A commercial chirp-sequence automotive radar sensor is reconfigured to allow for time-frequency analysis of the 77 GHz automotive radar bands. Compared to highend measurement equipment such as real-time spectrum analyzers, due to its low cost and compact size the proposed approach allows for in-situ interception and time-frequency analysis of traffic scenarios, e.g. by mounting the sensor in a test vehicle exactly at the positions where the radar sensors would be mounted. The theory behind the modification is discussed and compared with measurements. Time-frequency spectra of a rural road driving scenario obtained with the proposed system are shown to illustrate the practical relevance and usefulness of the approach.