On the VDOT traffic data website, you can search for AADT (Average Annual Daily Traffic), and in that list it has the AADT for every street in the county/city. Its tedious, but it has every street, and i think the latest is 2010. Link:
http://www.virginiadot.org/info/2010_traffic_data_by_jurisdiction.asp
Should be one for City of Fairfax and Fairfax County.