The Kitáb-i-Aqdas - The Most Holy Book
Index term: during fast, under Travel
k16.
O Pen of the Most High! Say: O people of the world! We have enjoined upon you fasting during a brief period, and at its close have designated for you Naw-Rúz as a feast. Thus hath the Day-Star of Utterance shone forth above the horizon of the Book as decreed by Him Who is the Lord of the beginning and the end. Let the days in excess of the months be placed before the month of fasting. We have ordained that these, amid all nights and days, shall be the manifestations of the letter Há, and thus they have not been bounded by the limits of the year and its months. It behoveth the people of Bahá, throughout these days, to provide good cheer for themselves, their kindred and, beyond them, the poor and needy, and with joy and exultation to hail and glorify their Lord, to sing His praise and magnify His Name; and when they end -- these days of giving that precede the season of restraint -- let them enter upon the Fast. Thus hath it been ordained by Him Who is the Lord of all mankind. The traveller, the ailing, those who are with child or giving suck, are not bound by the Fast; they have been exempted by God as a token of His grace. He, verily, is the Almighty, the Most Generous.
n30.
The traveller . . . not bound by the Fast
The minimum duration of a journey which exempts the believer from fasting is defined by Bahá'u'lláh (Q&A 22 and 75). The details of this provision are summarized in the Synopsis and Codification, section IV.B.5.a.i.-v.
Shoghi Effendi has clarified that while travellers are exempt from fasting, they are free to fast if they so wish. He also indicated that the exemption applies during the whole period of one's travel, not just the hours one is in a train or car, etc.
q22.
 
Question: Concerning the definition of a journey. [This relates to the minimum duration of a journey which exempts the traveller from fasting]
Answer: The definition of a journey is nine hours by the clock. Should the traveller stop in a place, anticipating that he will stay there for no less than one month by the Bayán reckoning, it is incumbent on him to keep the Fast; but if for less than one month, he is exempt from fasting. If he arriveth during the Fast at a place where he is to stay one month according to the Bayán, he should not observe the Fast till three days have elapsed, thereafter keeping it throughout the remainder of its course; but if he come to his home, where he hath heretofore been permanently resident, he must commence his fast upon the first day after his arrival.
q75.
 
Question: Concerning the limit of fasting for someone travelling on foot.
Answer: The limit is set at two hours. If this is exceeded, it is permissible to break the Fast.