A car ahead of you has its brake lights on near a freeway exit, as if the driver is unsure about taking the ramp. According to the CDL manual, at what point does this situation stop being just a hazard and become an emergency?

49 CFR 383.111(a)(13); AAMVA CDL Manual (nhcdm.pdf) Section 2.8.1, p. 2-18

Recognizing a hazard before it becomes an emergency is the entire point of hazard perception training. A driver who sees the clue early can slow down, check mirrors, or change lanes calmly; a driver who doesn't notice until the car cuts in front of them has to brake or swerve suddenly, which is far more likely to cause a crash.