## 4 Discussion  
This study intended to define a method of collecting unbiased information on the psychophysical state of the driver, with the aim of investigating the possible correlations between road safety and their  
Figure 14: The 15 areas with the highest stress scores. The areas identified with high accident rates according to road safety analysis are reported in red. The colors yellow and green identify zones with medium-high and medium-low accident rate. emotional state. In particular, this study focused on the possibility that either stressful or particularly relaxing conditions could be correlated with road accidents. The first phase of the research was oriented to the identification of physiological parameters related to stress, thus defining the methods of collection and comparison, which were later useful to define stress peaks. Data collection showed that the most interesting indicators were the LF component of heart rate variability, the LF/HF ratio (always linked to HRV), and the skin conductance (GSR). Following this introductory phase, two different experiments were carried out on the streets in the city of Milan. The first experimental phase represented a useful warm-up and made it possible to evaluate and test the tools, as well as revealing how to operate and manage them during the driving phases. At the same time, the test allowed the identification of stress points along a path and a comparison across data collected for each driver. These points were then spatially correlated with the locations where road accidents occurred, previously identified by techniques of road safety analysis. Taking into consideration the areas of linear influence around the incident point, it was found that 50% of the detected stress points also corresponded to incidental points; vice versa, about 56% of the incidental points were a part of the zones of influence around stress points. Another element of interest that emerged from the first experimental phase concerns the rise in stressful conditions associated with the presence of important road works (due to the construction of the new underground line M4). In fact, even during uncongested traffic hours, the user was stressed due to the continuous deviations. In particular, a considerable number of stressful points (identified as such on the basis of the monitored parameters) were recorded where the deviations and the alternative route were not adequately indicated. The second experimental phase represented a development and a deepening of the first phase; for this part, a significantly different approach was adopted, especially considering the data elaboration step. On the basis of the monitored parameters, it emerged that 38.7% of the cases reported stressful conditions in high-accident areas. On the other hand, the state of relaxation was registered in zones of low accidents in 40.8% of the overall sections considered. These results give an indication of the actual correlation between the psychophysical state of the drivers and the safety conditions. With the development of a scoring method, it was possible to combine the results of all the drivers undertaking the test, eliminating, as much as possible, the elements of uncertainty, and identifying the most stressful areas. Each area was given a score that conveyed information about the level of stress. It emerged that, among the 15 sections with the highest score, 12 of them were also sections with high accident rates. In terms of preventive road safety analysis, it is also possible to proceed by reversing the correlation. Out of 14 high-accident sections, 12 of them were also subject to high stress. This means that, if we want to apply the method for prevention, intervening on 25% of the route (i.e., limiting the possible interventions to a selection of more critical areas from the stress point of view) would identify 80% of the critical areas for car accidents, even without historical data.