At what point is a traveler permitted to commence shortening the prayers

The majority of the Muslim scholars assert the condition that a traveler must depart from all the housings and buildings of his/her home town relying on the fact that Allaah (glory be to Him) relates shortening to travel (specifically darb fil-ardh), and such travel only happens once departing one's home-town. Moreover, the Prophet Mohammed (peace be upon him) himself shortened at a place called (Dhul-Hulaiyfah) after departing from al-Madinah.