Praise be to Allah.
The majority of scholars are of the view that the distance at which a traveler may join prayers and not fast is forty-eight miles. Ibn Qudaamah said in al-Mughni:
The view of Abu ‘Abd-Allah [i.e., Imam Ahmad] is that it is not permissible to shorten the prayers for a distance of less than sixteen farsakhs, and a farsakh is three miles, so the distance is forty-eight miles. This was the estimation of Ibn ‘Abbaas. He said: From ‘Usfaan to Makkah, or from al-Taa’if to Makkah, or from Jeddah to Makkah.
Based on this, the distance at which it is permissible to shorten prayers is the distance of two days’ travel aiming directly for that destination. This is the view of Ibn ‘Abbaas and Ibn ‘Umar, and the view of Maalik, al-Layth and al-Shaafa’i.
The equivalent in kilometers is approximately 80 km.
Shaykh Ibn Baaz said in Majmoo’ al-Fatawa (12/267), explaining what is meant by traveling:
The view of the majority of scholars is that this is equivalent to approximately eighty kilometers for one who travels by car, plane or ship. This distance is what is called traveling according to the custom of the Muslims. So if a person travels by camel, car, plane or ship, for this distance or more, he is regarded as a traveler.
The Standing Committee was asked (8/90) about the distance at which a traveler may shorten his prayers, and can a taxi-driver who covers more than three hundred kilometers shorten his prayer?
The distance at which a traveler may shorten his prayers is approximately 80 km, according to the view of the majority of scholars. It is permissible for a taxi driver or anyone else to shorten his prayers, if he is going to cover the distance mentioned at the beginning of the question, or more.
Some scholars are of the view that traveling is not to be defined by a specific distance, rather it should be defined according to custom: whatever people customarily regard as traveling is the traveling to which the shar’i rulings apply, such as joining and shortening prayers, and not fasting.
Shaykh al-Islam said in al-Fatawa (24/106): The evidence supports those who regard shortening prayers and not fasting as being applicable to all types of travel and do not single out one kind of traveling to the exclusion of another. This view is the correct one.
Shaykh Ibn ‘Uthayemeen was asked in Fatawa Arkaan al-Islam (p. 381) about the distance at which a traveler may shorten his prayers and whether it is permissible to join prayers without shortening them.
The distance at which a traveler may shorten his prayers was defined by some of the scholars as being approximately eighty-three kilometers, and some defined it as being what is customarily regarded as traveling, even if the distance is not 80 km, and that what the people say is not traveling should not be regarded as such, even if it is as far as one hundred kilometers.
The latter view is the view favoured by Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him), because Allah did not state a specific distance for it to be permissible to shorten prayers, and neither did the Prophet (peace and blessings of Allah be upon him).
Anas ibn Maalik (may Allah be pleased with him) said: If the Messenger of Allah (peace and blessings of Allah be upon him) set out for a journey of three miles or three farsakhs, he would pray two rak’ahs. Narrated by Muslim, 691.
The view of Shaykh al-Islam Ibn Taymiyah is closer to what is correct.
There is nothing wrong, if there is a conflict between customary views, in going by the opinion which suggests that travel should be defined in terms of distance, because this was the view of some of the imams and scholars and mujtahids. So there is nothing wrong with that in sha Allah. But so long as custom gives a clear definition, then referring to what is customary is the right thing to do.