The man was found on the E45 outside Alafors, about 10 kilometres south of Gothenburg.
“Either somebody killed him deliberately or by misadventure, or it was an accident,” said Geir Hogstad of Västra Götaland Police.
The man was found dead at about 3:30am on Sunday.
“We have found a dead man in his thirties or forties. We have not confirmed his identity,” said Hogstad.
Around ten people who were in the area at the time have given witness statements.