A nonpaying passenger shall be transported by the driver of a taxicab only if both are in training, or if an off-duty sign is adequately displayed; provided, however, in the event a police officer requests the use of a taxicab in the performance of his official duty, then in such event, the driver shall transport said officer or assist him in any way possible. When a taxicab is in service, no persons other than the driver and passengers are allowed in the taxicab.