Rabbi Menachem M. Schneerson suggested in a letter that “as a segulah and protection against accidents, one should have a sidur (Prayer book), Tehilim (Psalms), Tanya and a tzedakah box (Charity box) in his car.” The Rebbe then mentions in the letter that it is appropriate to publicize this.
Most Chabad Chassidim will take along the above-mentioned even if they are traveling by plane, boat, train, or going anywhere for that matter.
I have long had the custom to travel with a mezuzah, one, for the extra protection, and two, because one should put up a mezuzah on their door post if they will stay at a fixed place for more than 30 days. It does occasionally happen that I will rent a room for more than a month.
In the attached picture with this post, I am on top of Mt. Kilimanjaro, the highest mountain in Africa. I am holding a picture of the Lubavitcher Rebbe which has Tefilas Haderech printed on the back, and the white thing is a mezuzah.
To read about my Mt. Kilimanjaro climb: