When a video camera records the scene in front of a moving road vehicle, then egomotion typically engenders an expanding optical flow field around the so-called focus of expansion (FOE). Additional FOEs correspond to objects that threaten to hit the vehicle. If the camera is oriented to the side, then the FOE due to egomotion typically lies beyond the image borders. In this contribution we assess whether or not the remaining FOEs observed within the image borders are reliable side impact indicators. Expanding flow fields are necessary, but not sufficient to unambiguously detect side impacts. However, the remaining ambiguities can be constrained in order to derive a reliable side impact alert. Our method is based on an estimate of the time to collision (TTC) derived for each FOE, and on additional constraints on the impacted area and on the velocity of the impacting object.