They are redoing the highway near my house, and running a few miles of
board-on-board fencing along the back of the yards that back up to the
highway.

Basically, this stuff:

http://www.outdoorstyle.ca/images/fe...oard-fence.jpg

I noticed that in some sections they alternated the boards like in the
picture so that you can't see through the fence, but other sections
have the boards directly opposite each other so the fence is open
between the uprights.

Similar to this, but with boards on both sides:

http://www.cedarfencing.com/images/d...st_550x300.jpg

There's a half mile or so of alternating, then some open, then some
alternating - mostly alternating.

There doesn't seem to be anything different about the locations as all
sections are along the back yards of residential houses.

Can you think of any reason why they did this? Bad workmanship comes
to mind, since I can't think of why they would want to leave the fence
open to the highway noise. I know if I lived in those houses I'd have
them "close" the fence for more privacy, less noise.