Thanks for all the responses to this question.  It was a much more complex
discussion than I originally imagined. It came originally from a posting on
alt.architecture (a spam-beleaguered site, with some nuggets of thoughtful
discussion).  It  seemed interesting at the time, but your responses have
proven that there are many facets to this question that depend on local
economic & political conditions, siting of the bypass, etc etc that seem to
make the difference between successful preservation of a historic town center
or its death. Thanks to all who responded!  It has been interesting!

M. Krugman