Thames Valley Police officers found a man who had died in a black Vauxhall Astra on the central reservation of the M40 between junctions 11 and 12.
Police intelligence suggests the incident happened at around 4.20am in Oxfordshire back on 5 April.
Police confirm the man who died was in his 40s and that his family have been informed and are receiving help from specially trained officers.
Investigating officer, PC Charlie Etheridge, of the Joint Operations Roads Policing Unit, said: “We are appealing for any witnesses, or motorists who may have dash-cam footage, of the incident to please come forward.
“Although this was reported to us yesterday, we believe it may have happened at around 4.20am on 5 April.
“Anyone who has information or dash-cam footage can make a report by calling 101 or via our website, quoting reference 43220157746.”