Licensed contractors and subcontractors shall conspicuously display their home improvement license number preceded by the designation "MHIC #' on both sides of all vehicles primarily used either by the licensee or under the licensee's direction and control for the performance of home improvement work. For purposes of this regulation, performance of home improvement work: Shall include, but not be limited to, transporting materials to and from the jobsite; May not include sales work. Contractors or subcontractors are required to display only their own license numbers while they are performing work as a subcontractor. Displays required by this regulation shall be at least 3 inches in height.