There is always enough space to maintain a braking distance, the traffic may just have to travel a little slower. The only issue I've seen with it is drivers cutting each other off because they don't understand braking distances or how to safely maneuver between lanes. I manage to maintain safe braking distances, I probably just average 5 mph slower than the traffic around me, which in reality is the result of matching speed with the car in front of me, then having to slow down when somebody gets between us to again increase the space to a safe distance. Doing that has saved me from an accident on exactly 2 occasions since I moved to the US; both times the traffic in my lane only (supposedly the "fast" leftmost lane), stopped very abruptly, then moved off again with no apparent indications as to what had caused the stop in the first place. On one of those occasions, had I not left an appropriate braking distance, I would certainly have hit the car in front of me as I was tired and reacted slowly.