A very useful basic guide to these laws is [“The Kohen's Handbook”](http://www.amazon.co.uk/Kohens-Handbook-Yochanan-Alexander-Lombard/dp/0976186241) by Rav Yochanan Alexander Lombard.

On p124, 5, Rav Lombard says that a metal car does not block tumah and a moving car is called an “ohel zoruk” that does not block tumah. (He reports an opinion that a car with closed windows may protect kohanim from tumas ohel of a non-Jew.)

Similarly to to your source, he reports that:

If the branches hovering over both vehicles at the same time are a tefach wide, they  conduct the tumah and cause the Kohen to become tomei. 

Even (p66) where the branches are less than a tefach wide, if there is dense foliage this combines to make a roof by rabbinic law and the Kohen should not go there.