A compact formula for the bounce-averaged magnetic drift velocity of trapped particles was derived by Roederer [Dynamics of Geomagnetically Trapped Radiation, Springer (1970)] for the case of curl-free flux tubes. It is shown in the present work that the formula is still valid for the more general case of non-zero electric current density and, hence, can be used for computations of particle drift motion in realistic model magnetospheres which include distributed currents with planar current sheets.