How many Virginia secondary routes retain their number when they cross county lines?
Most Virginia secondary roads that cross county borders
do not change route numbers (note that I exclude cases where a city is involved, since Virginia cities generally do not sign secondaries, nor cases where a county boundary is also a town boundary on one side for the same reason).
The two secondaries that I can immediately think of in Northern Virginia that cross the line are 620 (Hoo mentioned that one); 612 (Yates Ford Road) between Prince William and Fairfax Counties); and 659 between Prince William and Loudoun Counties.
621 (Freemans Ford Road) crosses the Rappahannock River (also the border between Culpeper County and Fauquier County).
Then there's the matter of 601 that roughly straddles the boundary between Clarke and Loudoun Counties at the crest of the Blue Ridge (and passes by the federal Mount Weather complex, much beloved by conspiracy theorists).
606 exists in Fairfax and Loudoun Counties, but it "gaps" at the county line, because it crosses into the Town of Herndon on the Fairfax County side, though Herndon is the
rare Virginia municipality that signs at least one secondary road (this one) within its corporate limits. On the other side of Herndon, the VDOT-posted route number happily resumes.
Post Merge: October 16, 2016, 09:57:45 PM
I believe most do, but I am sure there are exceptions that keep me from thinking of any sort of a percentage.
I cannot name a secondary route in Virginia that changes numbers at a county line, though there's probably somewhere a primary route that becomes a secondary at a county line, which means the route number has to change.