Illinois law requires motorists to give bicyclists at least three feet of space when passing a bicyclist. The "3-Foot Rule" comes from sub-paragraph (d) of Section 11-703 of the Illinois Rules of the Road. This statute provides as follows:
(625 ILCS 5/11‑703) (from Ch. 95 1/2, par. 11‑703) Sec. 11‑703. Overtaking a vehicle on the left. The following rules govern the overtaking and passing of vehicles proceeding in the same direction, subject to those limitations, exceptions, and special rules otherwise stated in this Chapter:
(d) The operator of a motor vehicle overtaking a bicycle or individual proceeding in the same direction on a highway shall leave a safe distance, but not less than 3 feet, when passing the bicycle or individual and shall maintain that distance until safely past the overtaken bicycle or individual.
Key language in the statute is obviously that there must be a minimum 3 feet between the bicycle and the motor vehicle when the motorist passes the bicycle. But note that the statute also requires that this distance of 3 feet must be maintained until the motorist is "safely past" the overtaken bicyclist. The goal of this law is to provide bicyclists with enough space to safely operate and to reduce the risk of bicyclists being "clipped" by a passing car. Ultimately, the goal is to reduce the number of bicycle accidents in Illinois.