A person who is traveling in a car when the time to pray arrives, if he is driving, he is forbidden to recite the Amidah. This is because he cannot concentrate properly, and there is even concern that he may endanger his life or the lives of others. Therefore, he must pull his car over to the side and only then begin to pray.
However, if someone else is driving, who is in a hurry to arrive at his desired destination, a passenger may recite the Amidah while sitting. The reason for this is because if they stop traveling in order to pray standing up, he will be distracted with wanting to finish praying quickly, and he will be unable to have the proper kavanah. Therefore, it is better that he pray Shemoneh Esrei while sitting, since, as we already learned (in halachah 4), b’dieved, the one who recites the Amidah while sitting fulfills his obligation.
Even one who is sitting when reciting the Amidah must put his feet together (Mishnah Berurah 95:2), and try to turn his face in the direction of Jerusalem (Mishnah Berurah 94:15). At the places that the Chachamim instructed to bow down, he must straighten himself slightly and bow as much as he can (Shulchan Aruch 94:5; Aruch HaShulchan 18).
When traveling by bus or train, both of which are more spacious than a car, if one can stand and concentrate properly, it is better that he stand for Shemoneh Esrei. However, if standing will disturb his kavanah, he must sit with his feet together, stand when he needs to bow down, and then sit again. At the end of his prayer, he stands and takes three steps backwards (see Shulchan Aruch 94:5).
When one is traveling at night by bus, and he knows that when he finishes his trip he will still have time to recite Ma’ariv, he should pray after the trip ends (Mishnah Berurah 89:42). However, if he knows that after the trip he will be unable to concentrate properly, because he will be tired and he will want to finish his prayer quickly, he is permitted to pray while traveling (based on Shulchan Aruch HaRav 94:5).
Reciting the Amidah individually while sitting is only permitted on a temporary basis, during a special trip, or in a set of circumstances completely beyond one’s control. However, even if it is difficult for a person who travels to work every day to find time to pray while standing, as long as he is not in circumstances beyond his control, he is not permitted to be lenient and pray on a regular basis while sitting. Still, it is permitted to pray regularly on buses that are specifically arranged for people to pray in a minyan while standing, although, l’chatchilah, one should pray in a synagogue. Nonetheless, if those people are hurrying on their way, or rushing to work, and they know that in the synagogue they will pray quickly, but that on the bus they will pray in a more composed manner, it is better that they pray on the bus.
. If one has two options, to stand with his feet apart or to sit with his feet together, it is preferable to stand, for the obligation to stand is more important. The proof for this is that some poskim maintain that if a person who prayed while sitting has an opportunity afterwards to stand, he must repeat the Amidah while standing (Shulchan Aruch 94:9), whereas regarding a person who prays with his feet apart, no such thing is said. See Mishnah Berurah 95:1 and Sha’ar HaTzion 1.Similarly, it seems that it is preferable to pray while standing, even if not in the direction of Jerusalem, rather than to pray while sitting facing Jerusalem. Again, this is because we have not found an opinion stating that if he recites the Amidah towards a different direction he needs to repeat his prayer. Further, in Bava Batra 25 we learn that there are opinions which maintain that a person may pray facing south or north, and although the halachah is not like them, b’dieved, we can take them into consideration (see Beit Yosef and Shulchan Aruch 94:1-2 and the Rama’s words).
A soldier who is in the middle of a long hike of endurance, and is worried about finishing, may pray while walking, for if he stops to stand in his place, he will not be able to concentrate properly, as explained in Shulchan Aruch 94:4. However, if the case concerns a group of religious soldiers, they should stand in their places and pray, for as a group they are not as distracted. Additionally, as it is only training and not actual combat, there is no reason l’chatchilah to slight the prayer, and they must arrange their schedules in such a way that the army training exercises do not conflict with the times of prayer.