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A critical traffic situation has been explored where a double-deck bus may roll over. The factors that may cause critical situations during a bus drive have been analysed. Based on this, they may be categorised as being related to vehicle construction, vehicle operation, and road infrastructure. A situation has been chosen for the analysis where the bus without passengers avoids an obstacle having suddenly sprung up in front of the bus at a distance within which the bus cannot be stopped. A similar problem occurring in the case of a passenger car drive was analysed in study [6]; it was indicated there that the manoeuvre of avoiding an obstacle is definitely more favourable in comparison with the braking process. A model of the bus motion dynamics with 12 degrees of freedom was adopted. The vehicle trajectory was given in the form of a predefined input generated by a driver model. The course of the rollover process depends to a significant extent on the interaction between the tyres and the road surface. A semi-empirical non-linear TMeasy tyre model was chosen. The calculations were carried out with the use of a computing program specially built for this purpose, where a double-deck bus model and a driver model were applied to investigate the traffic situation as described above. The calculation results obtained have shown that the process of rollover of a bus without passengers emerges at the final stage of the obstacle avoidance manoeuvre at a speed exceeding 108 km/h. This has been confirmed by the course of changes in the pressure of wheels on the road. Based on the simulation calculations carried out, an attempt was made to determine, from the observed courses of changes in physical quantities, the values that might warn the driver about a situation of imminent danger of rollover of the double-deck bus and thus might affect driver's actions.

Road traffic accidents involving coaches do not happen very often, but they are very dangerous because they affect a large number of passengers. Coaches (or intercity buses) are not equipped with safety belt harnesses. Valid regulations do not impose any obligation on coach manufacturers to provide intercity buses with either two- or three-point safety belts. This fact may result from the unawareness of risks and injuries that might befall the passengers with no safety belts during accidents. That is the reason why this work aims to compare the aftermath of coach accidents with no safety belts and the ones with safety belts. A detailed aim of this research is to analyse the results of dynamic loads during a frontal impact exerted on coach passengers travelling with and without (two- and three-point) safety belts. This objective was achieved by performing experimental studies and modelling which focused on the process of dynamic load transfer on the human body during a traffic accident. The research was conducted parallel on an adult and a child. The equivalent of a 50th percentile male was a hybrid III dummy (M50), whereas a child at the age of about 10 was represented by a P10 dummy. A numerical model was generated and verified in experimental testing in the scope of kinematics. Also, the comparison of the recorded courses of forces, acceleration, and moments was conducted. The results obtained from the tests were analyzed regarding the injury criteria for head, neck, and thorax. It was observed that both for the two-point safety system and the lack of safety belts, there were high values of acceleration recorded in the centre of gravity of the head. On the basis of the investigations conducted, it was ascertained that only a three-point safety belt system ensures the satisfaction of all injury criteria within admissible standards both in the case of criteria defined in the rules no. 80 and the rules no. 94 determined by the United Nations Economic Commission for Europe. It is the three-point safety belt system which should be obligatory in all intercity buses.

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