Agree with Chuck. County boundaries are relatively permanent, unlike
muncipal limits. The last time County boundaries were adjusted in
California was in the late 1940's, I believe, and even then the changes
were minor. A county was dissolved about 100 years ago. This seems
stable enough for travel forecasting work.
/Steve Colman
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"If you find something you like, buy a lifetime supply-- they're going
to stop making it." --Bob Dolfay