Within the limits of its budget, the Department of Transportation may erect one sign within each 100 miles of state highway mileage in each county, warning motorists of the penalties for littering or asking for their cooperation in keeping the highways clean. One such sign shall be located within a reasonable distance of all state highway entry points into this State from other states or countries. If the state highway leads to or from an international border crossing point, the sign shall be bilingual. [PL 1975, c. 739, §15 (NEW).]