Pedestrian injuries and fatalities are on the rise. Vulnerable road users are particularly difficult for transit bus drivers to see because of both the many obstructions to their view of the forward roadway and the limited visibility of events near the side of the bus (here we discuss only pedestrians but our discussion generalizes to other vulnerable road users such as bicyclists). In response to the rise in pedestrian collisions, systems have been developed to warn transit bus drivers of the potential presence and location of pedestrians whom they might strike if they (the drivers) do not take preemptive action. If the collision warning systems only alerted the driver of a crash threat when he or she was not aware of it and never warned the driver unnecessarily, they would seem to provide an unconditional benefit. However, warnings are often issued after a driver has observed the threat, possibly distracting the driver (here referred to as unnecessary warnings). In a highly trusted system, unnecessary warnings could lead drivers to respond abruptly in ways that increase the risk of injury to standing and seated passengers. In this paper, we review factors that influence when and how a driver responds to unnecessary warnings and identify the need for a framework for better understanding of the effects of imminent and cautionary warnings found in two-stage pedestrian warning systems.