Lincolnshire Police has referred itself to the Independent Office for Police Conduct after a man died following a crash on the A1 on Wednesday 21 December.
Officers were called to reports of a collision on the A1 at Colsterworth at just before 4pm, where an HGV had left the southbound carriageway.
The driver and only occupant of the lorry was arrested on suspicion of driving over the legal alcohol limit and was taken to Grantham Police Station.
A short time later he became unwell and was taken to hospital, where he subsequently died.
The A1 was closed to allow time for police to examine the scene and has now reopened to vehicles.
Officers are appealing for witnesses who were travelling southbound on the A1 in the Colsterworth area and saw the HGV travelling on the A1 shortly before or as it left the carriageway.
Police would also like to hear from anyone who has dashcam footage that will assist the investigation.
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