Bridge modal parameters play an important role in bridge engineering; they can serve as useful indices for many applications, such as numerical mode calibration and updating, and bridge structural health monitoring (SHM). Recently, the indirect monitoring of bridges has been increasingly investigated and developed because it avoids the need for instrumentation on all bridges of a network. The natural frequencies of bridges have been extracted from the dynamic response of a vehicle numerically and in laboratory experiments. This study proposes an algorithm to extract the bridge mode shapes and damping ratio using similar indirect measurements enhanced by the Hilbert transform (HT). The theoretical closed-form equations are derived for a vehicle-bridge interaction (VBI) model in which the vehicle is represented as a moving sprung mass passing over a simply supported beam at a constant low speed. Then, a numerical simulation with a quarter-car model is adopted to verify the proposed algorithm, including case studies on the influence of the vehicle speed and road roughness. In addition, a laboratory test is conducted to further investigate the feasibility of the proposed algorithm.