Both police and ambulances are on the scene for the incident, which took place between Bodmin and Fraddon. Speaking to Cornwall Live a witness commented: “Massive fire on the flyover just past Roche services heading for Indian Queens. Huge tailbacks both ways.
“Huge police presence and three ambulance services.”
Photographs from the scene show a small truck burning on the flyover, with police having blocked the A30 below.
Devon and Cornwall Police told Cornwall Live they were first notified about the incident at 6.32pm.
A spokesman said: “We are currently dealing with a vehicle fire on a road over the A30 near Trevose.
“We are on the scene with the fire service and awaiting more updates from the scene.”
Some of those caught up in the disruption expressed their concern and confusion via Twitter.
One posted: “In laws stuck in traffic – a car fire on a bridge in Cornwall.”
A second wrote: “Huge police presence on the A30 here…something’s up.”
On Thursday Boris Johnson held a meeting with Mr Biden, who is making his first overseas trip as president.
The US leader commented: “We had a good first full day in the UK.
“Prime Minister Johnson and I had a very productive meeting.
“We affirmed the special relationship — it’s not said lightly — the special relationship between our people.”
Mr Johnson returned the praise describing his American counterpart as a “breath of fresh air”.
The Prime Minister said: “The talks were great. They went on for a long time.
“We covered a huge range of subjects. It’s wonderful to listen to the Biden administration and Joe Biden because there’s so much they want to do together with us.
“It’s fantastic. It’s a breath of fresh air.”
The G7 consists of the UK, US, Germany, France, Japan, Italy and Canada with the EU also represented.
Britain, which is hosting this year’s summit, invited leaders from India, Australia and South Korea to join the main group.
Mr Johnson is hoping to use a meeting with Scott Morrison to finalise a post-Brexit trade deal with Australia.