Are U-turns legal in Fairfax County?

In general, you can make a U-turn, but there are significant restrictions. The statewide restrictions are spelled out in this law (Virginia Code Section 46.2-845).  

In addition, Fairfax County has a "No U-Turn Sign" at many intersections. These are there for several reasons, but mostly for safety and traffic flow.   

I once won a case where the charge was "Illegal U-Turn" under Virginia Code Section 46.2-845, but there was a sign at the intersection.   They should have charged "Failure to Obey a Highway SIgn" under Virginia Code Section 46.2-830, because the U-Turn statute doesn't prohibit U-turns at intersections.    Just one of the many quirks of Virginia Traffic law.