## 2 Materials and Methods
### Tests  
The methodological approach described was applied to a series of experimental driving tests, so as to evaluate its applicability to a real case, and to develop an initial calibration.  
The driving tests were carried out on the road network in the city of Milan. All participants were properly informed, and they signed a declaration of informed consent to take part in the study.  
In the first set of experiments, two tests were carried out, both with drivers between the ages of 25 and 35, who were used to driving in large urban centers and using their own vehicles, so as to avoid the arising of further stress during the test. The experiments were carried out on \"standard\" days, avoiding anomalous situations (holidays or strikes, and, as much as possible, rush hours). Data collection took place over a time interval of about 1 h and 40 min for each test, for about 26 km of overall travelled distance. The path is reported in Figure 5.  
Each driver was equipped and monitored by means of the previously presented wearable devices (T-shirt/top with electrodes, GSR bracelet, and video camera on the head or on the dashboard), and was instructed about the path to follow. The tests were carried out with other people on board to activate the instruments and to use the connected applications.  
In the second set of experiments, seven participants aged between 20 and 30 were involved, carrying out nine tests. Each driver drove his own car; however, unlike the first experimental set, some testers were not used to driving within an urban environment. As described before, the testers were instructed on the path to follow, and were monitored with the same wearable devices used in the first test. To record the environmental conditions, glasses with an integrated video camera were used instead of the video camera mounted on the head/dashboard. Before the actual driving test, the drivers were subjected to a test in rest conditions, lasting about 5 min, for the calibration of LF/HF ratio ranges for each driver. This baseline was fundamental to remove, as much as possible, the subjective components of the each driver's stress. In fact, knowing the range of values for each tester at rest allows obtaining objective results regarding values that exceed those recorded in rest conditions.  
The tests were carried out during the daytime hours, along an 11-km path, reported in Figure 6, for a duration of about 45 min.  
Both phases of experimentation allowed the identification of solutions to the operational problems that emerged. In particular, it was necessary to define an appropriate protocol for the synchronization of the different tools, since some complications in their simultaneous start-up arose.  
The developed test in rest conditions or the tests of the second experimental phase allowed, instead, referring all values recorded for the parameters to the specific feature of each driver.  
It was not possible, at this first level of research, to carry out long-term experiments, both because of the temporary availability of the used devices, and the complications that arose during the tests. In fact, the simultaneous activation of wearable tools, the GPS, and the video camera, as well as the checking of data collection, required the presence of at least two operators in the vehicle, making the independent management of tools by the drivers themselves difficult on various days.  
Figure 5: Path selected for the first set of experiments (QGIS elaboration of Google Maps).