Regarding tefilas Haderech, many times I am driving next to my wife on a trip and it is easier if she recites tefilas haderech and has me in mind during driving. Is this ok? I recently found a Yerashalmi in brachos 3:3 that says that one should not be yotzei through another person is birkas hamazon, shema, and tefilla. Do we say that tefillas haderech is a tefillah and therefore one should not ideally hear it from another person, or is it like any other bracha that we say brov am hadres melech and there is even an inyan to hear it from another person instead of saying it ones self?
The reason given by the Yerushalmi (concerning tefillah) is that each person should ask for rachamim for himself. This reason would seemingly apply even for tefillas haderech.
However, although the ruling is mentioned by poskim with regard to krias shema and tefillah (see Orach Chaim 59 and 124, and Magen Avraham 59:5, and Biur Ha-Gra), poskim mention that it does not apply to other blessings (see Magen Avraham; see also Magen Avraham end of 6, and Peri Megadim, Eshel Avraham, end of 6), and even when there is a ‘prayer’ involved, such as in Birkas Hatorah or in the blessing at the end of Birkos Hashachar, the common custom is for one to be motzi another (although this should not be done for a baki, unless there is reason for it). This would also apply to tefillas haderech.
Where there are more than two people, it would even be preferable that one should be motzi the others, on account of the pricniple of berov am hadras melech.
With regard to Birkas Hamazon, the Gemara writes against being yotzei with one’s wife’s blessing, but this is because a person should learn and know the blessing on his own, and in the circumstances you describe (difficult to concentrate while driving) it would be permitted to be yotzei from your wife.
Further to the above, see also Shulchan Aruch 298:14 concerning the blessing of me’orei ha’eish, and the same would apparently apply to tefillas haderech; see also Shulchan Aruch 8:5, Gra 12, and Mishnah Berurah 13, concerning the berachah for tzitzis.