Holidaymakers are facing delays as around 150 lorry drivers join a go-slow protest over the price of fuel.
A convoy set off along the A30 - the main route out of Cornwall - as thousands of holidaymakers headed home following the Whitsun half-term.
The main convoy of around 150 vehicles, mainly lorries, was travelling at less than 10mph from Truro, heading for Launceston, before returning to Truro later, Devon and Cornwall Police said.
But, despite reports of gridlock traffic from the scene, police said there were no reports of any major delays.
The Highways Agency warned drivers to expect "moderate delays" of 20 minutes along that stretch of the A30.
"There is a little bit of disruption down there, about 20 minute delays. We have been advising people to use the A38.
"Traffic is not gridlocked. It is busy down there but we are monitoring it and working closely with the police to make sure disruption is kept to a minimum."
A police spokesman advised drivers to set off after the protest or use alternative "A" class roads.
He added: "If the protesters drive normally and do not impede other road users they will be allowed to continue. If they engage in wilful obstruction of the highway or other offences against the Road Traffic Act, then we will gather evidence and prosecute."
The action follows fuel protests in London and Cardiff on Tuesday.
