Motorists on the M3 were shocked to find themselves driving through a sudden shower of £20 notes.
Only £500 out of an estimated £11,000 was recovered, police say
About £11,000 billowed out of the rucksack of a motorcycle passenger heading south near Winchester services in Hampshire last Tuesday night.
Several motorists pulled over to try and help the motorcyclists find the cash, but only £500 was recovered.
Police said they are making inquiries as to why the pair were carrying so much cash.
Hampshire Police say they had been concerned about safety as a number of cars screeched to a halt at the side of the motorway in a dash to get the cash.
"We are not saying that other drivers took it because it was a windy night and that would not have helped. It was over both carriageways," she said.
The motorcyclists, from the Isle of Wight, told police they were going to buy a car with the money.
"This raised our suspicions and we are making checks as to where the money came from at the moment but it's not an offence to carry money," the spokeswoman added.
The motorcycle broke down two junctions further down the motorway.