She said witnesses and people from Countdown quickly came out to help the woman, who was lying motionless on the ground. Some brought blankets to keep her warm until emergency services arrived.
She described the screams as "deafening".
"It was so scary. Family of the people were screaming and kneeling down beside them."
Meanwhile Mr Membery said the driver of the car was being spoken to by police but it was too early in the investigation to determine how the incident happened.