DRAFT MANUSCRIPT Title: Legibility of Text on Traffic Signs as a Function of Luminance and Size

This study investigates on a quantitative basis two strategies that can be used to increase the legibility distance of retroreflective traffic signs with negative luminance contrast legend or symbols at night. The first strategy involves the increase of the letter, numeral, or symbol size associated with a corresponding increase in the stroke width (representing the visual detail) and with an increase in the traffic sign area. The second strategy involves the increase of the traffic sign background luminance. The study also investigates the increase in the legibility distance when the sign legend/symbol size and the sign background luminance are simultaneously increased together. Further improvement of traffic sign legibility is needed to meet the needs of drivers with degraded visual capabilities, such as elderly drivers. A computer model has been developed to determine the legibility distance of a negative luminance contrast traffic sign with a dark legend on a bright background at night as a function of letter height (or stroke-width), sign background luminance, driver’s age and other factors. Looking at the legibility distance results based upon a negative luminance contrast traffic sign with a letter height to stroke-width ratio of 5 and an observer 50 years of age, the legibility distance increases by 13.2% (from 91.04 m to 103.08 m) if the background luminance is doubled (from 50 cd/m to 100 cd/m). Doubling the letter height (from 17.78 cm to 35.56 cm) or doubling the associated stroke-width results in an increase of roughly 100% (from 91.04 m to 182.09 m) in the legibility distance. Doubling both simultaneously, the background luminance (from 50 cd/m to 100 cd/m) and the letter height (from 17.78 cm to 35.56 cm), yields an increase of 126.5% (from 91.04 m to 206.20 m) in the legibility distance. Doubling the corresponding sign area results in an increase of 41.6% (from 91.04 m to 128.87 m) in the legibility distance. Doubling both simultaneously, the background luminance (from 50 cd/m to 100 cd/m) and the corresponding sign area, yields an increase of 60.2% (from 91.04 m to 145.82 m) in the legibility distance. For a given set of constraints, such as costs for sheeting materials of different retroreflectivity, costs for sign substrates, sign posts and/or supports for different sign sizes, etc, the user may determine with the help of the results of this study which strategy (sign size increase or background luminance increase) or which mix of the two strategies provides the most cost-efficient approach to increase the legibility distance.