognition of a traffic signal light may be seriously affected by sun phantom; this problem is discussed also. The recognition of figures and symbols which are used in traffic lights has become more important, and so the report examines some details of their shape and size. Some design considerations for traffic engineering are included as well. The report contains a discussion of flashing signal lights, but does not deal with flashing yellow lights used for warning only.