Directional, information, or public service signs such as those advertising availability of rest rooms, telephone or similar public conveniences, and signs advertising meeting times and places of nonprofit service or charitable clubs and organizations may be erected or maintained; provided, that such signs do not advertise any commercial establishment, activity, organization, product, goods, or services except those of public utilities. Directional and information signs provided for the guidance and convenience of the public within commercial establishments may also be erected. Such signs shall not exceed two square feet in area.