Officers stopped the vehicle on the M74 near junction 16 for Johnstonebridge, in Dumfries and Galloway, at around 3.30pm on Wednesday.
They discovered herbal cannabis in the vehicle.
Police said a 52-year-old woman and two men, aged 18 and 38, have been arrested in connection with the incident.
Detective Inspector Dean Little, from Dumfries CID, said: “This is a significant amount of illegal drugs that have been removed from circulation and can no longer cause harm.”
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