Abstract: A structure is considered to be irregular in this paper if its response is influenced considerably by higher modes. Herein, the effects of the higher modes on the geometric regularity of single-column-bent viaducts have been investigated by comparing modal parameters resulting from the elastic eigenvalue modal analysis with transverse displacements obtained from static modal pushover analyses. A class of bridges with equal span lengths, but with different pier heights has been considered in this study. For a better identification of the modal behaviour of this class of bridges, several regularity indices are discussed. Some of these indices were used previously in model updating, model validation and the determination of modal correlations in the field of experimental vibration analysis (modal testing) of mechanical and structural systems. Certain modifications have been applied to previous works in order to improve bridge regularity indices. New indices have also been proposed for the attainment of a more realistic representation of the structural particulars of such bridges to be consistent with the analytical methods addressed in the AASHTO Specifications, for practical design purposes. The results calculated with the aid of various indices are reported and compared.