# Does the Driver Understand the Warning? Comprehension of the Request to Intervene
## 1 Introduction  
Proper comprehension of signs has been found to be a significant factor for traffic safety improvement [1, 2, 3]. Due to the complexity of road information, there is a strong need to support the driver with homogenized pictorial messages [4]. What is more, such messages need to meet numerous conditions to support the driver effectively. One of these conditions is comprehensibility, recognized as one of the key requirements in the usability-based approach [5, 6]. In this context, comprehensibility, sometimes described as guessability, is a measure of cost to the user of a product when performing a task for the first time (the cost stems from uncertainty about how to do the task correctly). The higher the guessability, the lower the cost [6]. With the fast development of vehicle technologies, including powertrains and advanced driver assistant systems (ADAS), instrument cluster interfaces are also becoming complex, increasing the demand on drivers [7]. Guessability is especially important in those fields where symbols and signs do not have to conform to a worldwide standard. This is the case in the automotive industry, characterized by rapid technological changes and, consequently, requiring the introduction of new symbols and messages. While some of the ADAS symbols have already been a subject of extensive research and were standardized [3], most emerging technologies are independently developed by OEMs and producers and a significant lack of standardization is observed [8]. What is, therefore, particularly needed is research on icons. They can communicate information to the driver in a language-free and space-efficient manner [9], which is whythe use of pictograms and symbols is highly recommended in road traffic [10]. Symbols convey messages in a concise form. In a \"busy\" environment, they are more noticeable, more quickly understood, and, therefore, more effective than a written message, as well as more persuasive than a spoken message. This observation is supported by studies on the duration of fixation on signs, which revealed its positive correlation with user's difficulty in understanding the displayed sign [11; 12] and in extracting information from it [13]. These findings are of great significance for drivers because slow comprehension of the observed signal may be reflected in extended response time (as when excessively long fixations are needed to understand the meaning of the pictogram) and wrong reaction to an event (as when the icon is misunderstood). According to [PERSON] et al. [14], the focus in research on symbols should be on features such as familiarity, concreteness, complexity, meaningfulness, and semantic distance. The self-evident features, such as color and shape, or ones that concern relations to other symbols, such as distinctiveness, are of lesser importance. Familiarity is the frequency with which a given icon has been encountered. Icons are concrete if they depict real objects, materials, or people. Those that do not are abstract. Icons are regarded as complex if they are intricate or contain many details, and they are considered simple if they contain few elements or details. Meaningfulness refers, rather obviously, to how meaningful people perceive icons to be. Finally, semantic distance is the closeness of the relationship between what is depicted in an icon and what it is intended to represent [1]. Numerous methods of testing comprehension have been discussed in the literature [15; 16; 17; 18; 19], as this field developed intensively in the 1990s. Studies have been conducted in various contexts to improve not only the symbols used but also the methods for testing them. The multicultural test conducted in Austria, Hungary, the Czech Republic, and Spain including 33 symbols of variable-message signs for 243 variants revealed a problem with the cross-cultural interpretation of signs, as only 28 signs reached the 85% comprehensibility level [4].