# Does the Driver Understand the Warning? Comprehension of the Request to Intervene
## 2 Materials and Methods  
System-initiated transitions in automated vehicles have to be introduced via different Human-Machine Interfaces (HMIs) by issuing a Request to Intervene (Rtl). In the Trustonomy project, the effectiveness of individual HMI solutions was measured in two ways: Through the assessment of reaction time and through the assessment of ergonomics. The time-based approach, which is the focus of the article, is mainly based on comparing response parameters across different HMIs, complemented by additional subjective support measures based on the levels of message comprehension, perceived utility, and task load reported by participants. The authors conducted research on the comprehension of messages conveyed by icons used to inform drivers about the need to intervene while driving an automated vehicle. A survey based on a standardized ISO 9186 Comprehension Test was used for this purpose [19]. The evaluation procedure proposed by ISO was matched to the pilot requirements and served as an inspiration to develop a tailor-made comprehension test. Using the questionnaire thus developed, the authors examined the effect of different HMI symbols as perceived by the participants. More specifically, the comprehension test was used for testing different referents and variants of an HMI element. A referent is an idea or object that a graphical symbol is intended to represent, while a variant is an alternative graphical design for a given referent. The participants (present in the laboratory) were to complete an online questionnaire, responding to three types of close-ended questions: \"What do you think this symbol means?\", \"What action should you take in response to this symbol?\", and \"To what extent does this icon convey a Request to Intervene (Rtl)?\". Each participant completed the same questionnaire. Please see Supplementary Materials for the comprehension test used to examine the effect of the pictograms designed by the ITS team. In the beginning, the participants were asked to answer the first two questions, i.e., to guess what the symbol meant and what actions one should perform after noticing it. Then, after all pictograms were reviewed using the first two questions, the respondents were familiarized with the meaning of individual icons and asked to answer the third question, thus reporting the level of comprehension of each icon. All answers were provided in written form. The results were analyzed based on descriptive statistics. The study was conducted in two locations: Warsaw (Motor Transport Institute; ITS) and Bednary near Poznan (Solaris). Different groups of users and different types of pictograms were tested in each location. In the ITS laboratory, the sample consisted of 81 subjects (51% women, 49% men) aged 18-65. These people had a category B driving license and had no experience in driving a vehicle equipped with ADAS. On the other hand, Solaris invited their test drivers to take part in the research, namely, professional drivers with a bus driving license. All of these 10 subjects were men aged 30-40. In the course of their employment with a bus manufacturer, these people had prior contact with many new features implemented in the brand's vehicles. Nevertheless, they had not participated in the Trustonomy project before, did not know the research methodology, and, most importantly, had never seen the icons designed by the Solaris team before the experiment. As some Solaris buses were fitted with MobilEye Shield+, some of the test drivers may have used the device in the past. In Warsaw, the understanding of 3 different icons designed by ITS employees was examined. All of them were implemented in a driving simulator equipped with an L4 level automation system (according to SAE).