- This paper aims to characterize the effects that the velocity variations of the mobile stations (MSs) produce on the correlation properties of non-stationary time-frequency (TF) dispersive mobile- to-mobile (M2M) fading channels. Toward that end, we propose a novel geometrical model for non-wide-sense stationary uncorrelated scattering (non-WSSUS) M2M channels that incorporates such variations following a plane wave propagation approach. Capitalizing on the mathematical simplicity of this approach, we derive a general expression for the four-dimensional (4D) TF correlation function (TFCF) of the proposed channel model. From this expression, we analyze the influence of the MSs' acceleration/deceleration on the channel's correlation properties. Some simulation examples illustrating our findings are presented for the particular case of the geometrical one-ring scattering model. The proposed channel model can be used as a reference to study the performance of emerging vehicular communication systems in safety-threatening scenarios, such as when a MS is forced to break suddenly. © 2017 IEEE.