In what circumstances will an employer not be able to recover the amount of workmen's compensation awarded to an injured employee who was injured in a motor vehicle accident?

In Witt v. Jackson, Supra, 57 Cal.2d 57, 17 Cal.Rptr. 369, 366 P.2d 641, the case that declared the rule in California that the negligent employer may not recover the amount of workmen's compensation paid to an employee injured by the negligence of a third party and the employer's concurring negligence, the negligence imputed to the employer was that of a second employee which was of an active character in the operation of a moving vehicle that slowed down and changed lanes. ²