PDFWAC 204-32-050
Identification signs.
A sign must be affixed to every private carrier bus that is plainly visible and contains the words "private carrier bus." The sign must:
(1) Have letters not less than eight inches in height.
(2) Have letters that are at least three-quarter-inch stroke.
(3) Be located above the windshield on the front of the bus and above the rear windows on the rear of the bus.