I would be frightened to trust the data coming from a random car in front of me. Inferring road condition from another car behavior sounds reasonable, using data supplied from it, not so much.
However, if the car three cars in front of you just broadcast "I'm doing an emergency stop right now", that's really valuable data. The human in your car won't know anything is wrong for at least a second. The human driver behind you would know about it before the human driver in front of you.
Until some asshat implements a button that sends this message for "lulz", though that would probably constitute a crime under existing law.
A computer-driven car, though, wouldn't (shouldn't) just immediately slam on the brakes because of that signal. It would tighten seat belts and start slowing down, but it also would want to avoid getting rammed by the car behind it. It can make very accurate estimates about its stopping distance and use all of it.