Beginning July 1, 1988, all signs erected and used to designate the reservation of parking facilities for a person with disabilities shall be in a form and manner prescribed under Section 11-301 of this Code, and all parking spaces reserved for a person with disabilities, except those reserving on-street parking areas, shall be at least 16 feet wide. Non-conforming signs in use prior to July 1, 1988 shall not constitute a violation during their useful lives, which shall not be extended by means other than normal maintenance. Beginning October 1, 1992, all parking spaces reserved for a person with disabilities, except those reserving on-street parking areas, shall be at least 16 feet wide.