The distance by which the prayers are shortened; some scholars specified it to be around 83 kilometers. Other scholars specified it to be whatever is customarily considered traveling, even if he (the traveler) did not go beyond 80 kilometers. And whatever the people consider not to be traveling, then it is not traveling even if it reached 100 kilometers.
This last (opinion) is the opinion of Shaykh al-Islaam Ibn Taymiyyah (ÑÍãå Çááå) and that is because Allaah did not define a specific distance by which shortening the prayers is allowed nor did the Prophet (Õáì Çááå Úáíå æÓáã) specify an exact distance.
Anas ibn Maalik (ÑÖí Çááå Úäå) said:
“The Prophet (Õáì Çááå Úáíå æÓáã); whenever he went out for 3 miles or Faraasikh (a measurement of length) then he used to shorten the prayer and pray two Rak’aats.” 
The statement of Shaykh al-Islaam Ibn Taymiyyah is closer to what is correct. There is also no hardship, whenever there is a difference in what is customarily considered traveling, if the person acts according to the statement of the specific, defined distance because some of the Imaams and scholars sincerely striving towards a verdict (Mujtahideen) have said it. So there is no problem, In shaa Allaah. As long as the issue is left undefined, then acting in accordance to what is customarily considered travel is the correct opinion.
As for (the question), “Is combining (the prayers) permissible when shortening is allowed?” Then we say, “Combining (the prayers) is not associated with shortening. Rather, combining is associated with a need.” So whenever the person needs to combine whether he is traveling or not, then let him combine. For this, the people combine because of rain, which due to it; it would be a hardship to return to the masaajid. Also, the people combine if there is harsh, cold wind during the days of winter, which due to it, it would be a hardship to go out to the masaajid. He combines if he fears losing his wealth or any harm in it, or whatever is similar to that - the person combines (his prayers). It is reported in Saheeh Muslim from ‘Abdullaah ibn ‘Abbaas (ÑÖì Çááå ÚäåãÇ) that he said:
“The Prophet (Õáì Çááå Úáíå æÓáã) combined between Dhuhr and ‘Asr, and between Maghrib and Ishaa not due to any fear nor rain.” 
They asked, “Why did he do this?” He said that he did so in order not to cause any hardship upon his Ummah. Meaning, he did not want to place any hardship that would make one leave the congregation.
This is the condition - whenever the person finds a hardship which would lead him to leave off the congregation, then it is permissible for him to combine. If there is no hardship upon him, then he doesn’t combine. However, traveling is most likely to place a hardship which would lead to leaving the congregation. If this is so, then it is permissible for the traveler to combine whether he was in the middle of his journey or has stopped to reside (at his destination). If he is in the middle of his journey, then combining is preferable and if he has stopped traveling (reached his destination) then not combining is preferable.
The exception to this is when a person is residing (reached a destination) in a land where the congregation is established, then it is obligatory to attend the congregation. In that case, he would not combine nor shorten (his prayers). However, if he missed the congregation, then he shortens and does not combine, unless he needs to combine (for some reason).
 Reported by Muslim, The Book of The Traveler’s Prayer, Chapter: The Prayer of the Traveler, no. 691.
 Reported by Muslim, The Book of The Traveler’s Prayer, Chapter: Combining Between Two Prayers While Residing, no. 705.