Model of Overhead-Sign Luminance Needed for Legibility

Abstract

Photometric models can be used to estimate the sign luminance provided for different roadway scenarios. While photometric models provide specific luminance curves, no standards or design criteria exist that can be used to define how much luminance is needed by a driver. For instance, if one desired to design a sign based on an assumed design driver and other pertinent criteria, no guidelines or models are available. An analysis has been designed to develop a model for determining the minimum luminance needed to read overhead guide signs. The model was developed based on legibility data collected for defining the minimum luminance needed to read everyday words on overhead guide signs.

Legibility Index for Examining Common Viewing Situations: A New Definition Using Solid Angle

“…He found that detail discrimination is a function of distance rather than visual area. Shlaer, Smith, and Chase (1942), Moon and Spencer (1944), Zwahlen and Schnell (1995), and Holick and Carlson (2002) developed equations for predicting visual angles subtended by legible Landolt or single-bar type targets, or text presented on traffic signs and license plate or roadway signs. Blackwell (1946) used circular stimuli varied in diameter from 3.6 -121 arcminutes by projecting a spot of light on white screen for examining contrast thresholds of human eyes.…”

Legibility of Urban Highway Traffic Signs Using New Retroreflective Materials

“…The relationship between luminances required for adequate legibility are dependent upon factors such as distance (which affects the apparent size of the characters) (Graham et al 1997;Carlson and Hawkins 2002;Holick and Carlson 2002), the luminance contrast between the sign background and the characters (Schnell et al 2004) the type of characters on the sign (i.e., letters versus symbols) (Zwahlen and Schnell 1999), and observer's age (Graham et al 1997).…”