Curvature greatly complicates the behavior of horizontally curved plate girders used in bridge superstructures. The warping stress gradient across the width of I-girder flange plates results in a reduction in vertical bending stress at which the flange plate buckles. The non-compact section limit for curved I-girder flanges presented in the Guide Specifications for Horizontally Curved Bridges does not explicitly consider the effect of the warping stress gradient, but rather conservatively considers the peak stress as uniform across the flange width. In addition, the flexural strength curvature reduction factors presented in the Guide Specifications resulted from research conducted in the early 1970's based on curve fit of limited experimental data. Significant discontinuity occurs between the compact and non-compact section limits, and the reduction equations do not converge to those of the AASHTO standard bridge specifications as the radius of curvature approaches infinity. This paper presents a definition of flange slenderness based upon theoretical and analytical models that consider the effect of stress gradient coupled with the influence of rotational resistance provided by the web. The equations developed are continuous and reduce to the equations of the AASHTO standard bridge specification as the radius of curvature approaches infinity.