This sort of thing is supposed to be address in V2X in a more cross industry wide basis. I would expect that the tech would eventually be mandatory in all phones / vehicles as an OS feature, not a 3rd party app. I think its based on broadcasting position and direction vectors.
It should cut down on rear end collisions by detecting when vehicles ahead of you out of site suddenly brake. Or a when a vehicle from the side is going to blow a stop sign / red light.
As a cyclist I really looking forward to it. People have a habit of waiting until the last second to pass cyclists, screening you from the vehicle behind until they have completely switched lanes which ends up being 10-15 feet behind you, then blam-o zero time for driver behind to react. Would also be a huge help for pedestrians when drivers are making right (left in UK) turns and typically only look upstream for traffic not even looking the other way for anyone on the sidewalk.
It would also let traffic light know that a cyclist is approaching. The camera based detectors which are on 95% of our intersections only pickup cyclists 25% of the time. No detection, no green light, if a vehicle pulls up behind you they are too far back to trigger a green as well.
One obvious abuse would be to attach something with V2X to done and flying it into traffic to trigger auto braking in cars.