Swindon Police are appealing for witnesses following a fatal collision on the M4 in the early hours of Saturday, 1 October.
At about 2am officers attended an incident between junction 15 and 16, near the road bridge (A4361) near Wroughton.
A force spokesperson said: "Sadly, a young woman died at the scene.
"Our thoughts are with her family at this very difficult time. Please respect their wishes and do not contact them at this time.
"Any witnesses to the collision or anyone who may have dash cam are asked to contact the Serious Collision Investigation Team at Police Headquarters on 01225 694597.
"Alternatively they can contact the team directly - SCIT@wiltshire.police.uk."