Systems and methods to detect and warn proximate entities of interest are described herein. An example signal generation system for a vehicle capable of different modes of movement includes a detector to determine at least one property of vehicle movement and an output representative of that at least one property and a selectively variable signal generator includes an input to receive the at least one output representative of the at least one property of vehicle movement and, responsively, generates a selected signal based on the received output. In some examples, a detector on a pedestrian detects the selected signal from the signal generator and, responsively, provides an output indicative of a vehicle in proximity to the pedestrian. In some examples, a trajectory vector is generated for at least two entities of interest based on at least one characteristic of movement of each entity. Each entity's trajectory vector is expanded and each entity's expanded trajectory vector is analyzed for overlap with the other entity's expanded trajectory vector to assess the possibility of a collision between them.