## 4 Discussion  
The research interest in the described methods, techniques, and sensors is different, and this is indicative of their successful application in the very dynamic and continuously vibrating working environment in the vehicle. Contact techniques with wearable sensors for recording ECG and PPG signals are mainly used by research teams as benchmark (gold standard) signals. Their accuracy correlates highly with clinical medical equipment. The most popular sensor techniques and methods used in publications are measurements with capacitive electrodes, cameras, and radars. Interest in capacitive electrode techniques was  
Figure 9: Taxonomy of methods, techniques, and sensors for acquisition of physiological signals in PDMSs. most significant in the period 2008-2018. Its end coincided with the increased interest in wearable, personal devices, such as smart watches and wristbands. Although, the motion artefacts while driving may limit the use of wrist-based wearable devices in a driving environment, too. Camera monitoring is relevant for measuring HR, HRV, and RR parameters from video stream images. The ability to analyze facial expressions based on facial landmarks is a distinctive advantage of cameras. Despite significant (already mentioned) shortcomings in their operation, the research for implementation in real DMSs is devoted toward the application of increasingly complex deep learning algorithms to improve their accuracy and capabilities. In recent years, an increased interest has been observed in the use of radars for physiological monitoring of drivers. Unlike RGB and NIR cameras, radars can directly measure RR and HR parameters together. In addition, recent research has focused on monitoring blood pressure. A number of publications investigated their appropriate placement inside the vehicle to minimize the influence of motion artefacts, which are the main problem for radars. The rapid development of the technologies in the field is increasing their operating frequency range, which helps to reduce the impact of interference from surrounding electronic equipment on the radar's operation. This makes radars attractive for application in DMSs for continuous monitoring. The possible locations of sensors in a vehicle are limited by the location of the driver. In order to minimize the superposition of interference in the recorded signal, the determining factors when choosing a location are the maximum working (contact or non-contact) area, minimum movements, or existence of obstacles in the working area. This identifies the steering wheel, seat, backrest, center console, dashboard behind the steering wheel, windshield, and rearview mirror as suitable sensor locations. The application of BCG/SCG and inductive bioimpedance techniques for monitoring physiological parameters in vehicles is still limited to a few published studies. A potential reason is the mechanical movements of the driver's body in the moving vehicle, since in an environment of sudden acceleration/deceleration and continuous vibrations, the difficulty in using these types of sensors is greater. The potential for real-time blood pressure measurement using ultrasonic sonar, demonstrated in research in recent years, holds promise for the development of wearable devices or non-contact sensors integrated into the automotive environment. There is a lack of application of smart rings for physiological monitoring in vehicles in scientific publications. A likely reason is that they are not yet widely available, as they have only been on the market for the last few years. The available information on smart rings shows high measurement accuracy and long battery life. Compared to smart watches and wrist-worn devices, their operating area is more static, which would maintain higher accuracy during wrist and hand movements while driving. Monitoring of EDA parameters has proven to be a useful measure of the driver's emotional response to driving situations, as well as for stress and anxiety detection.