Kieran Currie, 19, was on the A75 Gretna to Stranraer road in the early hours of Saturday when he was struck by a passing articulated lorry.
He later died in hospital.
Shortly after the crash police traced and checked several heavy goods vehicles but they have yet to speak to the lorry driver involved in the collision about a mile east of Newton Stewart in Dumfries and Galloway.
Mr Currie was with two friends when he was struck at around 2.40am near the entrance to Calgow farm.
Police believe the occupants of a small blue car who offered assistance at the scene may be able to help them with the investigation.
Inspector Neil Hewitson, roads policing inspector for Police Scotland, said "This is a tragic set of circumstances which has left a young man dead.
"The lorry which appears to have struck Kieran has yet to be traced, and it may well be that the driver of that vehicle is unaware that he has been in a collision.
"We are appealing for anyone who may have been in the area at that time to get in touch and, in particular, we want to hear from the drivers of all heavy goods vehicles who were on the A75 driving east around that time.
"Likewise, we are keen to speak to the occupants of a small blue-coloured car who stopped at the scene of the crash shortly after it happened and offered assistance."