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.