The incident happened in the chain restaurant's parking lot in Elizabethtown, Kentucky.
Three people -- two women and one man -- were in the car at the time of the crash, according to the news outlet.
One passenger was critically injured and taken to University of Louisville Hospital, where she later died, police said. The woman was identified on Friday as Lillian Curtis.
The two other victims were transported to Baptist Health Hardin hospital with unknown injuries, WAVE reports.
Strong winds contributed to the sign's fall, police said. Witnesses say the wind blew the sign from its post, causing it to land on the parked car.
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A Denny’s spokesperson released a statement on Friday:
“Denny’s is aware of the incident that took place at our Elizabethtown location on Thursday. Safety is our top priority, and we are working with the authorities to better understand what led to this situation. Our thoughts are with all of those involved.”
The investigation into the cause of the sign's fall is ongoing.