605 ILCS 5/5-104
Except as otherwise provided in this Code, no mileage shall be added to a county highway system after the effective date of this Code, if such addition causes the total mileage of highways in the county highway system of the county to exceed 35% of the total rural public highway mileage in a county having less than 500,000 inhabitants or 75% of the total rural public highway mileage in a county having 500,000 inhabitants or more. When 80% of the maximum permissible county highway system mileage in a county is of proper width and alignment and has been satisfactorily improved with oiled earth, gravel, macadam, portland cement concrete, bituminous concrete or brick on a portland cement concrete base, or other hard-surfaced type of pavement, patented or otherwise, (including surface or subsurface drainage, grading, bridges and culverts thereon having adequate design and roadway width and satisfactory horizontal and vertical alignment and capable of sustaining highway traffic with safety), as determined by the Department, such county may, in the manner provided by Section 5-105, add to its existing highways in the county highway system, additional highway mileage to the extent of 10% of the permissible highway mileage of county highways in the county.
The total rural public highway mileage in a county shall be determined and published by the Department.
In determining the maximum permissible county highway system mileage of any county under this Section the mileage of county highways within the corporate limits of a municipality shall not be considered.
605 ILCS 5/5-104