# Driving-Style Assessment from a Motion Sickness Perspective Based on Machine Learning Techniques
## 2 Comfort Evaluation Methods  
Comfort is a subjective feeling, and there are no unified and clear definitions for it in academia. The passenger comfort discussed in the present article refers to the discomfort of passengers during a car ride, specifically regarding motion sickness. The level of ride discomfort is associated with the frequency of the vibration and is directly proportional to its intensity. Furthermore, it has also been observed that increasing the time of exposure to vibration means an increase in discomfort. With this in mind, we know that low-frequency vibrations, close to 1 Hz, are transmitted throughout the body by increasing malaise. Higher frequency vibrations are attenuated by the human body and reduce discomfort. Monotone continuous low-frequency vibrations increase fatigue, while transient vibrations produce stress [25]. The human body responds differently to vibration frequencies depending on the body part and in which direction the force acts. How the frequencies affect humans depends on the proportions of a person's body and the type of frequency that affects the person [26]. Thus, two types of discomfort are differentiated. In [27], discomfort (also named average discomfort, or simply discomfort) is defined as a general feeling of not well-being, while motion sickness is associated with dizziness, fatigue, and nausea. In this work, we will focus on the latter type--motion sickness. Most of the time, comfort is evaluated using a subjective rating test and/or using electrical accelerometers combined with comfort evaluation parameters. Many of these parameters (vibration dose value (VDV), estimated dose value (eVDV), vibration number (VN), motion sickness dose value (MSDV), vomit rate (VR)) are part of the essential standards in this area: ISO-2631 [28] and British Standard 6841 [29]. Two main standards are used to evaluate comfort, the British Standard 6841 (BS 6841) and the International Standard 2631 (ISO 2631). Both standards describe ways to evaluate vibration exposure to the human body. They define methods for the measurement of vibrations as well as how to process measurement data to standardized quantified performance measures concerning health, perception, comfort, and motion sickness. In this work, motion sickness-related frequencies will be studied; between the cited examples of the cited standards, only MSDV and VR can be used. In the bibliography, there are some alternative indices, such as motion sickness incidence (MSI) [30], and fast motion sickness scale (FMS) [31]. Quantitative methods based on questionnaires can also be noted, such as the motion sickness susceptibility questionnaire (MSSQ) [32]. Nonetheless, given its relevance in this work, the motion sickness dose value is used (MSDV, Equation (1)). MSDV is a measure of the likelihood of nausea and its evaluation is also determined in ISO 2631 Standard by the \(w_{f}\) filter seen in Figure 1. This standard only provides guidelines for the interpretation of the MSDV for the vertical direction, MSDVz. \[MSDV_{z}=\sqrt{\int_{0}^{T}(a_{z,w_{f}}(t))^{2}\,dt} \tag{1}\]  
[PERSON] provides an interpretation of the MSDV for the lateral direction in [33] and our previous real-time experiments [34] have shown that MSDV values obtained in the X and Y axes are substantially bigger than the Z-axis [35], this is a direct consequence of accelerations in X and Y being bigger than the vertical ones when measured in \(m/s^{2}\) without the added effect of gravity. The British Standard 6841 (BS 6841) [29] attempts to combine the influences of different axes to obtain a general approach toward the origin of motion sickness by quantifying the lateral and longitudinal contribution as the sum of the parts as seen in Equation (2).