Purpose:The purpose of the paper was to develop original letters size methodology for roadside information. Method: During moving on the roads, addressed to drivers and passengers is, among others, text information on information boards and signs including in the form of advertisements. Based on ergonomic principles and population data, optimisation can be made in the design of communicative messages. The author reviewed the issues concerning the visual process, optimal visual field, visual field defects, visual acuity characteristics and reading speed. This made it possible to select input data for the design of text information in terms of location in the visual field and the number of words on the sign board. Findings: Based on these analyses, computational algorithms were built to evaluate a proposed or already implemented information message. The developed formulae enabled the preparation of a prototype computer application to support the analysis of information systems. Practical Implications: The developed methodology makes it possible to integrate ergonomic criteria into the design process of text information. It can increase the effectiveness of information transfer and road safety. The most important areas of application are the design or evaluation of information systems: signs and road signs with text information, advertising for drivers. Originality/Value: The author has developed a method to evaluate the design and placement of text information systems dedicated to drivers (e.g., text-information signs, advertisements including text), which is based on quantitative data such as the size of the letters, the distance from the word board, the location of the text in the field of vision, the speed of the vehicle.