This paper investigates the effect of signalisation on safety of urban priority controlled four-arm junctions. Forty-eight such junctions were carefully selected to have different values for the geometric and traffic characteristics that may influence the safety effects of signalisation. Long periods before and after signalisation were considered so that the effect of the regression to the mean phenomenon becomes negligible. Signalisation was found either to increase the junction safety or to leave it unchanged. Discriminant analysis showed that among numerous variables considered those significantly influencing the effects of signalisation on safety are the flow weighted average width per arm, the flow weighted average grade per stream, and a function of the product of the conflicting stream flows of the junction. The discriminant function developed predicts quite satisfactorily the behaviour of junction safety after signalisation.