Each claims the other was going too fast for the circumstances and not maintaining a proper lookout entering the intersection. Betty was injured in the accident and sues John, alleging that John was negligent. For his defense, John argues that Betty herself was negligent. The case is tried and goes to the jury. In a contributory negligence jurisdiction, if the jury finds Betty was the least bit negligent and contributed to the accident, then Betty would recover nothing.
Comparative negligence differs among states.