Horizontally curved bridges have more complex geometries compared to straight bridges. Their design, structural analysis, and construction require specialized knowledge and expertise. The curved geometry of these bridges affects load distribution and structural behavior. Understanding these behaviors is critical for improving the safety and durability of these bridges. For this reason, many studies have been carried out in order to understand the seismic behavior of horizontally curved bridges and to take the necessary measures in order to design resilient structures. In addition, seismic design regulations have set restrictions on the maximum curvature of bridges and permit engineers to use an equivalent straight bridge for their analysis and design. This paper investigates the restrictions and reviews the AASHTO LRFD Specification Design Fundamentals concerning the seismic responses of horizontally curved bridges by using equivalent straight bridges. In this regard, the seismic responses of 27 horizontally curved and 3 straight bridges, for a total of 30 RC bridges with different span numbers and bridge lengths, are investigated. Numerical parametric structural models have been generated for the selected variables such as; bridge length, span number, span length, and subtended angle. By using the structural analysis program, multi-mode response spectral analyses have been performed for the maximum credible earthquake (MCE) excitation level. The modal periods and frequencies, modal mass participating ratios, maximum displacement of the pier, and internal forces of the structural elements are obtained from the structural analyses of the bridges. The analysis results are compared with horizontally curved bridges and equivalent straight bridges to determine the effect of subtended angle on the seismic behavior of the bridges. It was shown in the study that bridge length and span number had a significant effect on the seismic response of horizontally curved bridges compared to straight bridges. Besides, the subtended angle limitations that AASHTO LRFD specifications put forward regarding allowing the curved bridges to use an equivalent straight bridge should be reviewed again. It suggests that a bridge is considered regular if the subtended angle is smaller than 900. However, according to the analysis results, the dynamic modal quantities, the displacement and rotations of the pier, and the internal forces of the pier columns and the deck of the bridges could reach their maximum values at lower angles of curvature than 900. Therefore, the limitations of the subtended angle should be reviewed and re-evaluated for several variable parameters by using linear and non-linear analysis methods. The aspects that make this research valuable and different from other studies are firstly, that the parametric models had a wide range of different subtended angles of curvature, bridge span numbers, and bridge lengths. Besides, the analysis results evaluated for the wide scope of the determined bridge configurations to realize the curvature effect of the bridges are very important to designing resilient bridges under seismic excitation.