The radar cross-section of a target is a critical parameter for evaluating the performance of an automotive radar in terms of target detection, discrimination, and tracking. Furthermore, it has to be emulated accurately when applied in an over-the-air radar sensor validation methodology in a hardware-in-the-loop approach. Therefore, it is essential to study the RCS patterns of traffic participants under conditions with least influence of ground reflections and background artefacts. Given that in publicly available literature there are few reports of high-quality RCS-data measured over the entire frequency band at 77 GHz, we have performed wideband RCS measurements of a sedan car mock-up including both horizontal and vertical polarizations. The measurements were performed in a semi- anechoic chamber and analyzed with advanced post-processing for improving the accuracy of measured data. Results indicate that the RCS values of the sides were around 10 dB larger than the rear face, and 15 dB larger than the front face of the car. Comparison between horizontal and vertical polarizations show a high degree of similarity over all azimuth aspect angles except for the rear of the car, where the difference in RCS values amounts to as much as 7 dB. Further, averaging over the bandwidth reduced the fluctuations, nearly preserving the peak values at the favorable detection angles.