## 6 Conclusions  
The research conducted led to the formulation of the following conclusions:  
1. The studied processes differed significantly in terms of statistical properties. The highest coefficient of variation value was found for engine control and vehicle velocity and fuel consumption mass intensity. The lowest coefficient of variation value was found for nitrogen oxides and carbon monoxide emission intensity and particle number intensity. The range and standard deviation ratio was highest for particle number intensity and carbon monoxide emission intensity and engine control, as well as for nitrogen oxides and hydrocarbon emission intensity. The highest kurtosis value was noted for particle number intensity and hydrocarbon emission intensity, and the smallest negative value was recorded for engine torque, engine rotational speed and vehicle velocity. Particle number intensity had the highest skewness and relative engine torque had the lowest. 2. The strongest correlation was found between engine torque and effective power, as well as engine rotational speed and torque or engine rotational speed and effective power and between carbon dioxide emission intensity and fuel consumption mass intensity. A strong correlation was also observed between engine control and torque and useful power. There was also a strong correlation between hydrocarbon emission intensity and carbon dioxide emission intensity and nitrogen oxide emission intensity. The smallest value of the correlation coefficient for the studied processes was found for the particle number intensity related to the other studied processes. 3. The probability distributions of the studied processes showed significant differences and were strongly asymmetrical. In the case of the exhaust emission intensity values, the particle number intensity, the fuel consumption mass intensity and the probability density maxima for small values that were observed were expected outcomes. 4. The power spectral densities of the standardized vehicle velocity and standardized engine control and standardized engine rotational speed nearly monotonically decreased in the frequency domain, while for standardized engine rotational speed, standardized torque, standardized useful power, standardized exhaust emission intensity, standardized particle number intensity and standardized fuel consumption mass intensity, local spectral power density maxima occurred for frequencies around 580 Hz. As with all research studies based on the collection of empirical data, this study has possible limitations that need to be addressed when interpreting the results and conclusions presented above. To begin with, uncertainties related to measurement equipment and test procedures must always be taken into account. However, in this case, the main constraint is associated with the repeatability and reproducibility of the data obtained during measurements in real road traffic conditions. While the RDE testing procedure has been developed to more accurately represent actual driving conditions, two RDE tests carried out for the same vehicle may produce different results, even if all requirements imposed by legal regulations are met. Ensuring the repeatability of RDE tests has always been a challenge; on the other hand, it is the randomness of the driving conditions that makes this type of test unique and in many applications superior to, for example, chassis dynamometer testing, where the conditions are fully controlled and the repeatability of the results is ensured. Taking the above into account, further research directions might include a similar analysis with a stochastic approach for data obtained in RDE tests representing extreme conditions within the permitted limit--maximally favorable and maximally unfavorable in terms of pollutant emission and fuel consumption processes.