What is the burden of proving contributory negligence against a defendant in a motor vehicle accident?

Howard v. Worthington, 50 Cal.App. 556, 195 P. 709, is a somewhat similar case. In that case the appellate court found four erroneous instructions. The trial court had correctly instructed that the burden was on defendant to prove contributory negligence by a preponderance of the evidence, but it also instructed: 'In order, therefore, to find a verdict for the plaintiff, you must not only find from a preponderance of all the evidence that the defendant was negligent, but also that such negligence was the proximate cause of the injury to the plaintiff; and you must further find that the evidence shows, by a preponderance thereof, that the plaintiff was not guilty of negligence, however slight, contributing proximately thereto; otherwise, your verdict should be for the defendant.' 50 Cal.App. at page 559, 195 P. at page 710.