Burgess J.W. Raby & William L. Raby have published "Metropolitan Area" and "Away From Home," also available on the Tax Analysts web site as Doc 2005-4846, 2005 TNT 46-97.  Here is part of the Conclusion:

We started with the question, "Where is home?" The context, of course, was the deductibility of expenses when away from home. We were reminded that revenue rulings can be used against the IRS, as in Walker, but are often disregarded by the courts, especially the Tax Court, when they do not seem to be reasonable under the circumstances. They are also subject to interpretation by the courts and often the court will find it disagrees with the IRS interpretation of a ruling, as Judge Couvillion did with the IRS interpretation of "metropolitan area" in Wheir.

Home is a metropolitan area when you are an employee traveling to a temporary job location on business. If home is your principal place of business and you are self-employed, you can deduct the travel expenses whenever you travel on business, whether within or without your metropolitan area. Of course, what is an appropriate metropolitan area may well be open to debate. In Wheir, the IRS considered the taxpayer’s entire state to be such an area inasmuch as he lived in a city that the census bureau did not recognize as being part of a metropolitan area. That seems to be rank discrimination, of course, penalizing those who live in rural America as against dwellers in more populous cities. Perhaps the IRS reasoned that the psychic income from a more rural lifestyle needed to be subjected to income tax. (Of course, the metropolitan area approach also discriminates against those who live and work in extremely large metropolitan areas, such as Phoenix and Las Vegas, compared with smaller areas like Green Bay or Wausau.)

The tax practitioner faced with a client who wants to maximize deductions for travel while away from home will resolve the borderline questions based on the client’s risk tolerance as well as the practitioner’s own bias toward pushing the envelope or taking more conservative positions….