[caveat emptor]

That treaty the IBC relies upon for its authority does not actually authorize the regulation of that 20-foot-wide zone.

I spent a few minutes searching through their website and their provided links as well as and independent google search of "1960 international boundary commission act" (where I got only a single hit - to their website!) and could find nothing but their "interpretation", ie 20 ft. Have you found any source documents?

[chrisser]

From here: http://www.canlii.org/ca/sta/i-16/sec5.html (these are Canadian government documents, BTW)

"Construction of works

5. Except with the permission of the Commission, no person shall

(a) construct or place within ten feet of the boundary any work or any addition to a work; or

(b) enlarge any work that was on July 6, 1960 within ten feet of the boundary.

R.S., c. I-19, s. 5. "

[hocndoc]

Evidently, it’s 10 feet on either side, and has been for a while.
http://www.internationalboundarycommission.org/ibcpg2.htm