I was always under the impression that one person can say Tefilas Haderech and the others can answer Amein and you are Motzi them.
Today I was travelling and the fellow said Tefilas Haderech and when he finished he said he did not have me in mind, since he has heard that Tefilas Haderech is a Tefila one has to say it alone and can not be Motzi others. He did not know the source, and I was wondering who is the source of this ruling and what are the dissenting opinions?
It is based on the difference between a tefillah and a bracha. The difference is that one can be yotzi when someone recites a bracha (as with hamotzi) but must recite a tefila personally (as with shmona esrai) unless with a minyan and in extraordinary circumstances (which is the basis of chazaras Hashatz)
We see at Kollel Shaarei Horaah
HaRav Shmuel Kaminetzky shlita ruled that Tefilas HaDerech is a Tefila and not merely a Bracha. Consequently, he advised the Bochrim in Yeshiva that each must recite it himself, and not rely on a single individual saying it out loud for everyone. Of course, if it is not feasible for the driver to stop and look inside a Siddur and he cannot recite Tefilas HaDerech otherwise, hearing it from another is better than nothing. Especially considering that there are dissenting opinions.
Halachipedia says: How should it be said
It is preferable that each person say it themselves but the strict law permits one to say it and others to fulfill their obligation by hearing it. 21
Piskei Teshuvot 110:3
dinonline states that one is allowed to hear it from someone else (especially if one is the driver) though it is not preferred.
If there is another passenger in the car, the passenger should recite tefilas haderech for both of them. If this is not possible, he should insert the text of tefilas haderech into the bracha of Shema Koleinu during the Shemoneh Esrei immediately prior to his trip. (Sh.A. 4, M.B. 21-22, Aruch HaShulchan 11, Sefer Ishei Yisrael 50:2, Sefer Tefilah K’Hilchasah chap. 27, footnote 80)
Another option, as we mentioned earlier, is to rely on the opinion that tefilas haderech can be recited even if one is still in the city.
One who is traveling on public transportation, such as a bus or train, should stand in order to recite tefilas haderech, if at all possible, since in this case, standing will not be a cause of delay.
Although one fulfills his obligation of tefilas haderech by hearing it recited by someone else, he should preferably not hear it through a loudspeaker, since the sound heard is not a valid bracha according to some opinions. (Sefer Ishei Yisrael 50, footnote 1, Sefer Halichos Shlomo 22:15)
