Devon County Council is continuing its traffic survey on the A38 despite motorists complaining of delays. It began at 0700 BST on Tuesday and will continue until 1900.
Officials are stopping motorists at two interview bays between Lee Mill and Smithaleigh and at another on the southbound bridge at Deep Lane.
The Council says the survey is vital for improving the region's transport network but during morning rush hour tailbacks reached as far as Ivybridge.
The Council say that the interviews should only last 60 seconds and traffic will be kept moving. Motorists are being asked where they are travelling to, where they have come from and why they are making the journey.
Transportation Planning Officer Caroline Cozens said the work is essential to improve the region's transport network.
She said: "It's caused a few extra minutes on drivers' journeys but most drivers have been very helpful and that's going to help the success of this survey.
"We need to understand what journeys drivers are making."
Devon County Council is working in partnership with the Highways Agency and Plymouth County Council.
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We need to understand what journeys drivers are making 
