(a) Where destination or route information is displayed on the exterior of a vehicle, each vehicle shall have illuminated signs on the front and boarding side of the vehicle.
(b) Characters on signs required by paragraph (a) of this section shall have a width-to-height ratio between 3:5 and 1:1 and a stroke width-to-height ratio between 1:5 and 1:10, with a minimum character height (using an upper case ``X'') of 1 inch for signs on the boarding side and a minimum character height of 2 inches for front ``headsigns'', with ``wide'' spacing (generally, the space between letters shall be \1/16\ the height of upper case letters), and shall contrast with the background, either dark-on-light or light-on-dark.