State police said it happened around 1 p.m.
The suspect arranged to meet the victim at a South Front Street lounge, then entered the victim's car and robbed him at gunpoint when he arrived, authorities said.
The victim was forced to drive I-95 while the suspect threatened to kill him and his girlfriend, according to police. Eventually the victim pulled onto the the shoulder and fled the car on foot, troopers said.
The suspect also exited the car and tried to get in the driver's seat, but the door was locked, officials said. He shot out the passenger's side window then ran westbound across four lanes of the Delaware Expressway, state police wrote.
In the far left northbound lane, a passing driver struck the suspect, launching him into the air before he landed back in the roadway, authorities said.
He was taken to Jefferson University Hospital with "damage to his brain, a broken foot and other injuries" and placed in "very critical condition," state police said.
Neither the victim nor the driver who hit the suspect were injured, they added.
Troopers did not name the suspect but said he is a 42-year-old man from Quincy, Massachusetts. The incident remains under investigation and anyone with information is asked to call PSP's Troop K at 215-452-5216.
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