The Chicago Municipal Code states, in relevant part:
9-52-080(a) Every bicycle when in use at nighttime shall be equipped with a head lamp which shall emit a white light visible from a minimum distance of 500 feet from the front and with a rear red reflector capable of reflecting the head lamp beams of an approaching motor vehicle back to the operator of such vehicle at distances up to 200 feet or a rear lamp emitting a red light visible from a distance of at least 200 feet from the rear.
This language mirrors the language from the Illinois Vehicle Code:
625 ILCS 5/11-1507
Lamps and other equipment on bicycles.
(a) Every bicycle when in use at nighttime shall be equipped with a lamp on the front which shall emit a white light visible from a distance of at least 500 feet to the front and with a red reflector on the rear of a type approved by the Department which shall be visible from all distances from 100 feet to 600 feet to the rear when directly in front of lawful lower beams of headlamps on a motor vehicle. A lamp emitting a red light visible from a distance of 500 feet to the rear may be used in addition to the red reflector. (from Ch. 95 1/2, par. 11-1507)
In Illinois and specifically Chicago you are required to have a white headlight and rear reflector. If you get into an accident at night and you don't have a light you can count on being blamed for the accident almost regardless of what the driver did.
Let's say you are approaching a car that is traveling in the opposite direction - coming toward you. Let's say it is at night and you have no light. Finally, let's say the car makes a left turn in front of you (left cross). They strike you and break one or both of your legs. In this instance you might expect that the driver's insurance would pick up your medical bills since the accident was the driver's fault, right? WRONG. The driver is going to say he didn't see you, and he will point out that you didn't have a light. The driver's insurance will deny your claim outright and may threaten to pursue a property damage claim against you for the damage to their insured's car.
Get a headlight. It is insufficient and illegal to have a rear light only.