The most dangerous bus accident is the rollover. An accident statistics - collected by the author containing more then 300 accidents - shows that the average casualty rate is 25 casualties/accident. There are four major injury mechanisms in a rollover which may endanger the occupants: intrusion, projection, total and partial ejection. Different ways of protection may be used to avoid these kinds of injuries, which are shown in the paper. The severity of a rollover accident may be specified on two different ways: one is based on the number of casualties (this is mainly used by the public opinion) and the other one evaluates the circumstances of the rollover (turn on side is less severe than roll down into a precipice) The severity is a basic parameter when specifying the protectable rollover accidents (PRA) in which the occupants may be and shall be protected. This severity limit may be defined on the basis of the accident statistics mentioned above and on the basis of in depth analysis of real world rollover accidents and different rollover tests. These methods are used and discussed in this paper. All bus passengers using different bus categories (traditional buses, high decker tourist coaches, double deckers, and small buses) shall have the same safety level which shall be guarantied by international regulation. This paper is a contribution to the international effort specifying a general regulation about the safety of buses in rollover accidents.