Kohanim in Cars in Cemeteries

A poster on a discussion list, commenting on a psak a few years ago by R' Zev Cohen that it's prohibited for kohanim to travel along a certain block of W. Pratt Ave. in Chicago because of trees that overshadow a Jewish cemetery and one side of the street, wondered why the car would not insulate the kohen. Another poster suggested that perhaps the prohibition is only if one were walking, or if a window was open.

According to the Rambam, Hil. TM 20:1, there are only three things that can protect against tum'a transmitted by an ohel: Tzamid p'sil, Ohalin, and Belu'in.

Acc. to Ch. 21, Tzamid p'sil (a tight seal - which would be negated by an open window) only works by earthenware vessels or, by extension, vessels that are susceptible to tum'a at all. Hence, this would not work for most cars, except perhaps the ones that are made of plastic. The protection of ohel b'soch ohel (a tent in a tent) would not work because a moving car constitutes an ohel zaruk (movable tent) that is not halachically considered an ohel (11:5/6), while the hatzalah of balu'a (swallowed object) only works for something inside a living creature (20/5:6).

Hence, I don't see how being in a moving car would prevent a kohen from becoming tamei. Are any of these restrictions subject to machlokes?

Update, 7/20: There are numerous fascinating articles on the issue of kohanim in airplanes available online, including this one by R' Daniel Wolf of Yeshivat Har Etzion and an article in Tradition by R' JD Bleich (only available to subscribers). To summarize a few points: The Rashba paskens like the opinion on Eruvin 33b which says that an ohel zaruk is a valid ohel, but Rambam and Tosefos argue. The P'nei Yehoshua and Shevus Yaakov hold that even the opinion which says that it's not an ohel zaruk is only based on a rabbinical decree, so that there would be a basis to be lenient in a case of doubt, but there wouldn't seem to be any doubt to base such a leniency on. There's also an interesting discussion regarding whether or not the aluminum that comprises the bulk of most planes is m'kabel tum'a (machlokes Rashi and Rambam regarding whether the list of metals in this week's parsha is exhaustive) and, were aluminum not to be m'kabel tum'a, whether the steel bolts, rivets, etc. that hold together the plane (ma'amid) would still render it m'kabel tum'a and whether the plane's seal satisfies the requirements of tzamid p'sil.


Update, 7/23: Another poster asked if cars would be exempted from the laws of tum'a because they are keilim ha-ba'im b'middah, vessels which are larger than 40 sei'ah in volume (3 cubic amos), and hence are considered to be immobile.

K'li ha-ba'ah b'midah (aka k'li he-asui l'nachas - a vessel intended to be stationary) is only a law by wood, hide, or cloth, as is learned out from a juxtaposition to a sack (which is intended to be movable even when full) in Vayikra 11:32 - see Chagiga 26b and Rambam Hil. Keilim 3:2 and 3:3.

RJDBleich in his article on airplanes that I previously mentioned (Tradition 36:4) mentions several other reasons why this heter would not apply to a vehicle, including that it has wheels (so is movable) and that it is used for seating (as excepted by Tos' Shabbos 44b DH Mucheni).