This paper presents a probabilistic framework to derive the safety factors for fatigue of steel and composite steel concrete road bridges. Engineering models are used for the design and the safety factor is derived in such a way that the design meets the target reliability set by international Eurocode and ISO standards, estimated using measured data and advanced probabilistic models. Engineering model uncertainties and dynamic amplification factors are established through comparison of measurements and models. The value of visual inspections is quantified based on observations from practice and expert opinions. The safety factors are derived for Eurocode’s Fatigue Load Model 4 and Eurocode’s tri-linear S-N curve. The study shows that the safety factors for fatigue as currently recommended by the Eurocodes need to be raised. •A method is proposed to derive safety factors for the fatigue design of bridges.•Engineering model uncertainty is determined by comparing modelled with measured strain.•Dynamic amplification through road bridge–vehicle interaction appears small.•The safety factor of Eurocode’s Fatigue Load Model 4 must be raised from 1 to 1.1.•Visually inspected bridges demand 10% lower safety factors than noninspected bridges.