Question: What is the safar (travel) distance in Islam?
ANSWER
Even if one travels a way of three days on a fast vehicle, such as a train, which will naturally take much less time, one still becomes a safari (traveler). (Sharh al-Majalla)

If a new vehicle were made and if it were possible to travel to the USA in a second, one would still become a safari. In ancient times, pious people who traveled to faraway countries in a second performed their salats by shortening them. It is written in fiqh books:

If there are two ways of going to a place, one of the ways being shorter than the other, the person who travels the shorter way does not become a safari. If the other way is longer than 104 km, a person who travels that way on any vehicle [by bus, by plane, or by karamah] becomes a safari. In the past, of course there were horses that run fast, but our religion based the ruling on the camel's walking. For example, it is written in the book Fatawa-i Hindiyya:

If one arrives at a place that is at a distance of three days' way [104 km] in two days or less than it with a fast running horse, one will still be a safari, so one must shorten four-rak'at fard salats to two rak'ats. The same is written in the book Jawhara. (15th part)

Hadrat Ibn Abidin states:
All scholars have described “a way of three days” by a unit called farsah (parasang), the distance traveled in one hour. Some of them said a way of three days was 21 farsahs; some said it was 18 farsahs; and others said it was 15 farsahs. The fatwa has been given according to those scholars who said it was 18 farsahs. (Radd-ul-Mukhtar)

One farsah is equal to about 6 km. It is exactly 5.8 km. 18 farsah is equal to 104 km approximately. In the madhhabs of Maliki, Shafi'i, and Hanbali, one who travels to another place that is above 80 km becomes a traveler. (Al-Fiqh 'ala al-Madhahib al-Arba'ah)

Question: The Prophet said that one who traveled more than 104 km from the place one was in became a traveler, for there were not buses or planes then. However, in today's conditions, this distance is very short. Instead of it, what matters must be time, not distance. How can it be correct to shorten salats for such a distance as 104 km? Does it not mean altering the religion in a way it will be easier?
ANSWER
Your statements mean:
"Allah did not know [hasha] that people would make buses, trains, planes, motorboats, or nuclear missiles. His Prophet solved only the problems that existed during his time in his religion, which will be valid until the end of the world."

This is a very ugly accusation. Our Master the Messenger of Allah stated rulings that were applicable in every century because Allahu ta'ala informed His Prophet about all necessary events that would take place until the end of the world.

In truth, the suggestion you have made based on your personal view means altering the religion. Our religion says that what matters is distance, not time or duration.

Ibadah means doing a thing ordered by Allah and His Messenger in the time, amount, and conditions prescribed by them. Increasing, reducing, or altering it in any way is to change the religion, which will be no longer ibadah.

Question: In safar distance, three days' way is practiced upon, which is equal to about 104 km. If a road that is below 104 km is very bumpy, and therefore, one reaches one's destination in 4 or 5 days, will one still become a safari?
ANSWER
No, one will not be a safari because what matters is distance. Similarly, if one reaches there in an hour by plane, what matters is still distance. One who travels to another place that is below 104 km will not be a safari no matter by which vehicle and by which way one travels. Even if it were possible to travel to another place that is above 104 km in a second, one would still be a safari there.

Question: I set out from the town of A, which is my watan al-asli, with the intention of traveling to the city of B in an easterly direction. I thought that the distance between them was no more than 90 km. For this reason, I performed Salat az-Zuhr without shortening on my way to it. I later learnt that the distance between them was more than 104 km. How many rak'ats should I offer for Salat al-Asr?
ANSWER
One who sets out with the intention of traveling to somewhere at a safar distance becomes a safari, even if one does not know the distance between two places. If one does not know that one has become a safari, then it will not be a sin to perform salats without shortening them. Suppose that one thinks that the distance between two places is less than 100 km. One has learnt later that it is, in fact, more than 104 km. In this case, one must consider oneself a traveler from then onward.

Question: I set out from the town of A, which is my watan al-asli, with the intention of traveling to the city of B at noon. I thought that the distance between them was more than the safar distance. Accordingly, I performed Salat az-Zuhr as two rak'ats on my way to it. I later learnt that the distance between them was not more than the safar distance. How many rak'ats should I offer for Salat al-Asr?
ANSWER
You did not incur a sin by performing Salat az-Zuhr as two rak'ats because you thought that you were a traveler. However, you must repeat it as four rak'ats if the time for it has not expired. If it has expired, then you must perform a qada (make up) salat. You must perform Salat al-Asr as four rak'ats.

Question: My watan al-asli is the city of C, and I reside in the city of B that is at a distance of 312 km in a westerly direction. I every day shuttle between the cities of B and D, which is at a distance of 230 km in a westerly direction. Am I considered a traveler when I am at home in the city of B and when I am in the city of D?
ANSWER
Yes, you are considered a traveler when you are on the way, when you are in the city of D, and when you are at home in the city of B.