In what circumstances will a plaintiff be considered a proximate cause of injury in a motor vehicle accident?

In Rosa v. Pacific Gas & Elec. Co., 133 Cal.App.2d 672, 284 P.2d 844, the court of appeal held that an intersection accident was, as a Matter of law, proximately caused by a taxicab driver who was exceeding the speed limit at the time. The court said (p. 676, 284 P.2d p. 846): 'If the cab driver had lessened his speed to any appreciable extent the collision would not have occurred. His failure to do so was the sole proximate cause of the plaintiff's injuries.'