*3.3. Environments*

Temperature, humidity and CO2 concentration are variables that influence comfort and safety [70]. Vehicles tend to circulate in areas that are heavily contaminated. Many drivers close the windows and use the air conditioning in order to avoid polluting gases. However, the air that comes from this system is not clean. Besides, the reduced space of the vehicle causes a high amount of CO2 to accumulate due to the passengers themselves. If the level of CO2 inside the vehicle is very high, the driver may suffer from dizziness and nausea [71].

Temperature and humidity are other factors that can induce fatigue in the driver [72]. In the past, temperature was a significant cause of traffic accidents [73]. Currently, most vehicles integrate an air conditioning system. However, it is very difficult to adjust it correctly because the thermal sensation is different for each passenger [74]. An inadequate temperature, either too high or too low, causes a significant worsening of driving performance [75]. In order to monitor the interior of the vehicle, we used a Netatmo Healthy Home Coach [76]. This system allows us to obtain the air temperature, relative humidity and CO2 concentration. The measurements are taken every five minutes, and are uploaded to the cloud instantly. The data are processed internally using proprietary Netatmo algorithms. We directly downloaded the temperature, humidity and CO2 values using a Python script. We could not obtain the raw data. Regarding the validity of the use of the device to measure the CO2 concentration, there are several works where it has been verified, providing that a calibration has been previously performed [77,78].

The Home Coach Netatmo device allows manual calibration of the CO2 and temperature sensor. The calibration of the CO2 sensor of the device was carried out for 8 h inside a room without any occupants following the manufacturer's instructions [79]. The temperature of the room during calibration was 25 ◦C. The temperature sensor was calibrated by calculating the average di fference between the measurement obtained by the Netatmo device and the value o ffered by a weather station belonging to the State Meteorological Agency (AEMET), located in Asturias. The samples were collected over 7 days at 11 A.M. As a result, we fitted the temperature by +0.1 ◦C. In order to obtain the temperature reading using the Netatmo device at the location of the weather station, an external Samsung power bank was used. The Netatmo device has been validated by other authors previously obtaining good accuracy when the value of the air temperature under investigation is close to the air temperature in which the manual calibration occurs. In our case, the temperature during the calibration and test drive was very similar. Table 3 shows the specifications of the air quality sensor.

In our experiment, the temperature and humidity remained constant and we analyzed the CO2 concentration. In the literature, we find works where the CO2 concentration inside the vehicle cabin is analyzed [80]. However, we have not found works which study how a high CO2 concentration influences driving stress and driving behavior.



## *3.4. Driving Simulator*

This experiment was carried out using the "City Car Driving" simulator [82]. The simulator uses advanced car physics to achieve a realistic car feeling and a high-quality render engine for graphical realism. The simulator implements German tra ffic rules and warns drivers if they fail to comply with some of these. Tra ffic density can also be adjusted with the simulator. The drivers' behavior and pedestrians' behavior are sometimes erratic as in a real environment. The vehicles can collide with the player's car or with each other. Pedestrians sometimes cross the road in the wrong places. The scene selected for the experiment is named "Old District" and is characterized by narrow streets with simple crossing places and clear tra ffic patterns. This driving simulator was developed to train novice drivers in driving schools. It saves a log file with all the tra ffic rules that the driver violated as well as events such as tra ffic accidents.

The execution of the driving simulator on a computer and features are included in Table 4. Three 27-inch screens were connected to the computer. To operate the vehicle, we employed a Logitech G29 [83]. This device is an electronic steering wheel designed for driving video games with realistic

force feedback. It includes a set of three pedals and a gearbox, and it allows us to archive an immersive perception in the virtual environment. Table 5 shows the specifications of the device.


**Table 4.** Specifications of the PC on which the driving simulator is run.

> **Table 5.** Specifications of the G29. Data from [84].


In order to evaluate driving performance, we have developed a program based on the SFML library [85] that captures the angle of rotation of the steering wheel and the pressure applied by the participant on the pedals. In this study, we have defined the following variables to assess driving behavior:

