Microstrip traveling-wave series-fed antenna arrays (MTSAAs) have been popularly developed for radar systems in the millimeter-waveband. Because the antennas are connected and fed in series in MTSAA, in-phase radiation of the antenna elements with properly tapered amplitude for achieving a broadside beam with a low sidelobe level (SLL) is challenging. Although the reported series-fed patch and comb-line arrays accomplished the goal, design equations for the optimum distances between antenna elements in the series-fed array do not guarantee in-phase radiation due to the mutual coupling in array structures. Therefore, optimization of the distances in the array using full-wave simulations is inevitable. In this article, we propose the first universal and non-iterative design equation that compensates for the phase offset generated in patch and comb-line MTSAAs and successfully found optimum distances for broadside beams with low SLLs without optimization. Finally, we fabricated both MTSAAs with ten elements operating at 76.5 GHz and demonstrated almost 13 dB boresight gain and low SLLs near −18 dB without a beam tilt.