No impact fee shall be imposed by a unit of local government within a service area or areas upon a developer for the purposes of improving, expanding, enlarging or constructing roads, streets or highways directly affected by the traffic demands generated from the new development unless imposed pursuant to the provisions of this Division. An impact fee payable by a developer shall not exceed a proportionate share of costs incurred by a unit of local government which are specifically and uniquely attributable to the new development paying the fee in providing road improvements, but may be used to cover costs associated with the surveying of the service area, with the acquisition of land and rights-of-way, with engineering and planning costs, and with all other costs which are directly related to the improvement, expansion, enlargement or construction of roads, streets or highways within the service area or areas as designated in the comprehensive road improvement plan. An impact fee shall not be imposed to cover costs associated with the repair, reconstruction, operation or maintenance of existing roads, streets or highways, nor shall an impact fee be used to cure existing deficiencies or to upgrade, update, expand or replace existing roads in order to meet stricter safety or environmental requirements; provided, however, that such fees may be used in conjunction with other funds available to the unit of local government for the purpose of curing existing deficiencies, but in no event shall the amount of impact fees expended exceed the development’s proportionate share of the cost of such road improvements. Nothing contained in this Section shall preclude a unit of local government from providing credits to the developer for services, conveyances, improvements or cash if provided by agreement even if the credits are for improvements not included in the comprehensive road improvement plan, provided the improvements are otherwise eligible for inclusion in the comprehensive road improvement plan.