Question:  Caltrans publishes traffic count data that is organized by Route number and post mile.  The database sometimes includes counts on either side of a cross street, labeled as “Back” and “Ahead”.  The Ahead data for one posting should match the Back data for the next one, but sometimes they don’t.  What number should be used when they don’t match? 

 

Answer:  Caltrans traffic data is available on their website at http://www.dot.ca.gov/hq/traffops/saferesr/trafdata/2000all.htm.  An example of your question is Route 101 in Santa Clara County.  The Ahead data shown for the Great America interchange (in this case, Ahead is toward San Francisco) is 12,300 vehicles per hour.  The Back data for Lawrence Expressway, the next interchange to the north is exactly the same.  This makes sense because it is a restricted access freeway and no one should be entering or exiting between these interchanges. 

 

A second example is Route 82 in the same general area.  The Ahead data at Scott Lane is 3,150 vehicles per hour.  The Back data at Lawrence Expressway is 4,000.  El Camino Real (Route 8) has no access restrictions.  There are several driveways and cross streets along its length, so it is not too surprising that the volumes are different. 

 

The best thing you can do is contact the Traffic staff in your local Caltrans office and ask for the detailed traffic count data.  That will tell you the date of the count and specific location.  From there you should be able to judge which number to use.  If you don’t know who to call, start with the Public Affairs manager for your County, found at http://www.dot.ca.gov/dist4/publicaffairs/pastaff.htm