What is the test for determining whether an assault by an oil company truck driver was an assault on a motorist?

Where the driver of an oil company's truck disputed with a motorist after the latter's car had been run off the highway by the truck and concluded his argument by striking the motorist with a 'large instrument,' the trial court properly instructed the jury that the assault by the truck driver was the conduct of the oil company. The court held that 'the fact that the questioned act was unauthorized or, if wrongful, that it was not committed in order to further the interests of the principal, will not show such a departure from the service of the principal as will absolve the latter if the act was committed while the agent was still occupying himself with the principal's business within the scope of his employment. * * * Sanders' entire association with plaintiff arose out of his use of the public highway on his employer's business.' Fields v. Sanders, 29 Cal.2d 834, 839, 840, 180 P.2d 684, 688, 172 A.L.R. 525.