Most drivers, when passing an emergency vehicle stopped on the side of the road, probably shift to the other lane or slow down because of courtesy or common-sense safety. In Illinois, it's the law, and, according to an article in today's Southern Illinoisan, state police are starting to crack down on violators.
On a typical four-lane highway, a driver "approaching a stationary authorized emergency vehicle, when the authorized emergency vehicle is giving a signal by displaying alternately flashing ... warning lights" is required to "yield the right-of-way by making a lane change into a lane not adjacent to that of the authorized emergency vehicle, if possible with due regard to safety and traffic conditions" or "reduce the speed of the vehicle, maintaining a safe speed for road conditions, if changing lanes would be impossible or unsafe." 625 ILCS 5/11-907(c).
Illinois law also requires drivers to "yield the right of way to any authorized vehicle or pedestrian [working in a] highway construction or maintenance area," and to change lanes or reduce their speed when "entering a construction or maintenance zone when workers are present." 625 ILCS 5/11-908(a) and (a-1).
Sections 11-907 and 11-908 are exceptions to the general rule in Illinois that "[u]pon an Interstate highway or fully access controlled freeway, a vehicle may not be driven in the left lane, except when overtaking and passing another vehicle." 625 ILCS 5/11-701(d) and (e).