The attack happened in the early hours of Saturday 23 November at some time between midnight and 12.45am.
The victim, a woman in her twenties, was targeted by an unknown man on the Wroughton M4 overpass.
Officers are appealing for witnesses and dash cam footage.
A force spokesperson said: "Our enquiries into the incident are ongoing.
"Anyone who witnessed the incident, or drove over the overpass at around the times above, is asked to contact us on 101 quoting reference number 54240138383."