Emergency services attended the scene but he wasn’t in attendance and extensive enquiries are now ongoing to trace him.
Police are concerned he may have been injured in the incident, which took place on the A7 near to the Lindean Junction, north of Selkirk.
Shane is described as 6ft tall, with brown hair and facial hair and blue eyes but it is unknown what he was wearing at the time.
The female driver of the second car was taken to Borders General Hospital for treatment.
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Police Inspector David Aitchison said: “We are concerned for Shane’s welfare, as we believe he may have suffered injuries, and is imperative that we trace him as soon as possible.
“Significant resources, including the police helicopter and dog branch, have been involved in searches of the local area and enquiries are continuing.
“Shane has links across the Scottish Borders and I am keen to hear from anyone who may have information about Shane’s whereabouts. Please get in touch, as we need to ensure his safety.
“I am also appealing directly to Shane, if you are reading this, please let us know you are safe and well.”
Anyone with any information is asked to call 101, quoting reference 3784 of 14 January, 2025.