Officers are appealing for drivers to contact them after a white Ford Transit Tipper van left the M3 northbound carriageway between junctions 7 and 6 and collided with the mile marker post for junction 6 shortly after 19:10pm last night (Friday, July 29).
No other vehicles are believed to have been involved in a collision with the van, but other motorists may be able to help establish exactly what happened.
The driver, a 25 year-old man from Bournemouth, was taken to Southampton General Hospital by road with a serious leg injury. His condition today is described as critical, but stable.
The northbound carriageway was closed for around 6 hours overnight, and the southbound carriageway was closed for a short period to allow the air ambulance to land.