Optimizing Comprehension of Changeable Message Signs (CMS)

The goal of this research was to assist the California Department of Transportation (DOT) in optimizing the message content and presentation within changeable message signs (CMS). Optimized content will improve information transfer while at the same time minimizing the likelihood of congestion owing to slowing by motorists attempting to read the message. The research was restricted to simulated signs displaying 16 characters in each of three lines, representing permanent CMS displays, or signs containing only 8 characters in each of three lines, as is the case for portable CMS displays. While all information can be contained in a single screen for the permanent signs, multiple screens are often required for the portable CMS displays. This study specifically focused on “early vision” which is distinct from “cognitive processes”. Early vision problems are those relating to the limitations of the first several stages of the visual system. An example question of early vision is whether flashing the letters ‘NB’ on a CMS message is more effective than leaving them steadily on. For these questions of recognition, accuracy is used as a measure of intelligibility. Cognitive process limitations, by contrast, involve events that are due to thinking on the part of the observer. An example of a question involving a cognitive limitation is whether observers interpret the abbreviation ‘NB’ as meaning ‘north bound’. The project was divided into two phases, a laboratory phase and then a field test.