Owing to the successful application of grounded-source in recent years, roadway detection using the grounded-source transient electromagnetic (TEM) method has attracted significant academic interest. Using the three-dimensional finite-difference time-domain (FDTD) method, this study simulated the grounded-source TEM responses of the anomalies in the roadway floors and between roadways. The simulation results are as follows: (1) For anomalies in the roadway floors, their surfaces show violent changes in the horizontal and vertical components of the electric field subjected to the surface charge of anomalies. The horizontal electric field exhibits a strong ability to discriminate low-resistivity bodies but a weak ability to discriminate high-resistivity bodies. The vertical electric field displays opposite response signs from the low- and high-resistivity bodies. The low- and high-resistivity bodies correspond to opposite ∂B_y/∂t responses in the early and late stages. Specifically, the response signs are negative in the early stage and positive in the late stage for low-resistivity bodies, whereas the results are contrary for high-resistivity bodies; (2) For anomalies between roadways, the horizontal electric field also exhibits a strong ability to discriminate low-resistivity bodies but a weak ability to discriminate high-resistivity bodies. The ∂B_z/∂t response curves are above the background curves in the model of high-resistivity bodies but below the background curves in the model of low-resistivity bodies, suggesting the discrimination ability of the ∂B_z/∂t response for both low- and high-resistivity bodies.