Final answer:
Illinois law requires that headlights be used when atmospheric conditions impair visibility and necessitate the use of windshield wipers.
Step-by-step explanation:
The question asks whether Illinois law requires that headlights be illuminated when atmospheric conditions require the use of windshield wipers. This is true.
In the context of Illinois law, there is indeed a requirement that mandates the use of headlights during times when visibility is impaired by atmospheric conditions such as rain, fog, snow, or sleet. This typically means that if weather conditions necessitate the use of windshield wipers, headlights should also be on to improve visibility both for the driver of the vehicle as well as for others on the road.