Traffic counts...

The Department for Transport counts traffic at a large number of locations around the UK's road network.
These can be accessed via the website here.

They are available for each county. Here's part of the Norfolk map showing the area around Swaffham.

Click a marker, and then choose a section of the road and you can download a CSV file of the data for that stretch of road.

This will show you the average annual daily flow of cars, lorries etc. for a number of years. Choose a series of roads, and you can assess the rate of traffic flow, changes over time, and the influence of particular junctions.

I came across this data while I was working on a forthcoming resource for the Chartered Institute of Logistics and Transport (CILT) in association with the GA.