Public transportation agencies commonly use drilled shaft foundations as support of mast arm traffic signs and signal pole structures. These structures and their foundations are subjected to wind-induced torsion. Design provisions can be found in AASHTO specifications for structural supports for highway signs, luminaires and traffic signals; nevertheless, those standards do not provide guidance to estimate the torsional resistance of drilled shaft foundations, or what an appropriate factor of safety (or resistance factor) for design could be. Although load and resistance factors format is desired because AASHTO is moving in that direction, still many Departments of Transportation design requirements are based on factors of safety. In this study, a probabilistic approach is used to recommend a rational procedure to determine factors of safety that consider the uncertainties and the consequences of failure. This procedure can be modified for load and resistance factors design calibration, as well. The skin friction approach was calibrated employing reliability analysis, available statistics, published experimental data, and simulations. However, a lack of field test data has been noticed. Factors of safety for cohesive, cohesionless, and layered soils are recommended. They are presented as a function of the target reliability index, and which in-situ test is performed to obtain the soil strength properties. Three alternatives were considered: standard penetration test, cone penetration test, and vane shear test. The procedure described can be used by practitioners to select appropriate factors of safety based on local conditions when statistical parameters from a particular site investigation are available.