The census, being conducted on behalf of the city council, has been carried out in various locations including the Wells Road and M32, and looks set to continue until Monday.
Drivers are asked to fill in a short survey which includes questions about their journey.
Motorist Iain Wiltshire said: “I wondered what the problem was this morning when I passed through Keynsham and crossed the bridge near the High Street over the bypass; traffic was stationery on the Bristol-bound carriageway.
“It looked like the queue went back to the Broadmead roundabout by the Co-op. It was also stationary towards Hicks Gate and queued back to where the A431 crosses the ring road near Hanham.”
Fed-up commuters vented their frustration on Twitter as they sat in traffic jams.
“Been on the A4 from Bath to Bris for an hour and only just reached Keynsham RFC playing fields,” said one.
“Never have I had an opportunity to study Keynsham bypass in such detail,” said another.
“Traffic is at a standstill on dual carriageway between Saltford and Hicks Gate, avoid if you can,” said another driver tweeted.