1. A person is required to say Tefilas Haderech whenever he takes a “long-distance” journey. The mode of travel is insignificant, whether by foot, animal, wagon, ship, automobile, railroad or even airplane. Even traveling on highways or by rail also involves a certain degree of danger
2. A journey involving the obligation to say Tefilas Haderech with a concluding berachah must be to a destination at least one parsah beyond the city limits
Uninhabited areas, such as gardens and orchards, which are on the outskirts of the city and beyond houses are considered part of the required parsah .
3. If one is traveling less than a parsah [though there is no requirement], he may still say Tefilas Haderech, but without the concluding berachah.
It seems that he may also conclude with the berachah, if he wants, provided he omits G‑d’s name .
4. If the area in which one is traveling is dangerous, then Tefilas Haderech is in order even for a journey less than a parsah .
5. The distance of a parsah is equivalent to 8000 amos which in our terms range from 3.840 km. / 2.385 miles to 4.640 km. / 2.9 miles to 4.800 km./3 miles .
6. The distance of a parsah according to a majority of poskim refers to a spatial distance, even though it may take just a few minutes to travel .
Some poskim, however, take it to mean a distance based on the time that it would actually take to walk a parsah, which is 72 minutes. Thus, any journey that takes less than 72 minutes would not require a Tefilas Haderech. However, if one wants to say Tefilas Haderech without the concluding berachah, he may do so .
7. When one is traveling more than a parsah, but there are inhabited areas along the way, on either side, within a parsah, as is common on many of today’s highways, it is questionable whether Tefilas Haderech should be said or not .
Therefore it is better that the Tefilas Haderech be recited without the concluding berachah .
It would seem therefore according to this, that if the highway cuts through the city, even if the highway is long, Tefilas Haderech is not said, since the traveler really never left the city.
8. Once one has left the city, even though there are houses constantly along the way, he should still say Tefilas Haderech immediately and not wait until the houses cease. Nowadays, the main danger is accidents, especially due to speeding, and therefore Tefilas Haderech should be said once he is on the highway outside the city limits. [Because of these dangers, one may say it without the closing berachah, even when driving within the city.]
9. If one is traveling between two cities and the distance is less than 2 parsah, then Tefilas Haderech is not said .
10. Even though there is a principle that those who are on a mission for a mitzvah are protected from harm, they must still say Tefilas Haderech .
Our sages comment that the two verses starting with “Vayehi binso’a” (Bamidbar 10:35-36) are considered as a separate book. The Yetav Lev “jokingly” explains that there are people who are very meticulous and scrupulous in the observance of kashrus and other religious matters when they are at home, but who manage to find leniencies for everything, when they are away on a journey rationalizing their actions and arguing that on a journey everything is permitted. This, says the Yetav Lev, is the meaning of the comment that “Vayehi binso’a” is a separate book — that “binso’a” — when one travels — he thinks that there is a different Torah than the one he practices at home.
With this in mind, one can also explain the verse “al pi G‑d yachanu ve’al pi G‑d yisa’u” — “according to the word of G‑d they would encamp, and according to the word of G‑d they would journey” . Just as when they “camp” at home, they are guided completely by the word of the Torah, so too, when Jews travel they should be guided and observant of all the laws of the Torah.
In the same vein, the verse concerning the prophet Shmuel which states that wherever he traveled to judge the people “for there was his home” can be explained as follows: “Wherever he went, his home was with him,” i.e. that he always conducted himself, wherever he may have been, with the same consistency and observance as if he was at home.