When can you legally turn left on a red light?

Turning left on a red light is only legal under specific circumstances. The correct answer highlights that this maneuver is permissible when you are turning from a one-way street to another one-way street. This allows for a smoother flow of traffic in certain urban settings, where one-way streets are prevalent.

In this scenario, it's essential to ensure that the left turn is made safely, yielding to any oncoming traffic and pedestrians. This regulation helps maintain safety while allowing drivers flexibility in navigating through intersections.

The other options do not align with Illinois law regarding left turns on red. Turning at any red light if safe or whenever no cars are coming disregards traffic control signals, while the necessity of a sign permitting the left turn clarifies that this maneuver is not universally permitted. Understanding these nuances aids in ensuring compliance with traffic laws and promoting road safety.