Sgt Brian Simpson said the force’s thoughts were with the victim’s family.
He added: “We are carrying out extensive inquiries to establish the full circumstances and we continue to appeal to anyone who witnessed the crash, or has dashcam footage from the area to please come forward.
“We know that motorists drove through the scene immediately after the crash.
“We are particularly keen to speak with the drivers of a dark-coloured saloon vehicle and a teal-coloured pick-up truck with a cab on the back.
“These drivers may have information which could assist with our investigation.”
Anyone with information is asked to contact Police Scotland.