605 ILCS 5/4-403
Whenever local traffic conditions within any municipality through which or to the corporate limits of which any State highway is located, in the discretion of the Department, are such as to interfere with or impede through or State traffic, the Department is authorized and directed to locate and construct a durable hard-surfaced highway in the nature of a belt-line to connect State highway routes entering such municipality, so as to avoid congested traffic districts in the municipality. Such belt-line routes may be wholly without the corporate limits of the municipality or partly within and partly without such limits.
The type of construction and width of such belt-line routes shall be sufficient to care for present or reasonably expected future needs of through or State traffic.
Such belt-line routes shall be a part of the State highway system.
605 ILCS 5/4-403