The only drop in 'traffic' I've ever noticed was in my former life in
the military. Retreat policy on base was that traffic pulled to the side
of the road and the driver got out, faced the music and rendered the
appropriate salute through the end of the anthem. Since this occurred at
a regularly scheduled time one indication of its approach was a
noticeable drop in traffic preceding retreat.
Sad commentary on our allegiance at the time.