When a trip serves both business and personal purposes, it is a personal trip if the trip would have been made in spite of the failure or absence of the business purpose and would have been dropped in the event of failure of the private purpose, though the business errand remained undone; it is a business trip if a trip of this kind would have been made in spite of the failure or absence of the private purpose, because the service to be performed for the employer would have caused the journey to be made by someone even if it had not coincided with the employee's personal journey.