Continuing obligation. All required parking facilities shall be provided and maintained as long as the use exists which the facilities were designed to serve. Off-street parking facilities shall not be reduced in total extent after their provision, except upon the approval of the Zoning Hearing Board and then only after proof that, by reason of diminution in floor area, seating area, the number of employees, or change in other factors controlling the regulation of the number of parking spaces, such reduction is in conformity with the requirements of this chapter. Reasonable precautions shall be taken by the owner or sponsor of particular uses to assure the availability of required facilities to the employees or other persons whom the facilities are designed to serve. Such facilities shall be designed and used in such a manner as to at no time constitute a nuisance, a hazard, or unreasonable impediment to traffic.