Cell phones for pedestrian safety

Gizmag has a post that Nissan is working on a system that would use the GPS signal in your cellphone to prevent car v. pedestrian collisions.

Pedestrians' cell phones would transmit their coordinates to a central server. The central server would send a warning to approaching drivers about pedestrians in the vicinity.

Interesting concept and worthwhile goal (to reduce collisions with pedestrians), but I don't see how they filter the noise out of the system. Where there is even a moderate number of pedestrians on the sidewalk, how is the system going to help warn a driver about a pedestrian that is about to walk into the street?