I meant some sort of visual system and/or possibly vibration. This would some how work in coordination with other helmets to be aware of distances and speeds of other helmets. Maybe even something like HUDs in games that have the indicator overlays in the corners or mini map type system for receivers. If we are alerting in our cars of collisions why not the same type of impact on the field? These can be life changing moments, split seconds matter.
The NFL should do some of the other items for safety/health (weight/padding/indicators) if they truly care more about the player on the field than treating them like a child off the field. I think many of the problems with athletes would be solved if they relaxed their harshness off the field and used that on the field. If every lineman that plays gets knee and ankle troubles for life, maybe they should limit weight and require braces and better equipment.