The crash occurred at Addison Street and involved a motorcycle and another vehicle.
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Illinois State Police said a 38-year-old Chicago man was riding a motorcycle in the left lane when it left the roadway and then crossed all four lanes of traffic to the right.
The motocyclist was then hit by a car that was merging onto the Kennedy northbound from Addison street, police said.
The motorcyclist was transported to a hospital with life-threatening injuries. Police said he was not wearing a helmet.
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THe driver of the car refused medical treatment.
LIVE TRAFFIC: Check out conditions on our live traffic map
All lanes of the outbound Kennedy Expressway wereblocked at Addison. The Addison entrance ramp was also blocked. By 8:45 a.m., lanes were back open with traffic moving outbound again.