The Night-Time Visibility of Vehicle Number Plates

Abstract

From time to time complaints and doubts have been expressed about the standard of visibility of vehicle number plates at night. This paper is the second stage of a general experimental study on the visibility of British vehicle number plates and describes tests made after dark to determine the greatest distance at which the number on a plate could be read correctly. Twenty-one observers made 270 measurements of visibility for three different types of number plates and six different illumination schemes. The results give average distances at which plates can be read correctly varying between 35 feet and 130 feet. The lower distances are given by poor illumination schemes and plates where the digits have a low luminance factor for 45° illumination and 90° (normal) viewing. Conversely, the greater distances are given by better illumination schemes and plates with digits of higher luminance factor. Plates with clear white digits have a visibility about 50 feet greater than those with shiny chrome digits and this difference is highly significant statistically. It is evident that there is an optimum (and reasonably even) illumination level of about 10 to 20 lumens per square foot for good visibility. At both higher and lower levels of illumination visibility is impaired.