Systems, methods, and other embodiments described herein relate to improving alert activation for rear collision avoidance. In one embodiment, a method includes responsive to detecting a target object that is located behind a subject vehicle, determining, by the subject vehicle, characteristics about a surrounding environment of the subject vehicle, including of the target object. The method includes analyzing the characteristics to identify lateral free space next to the subject vehicle that is an area without obstruction beside the subject vehicle. The method includes modifying a collision threshold for activating an alert to the target object according to the lateral free space.