From the report:
"3. Recent trends in traffic estimates
A number of factors may affect traffic volumes. For example, traffic levels would normally be expected to change in line with economic growth, as well as widespread weather events. When comparing changes between quarter one of 2012 and quarter one of 2013:
An important factor affecting traffic appears to have been the heavy snowfall and icy conditions in many areas of Great Britain in the first quarter of 2013, dampening the traffic volumes. Initial analyses suggest that that this has not been to the extent of that seen in quarter four (October to December) of 2010.
Preliminary estimates show GDP to have increased slightly overall between quarter one 2012 and quarter one 2013, by 0.6 per cent, with increases in the sectors we would expect to have the greatest influence on traffic volume. For example, the index for transport, storage and communication increased by 2.5 per cent between the first quarters of 2012 and 2013 and index for distribution, hotels and restaurants increased by 0.7 per cent during the same period."
If you look at the graph there, it shows traffic growing from 1993, then plateauing from the end of 2007 onwards: presumably something to do with the global financial crisis and subsequent economic recession/stagnation?