Example. A, an employee of the M Company works and maintains his principal residence in Boston, Massachusetts. Upon receiving orders from his employer that he is to be transferred to M's Los Angeles, California office, A motors to Los Angeles with his family with stopovers at various cities between Boston and Los Angeles to visit friends and relatives. In addition, A detours into Mexico for sight-seeing. Because of the stopovers and tour into Mexico, A's travel time and distance are increased over what they would have been had he proceeded directly to Los Angeles. To the extent that A's route of travel between Boston and Los Angeles is in a generally southwesterly direction it may be said that he is traveling by the shortest and most direct route available by motor vehicle. Since A's excursion into Mexico is away from the usual Boston-Los Angeles route, the portion of the expenses paid or incurred attributable to such excursion is not deductible. Likewise, that portion of the expenses attributable to A's delays en route not necessitated by reasons of rest or repair of his vehicle are not deductible.
26 C.F.R. §1.217-1