## 2 Materials and Methods
### Parameters Related to the Psychophysical State of the Driver  
Within the first set of experiments, to gather information about the psychophysical state, and particularly, to investigate the stressful situations of a user driving their vehicle, the physiological parameters of heart rate variability (HRV), respiratory rate (RR), and galvanic skin response (GSR) were selected. Figure 1: Procedural diagram of the study. Heart rate variability (HRV) is the natural variability of the heartbeat, thus representing the change in heart rate as a consequence of emotional factors, including stress. It can be divided into three frequency bands: very low frequency (VLF; between 0.01 and 0.04 Hz), low frequency (LF; between 0.04 and 0.15 Hz), and high frequency (HF; between 0.15 and 0.4 Hz). For the purposes of the first test phase, the component considered as most significant was LF, since several studies indicated it as a parameter closely related to the stress state of a person [12; 13]. Respiratory rate (RR) represents the number of breaths performed by a subject in one minute. In the case of abnormal conditions, particular moods, or physical states, an increase in respiratory rate may be recorded [20]. Typically, while a subject is in rest condition, the value of this parameter is around 12-20 breaths per minute [21]. The data collection and the analysis of HRV and RR were developed through the use of a wearable instrument consisting of a T-shirt (or a top for the female version) equipped with two sensors placed at the lower limit of the chest, where the tool equipped with a battery was fixed. This device allowed the measurement of an electrocardiogram (ECG) and the transmission of data collected via Bluetooth to a smartphone application, or their collection in a dedicated internal storage. The devices, shown in Figure 2, were developed by a working group coordinated by the Sensibilab Laboratory of the Design Department--Politecnico di Milano. These tools were created for research purposes, and therefore, they are not directly comparable with commercial devices, as the precision and reliability of the outputs allow overcoming some limits of commercial tools, well described in the literature [32]. Another parameter considered significant is the galvanic skin response (GSR), which represents the conduction ability of the skin; in the case of emotional agitation or stress, the electrical resistance of the skin decreases progressively, while, on the contrary, the conductance increases [33; 34]. This parameter, together with HRV low frequency and the LF/HF ratio, appears to be one of the most valid indicators for the acquisition of information regarding the autonomic nervous system and stress levels [24]. The measures of GSR are usually performed with sensors placed on the fingers of the hand and/or on the feet; however, to avoid interference with driving maneuvers, a bracelet with electrodes for the measurement of skin conductance was adopted (Figure 3) [33]. The values recorded by the instrument during the test can be displayed on a smartphone application called MAXREFDES#73, developed by Maxim Integrated (San Jose, CA, USA), and they were collected in a text file. For the second set of experiments, the HRV parameter was confirmed as a fundamental parameter for the assessment of the psychophysical state of the driver. In particular, in this second stage, the LF/HF ratio was considered, being able to provide information on the balance between the sympathetic and parasympathetic activity of the autonomic nervous system. High values of this  
Figure 2: (a) T-shirts used for the tests, equipped with sensors; (b) female version of the wearable sensor. ratio are linked to stressful situations involving greater activity of the sympathetic system, while low values are related to relaxed conditions, with a predominance of parasympathetic activity [15; 17; 18; 19].