A vehicle safety system comprising: a force determining arrangement to detect or calculate a lateral force acting on at least one wheel of the vehicle; a slip angle measuring arrangement to measure the slip angle of the at least one wheel of the vehicle; and a processor arranged to receive signals from the force determining arrangement and the slip angle measuring arrangement, and to analyse the signals to determine a relationship between the lateral force acting on the vehicle and the slip angle of the at least one wheel, wherein the processor is configured to: determine a first range of values of lateral force over which the relationship is substantially linear, and to determine a second range of values of lateral force over which the relationship is substantially non-linear, the second range of values being higher than the first range of values; determine a transition value or range of values at the boundary between the first range of values and the second range of values, at which the relationship changes from being substantially linear to being substantially non-linear; characterized in that the processor is further configured to estimate a slip value of lateral force acting on the vehicle, at which slip will occur between the at least one wheel and a surface over which the vehicle is travelling, the slip value being estimated by multiplying the transition value or range of values by a predetermined factor. a force determining arrangement to detect or calculate a lateral force acting on at least one wheel of the vehicle; a slip angle measuring arrangement to measure the slip angle of the at least one wheel of the vehicle; and a processor arranged to receive signals from the force determining arrangement and the slip angle measuring arrangement, and to analyse the signals to determine a relationship between the lateral force acting on the vehicle and the slip angle of the at least one wheel, wherein the processor is configured to: determine a first range of values of lateral force over which the relationship is substantially linear, and to determine a second range of values of lateral force over which the relationship is substantially non-linear, the second range of values being higher than the first range of values; determine a transition value or range of values at the boundary between the first range of values and the second range of values, at which the relationship changes from being substantially linear to being substantially non-linear; characterized in that the processor is further configured to estimate a slip value of lateral force acting on the vehicle, at which slip will occur between the at least one wheel and a surface over which the vehicle is travelling, the slip value being estimated by multiplying the transition value or range of values by a predetermined factor. determine a first range of values of lateral force over which the relationship is substantially linear, and to determine a second range of values of lateral force over which the relationship is substantially non-linear, the second range of values being higher than the first range of values; determine a transition value or range of values at the boundary between the first range of values and the second range of values, at which the relationship changes from being substantially linear to being substantially non-linear; characterized in that the processor is further configured to estimate a slip value of lateral force acting on the vehicle, at which slip will occur between the at least one wheel and a surface over which the vehicle is travelling, the slip value being estimated by multiplying the transition value or range of values by a predetermined factor.