## 4 Conclusions  
This paper presents the experimental investigation of the driver's reaction time and the accuracy of speedometer reading as a function of the instrument cluster layout and speedometer design. The research was carried out in laboratory conditions. Seven different instrument clusters of real-world passenger cars with various types and designs of speedometers were used to test the reaction time and reading accuracy of 32 participants. In addition to the speedometer characteristics, the integral design of the instrument cluster was included in the analysis in order to capture some additional effects on the results. Reaction time and reading accuracy were generally not correlated. It must be taken into account that in real driving conditions, there would be no pressure to perform the task as it was in this laboratory experiment and that the driver can at all times return his gaze to the road. No significant differences were found in either reaction time or accuracy between male and female participants. A certain dependence of the reaction time on the age of the participants was found to be a typical increase in the reaction time in older participants. The following design attributes of the speedometer, as well as the instrument cluster, were identified as factors that affect the results:  
* The shortest mean reaction time and the lowest error rates were recorded on the instrument clusters with a centrally positioned speedometer that stands out from the other instruments, regardless of the type of the presentation: analog (\"classic\") or digital (\"modern\") speedometer;
* The reading from the digital speedometer was not error-free despite preferable characteristics;
* A typical cause of coarse errors in this experiment for both analog and digital speedometers was the reading from another instrument that resembles a \"classic\" speedometer (for example, a tachometer);
* The tachometer or any other instrument graduation in hundreds of RPM (i.e., 10, 20, 30, etc.) has been identified as the cause of coarse errors. In contrast, participants did not make the coarse errors if a tachometer was graduated in thousands of RPM (i.e., 1, 2, 3, etc. ), even when the instrument panel had a visually symmetrical instrument layout\". As the presented work shows, instrument cluster design and its interaction with human factors is a complex area with potential for future research. The speedometer, or any other instrument, cannot be evaluated as an individual component but as an integral part of the instrument cluster. The method used in this study is suitable for initial studies of Driver-Vehicle Interface (DVI) design due to the simple setup and equipment, as well as the unrestricted variations of the instrument clusters being evaluated. In later stages, other influences of boundary conditions (driving tasks, vibrations and movements, other visual/auditory inputs), as well as additional monitored quantities (eye tracking, lane keeping monitoring, etc.) can be included in the experiments. This method is also applicable to other types of instruments on vehicles, as well as to displays in other systems where there is an HMI and where user performance is important. The space for further investigation in this project is still open, and it will be expanded with the results of subjective assessment. Although the experiment was conducted under simplified laboratory conditions, the proposed methodology and the data collected in this study contribute to the understanding of Human-Machine Interface (HMI) design and can serve as a useful resource for future research in this field. Conceptualization, D.B. and D.R. ; methodology, D.B. and D.R. ; formal analysis, D.R. and D.B. ; investigation, D.B. ; resources, D.B. ; data curation, D.R. ; writing--original draft preparation, D.R. and D.B. ; writing--review and editing, N.N., B.S. and S.M.-S.; visualization, D.R. and D.B. ; funding acquisition, N.N. and B.S. All authors have read and agreed to the published version of the manuscript.