I just stumbled upon the NFDC Preferred Routes Database at the Air Traffic Control System Command Center Website. If you plug in you departure and arrival airport, it gives you the preferred IFR route. This information is already contained in the
A/FD Chart Supplement but it is easier to access here. You can also get them on your preferred EFD.
This is a sample output for the Tower Enroute Control (TEC) route (TEC) from San Luis Obispo (SBP) to Orange County (SNA).
NFDC Preferred Routes Query Results
Effective 03/12/2009 until 05/07/2009
Pilot Controller Glossary
PREFERRED IFR ROUTES− Routes established between busier airports to increase system efficiency and capacity. They normally extend through one or more ARTCC areas and are designed to achieve balanced traffic flows among high density terminals. IFR clearances are issued on the basis of these routes except when severe weather avoidance procedures or other factors dictate otherwise.
Preferred IFR Routes are listed in the Chart Supplement U.S. If a flight is planned to or from an area having such routes but the departure or arrival point is not listed in the Chart Supplement U.S., pilots may use that part of a Preferred IFR Route which is appropriate for the departure or arrival point that is listed. Preferred IFR Routes are correlated with DPs and STARs and may be defined by airways, jet routes, direct routes between NAVAIDs, Waypoints, NAVAID radials/ DME, or any combinations thereof.