(A) Passing on the left.
(1) No vehicle shall be driven to the left side of the center of the roadway in overtaking and passing another vehicle proceeding in the same direction unless authorized by the provisions of this chapter, and unless the left side is clearly visible and is free of oncoming traffic for a sufficient distance ahead to permit the overtaking and passing to be completely made without interfering with the safe operation of any vehicle approaching from the opposite direction, or any vehicle overtaken.
(2) In every event, the overtaking vehicle must return to an authorized lane of travel as soon as practicable, and in the event the passing movement involves the use of a lane authorized for vehicles approaching from the opposite direction, before coming within 200 feet of any vehicle approaching from the opposite direction.
(ILCS Ch. 625, Act 5, § 11-705)
(B) Conditions where passing on the left is prohibited.
(1) No vehicle shall be driven on the left side of the roadway under the following conditions:
(a) When approaching or upon the crest of a grade or a curve in the highway where the driver's view is obstructed within a distance as to create a hazard in the event another vehicle might approach from the opposite direction;
(b) When approaching within 100 feet of or traversing any intersection or railroad grade crossing; or
(c) When the view is obstructed upon approaching within 100 feet of any bridge, viaduct, or tunnel.
(2) The limitations in division (B)(1) above do not apply upon a one-way roadway, nor upon a roadway with unobstructed pavement of sufficient width for two or more lanes of moving traffic in each direction, nor to the driver of a vehicle turning left into or from an alley, private road, or driveway when the movements can be made with safety.
(ILCS Ch. 625, Act 5, § 11-706) Penalty, see § 71.999