Gemara

(R. Elazar): One should not pray for three days after traveling (his mind is not settled) - "v'Nachaneh Sham Yomim Sheloshah va'Avinah va'Am" (only then he had Binah (understanding).

Shmuel's father would not pray for three days after traveling.

Berachos 54b (Rav Yehudah): Four categories must thank Hash-m [after being saved from danger]: one who was at sea, one who was in the Midbar (wilderness), a sick person who recovered, and one who was released from prison;

We learn one who was at sea from "Yordei ha'Yam ba'Aniyos... va'Yitz'aku El Hash-m ba'Tzar Lahem... Yodu la'Shem Chasdo..."

We learn one who was in the Midbar from "Ta'u ba'Midbar bi'Shimon... va'Yitz'aku El Hash-m... Yodu la'Shem Chasdo..."

Rishonim

Rosh and R. Yonah (Berachos 9:3 and 43a DH Arba'ah): In Ashkenaz and France, the custom is not to bless ha'Gomel when traveling from one city to another. They hold that it is obligatory only for those who traverse Midbaros, in which wild animals and bandits are common. The Yerushalmi says that all roads are established to be dangerous. This refers to Tefilas ha'Derech. One should request for himself whenever traveling.

Rosh: Ha'Gomel is in place of a Korban Todah.

Rambam (Hilchos Berachos 10:8): Four categories of people must thank Hash-m: a Choleh who recovered... and one who traveled, when he reaches a settled area.

Rashi (54b DH Tzerichin): The four must bless when they leave danger.

Poskim

Shulchan Aruch (OC 219:1): Four categories must thank Hash-m: one who was at sea, when he alights; one who was in the desert, when he reaches civilization...

Birkei Yosef (3): If one passes through Midbaros, and during his journey he comes to a big city, he does not bless until reaching his destination. Kol Yakov (3) says so. Mikra'ei Kodesh (53b) disagrees. He learns from Rashi says that one blesses when he leaves danger, i.e. each night, when he reaches a village. Really, there is no proof from Rashi. One can explain that Rashi means when he totally leaves danger, and not when he is partially saved.

Mishnah Berurah (1): This is when he is totally out of danger, but not if the boat comes to a port and the passengers go onto the land for a day or two until the boat leaves again. They do not bless, for they were not totally saved from the danger. The same applies to those who go through Midbaros, and pass through a city.

Kaf ha'Chayim (4): Even if a miracle occurred, he does not bless until the end of the trip. If he will delay in the port several days, this requires investigation. The Eliyahu Rabah and Birkei Yosef connote that he does not bless until the end.

Bi'ur Halachah (DH Yordei): If one went in a boat on a river, this depends on the custom of Sefard and Ashkenaz (Sa'if 7). According to the opinion that one blesses for Stam roads, one blesses also for big rivers. It is no worse than a road. However, the custom in Ashkenaz is not to bless for Stam roads. Perhaps the same applies to rivers. They are not as dangerous as seas. All agree that one who crossed a sea or Midbar blesses, even if he encountered no danger, e.g. there was no storm, he did not get lost, and he did not lack water. Even so, Chachamim enacted to bless. The verse discusses one who got lost in the Midbar or there was a storm at sea, but this does not mean that it occurred to him. Rather, such things are prone to happen, so one who was saved from them must thank.

Shulchan Aruch (6): If one delayed, he may bless whenever he wants. It is proper not to delay three days.

Beis Yosef (DH v'Im): The Tur holds like this. Orchos Chayim brings from the Ramban that one may bless only until three days. I found that we learn from Eruvin that until three days is called 'coming from the road.' The Rashba received from R. Yonah that one may bless until five days.

Magen Avraham (6): If one left (i.e. arrived - PF) on Monday, he should bless in front of 10 without a Sefer Torah, and not wait until Thursday.

Eliyahu Rabah (12): Our custom is to pray within three days of coming from the road, for we do not have such great Kavanah in Tefilah. However, even so within three days is called 'coming from the road.'

Mishnah Berurah (8): It is only l'Chatchilah to bless in front of 10. If one cannot bless in front of 10, he blesses not in front of 10 l'Chatchilah. The Ro'oh says that he should wait up to 30 days, for perhaps he will happen to find 10 and be able to do the Mitzvah properly. He should not wait longer.

Kaf ha'Chayim (37): Tzedah l'Derech says that since the Berachah is in place of a Korban Todah, which one may bring whenever he wants, he may bring it at any time. It is proper to read the Parshah of Todah (Vayikra 7:12-15).

Kaf ha'Chayim (38): The custom is that there is always compensation. We do not say Safek Berachos Lehakel against a custom.

Shulchan Aruch (7): In Ashkenaz and France they do not bless for going from city to city. Chachamim obligated a Berachah only for going through a Midbar, in which wild animals and bandits are common. In Sefard the custom is to bless, for all roads are assumed to be dangerous.

Beis Yosef (DH u'Mah): The Tur says that the Rambam says that one must bless for every journey or sickness. The Rambam did not say so explicitly. However, he changed from the Gemara's words 'those who traverse Midbaros', and wrote 'those who travel on the road.' This connotes that all travelers must bless. The Tur derived that any Choleh who was healed must bless. Perhaps the text in the Tur should say Ramban, for the Ramban says in Toras ha'Adam that anyone who was bedridden and recovered must thank, for this is like one who was tried for a capital case. Also the Rashba says so.

Beis Yosef (DH u'Mah she'Chasav Rabeinu d'Garsinan): Why does the Tur say that the Yerushalmi says that all roads are established to be dangerous? He brought this above, and that Tosfos said that this is only regarding Tefilas ha'Derech! Some texts of the Tur bring from the Yerushalmi that all sicknesses are established to be dangerous. This supports the Rambam. Or, perhaps he learns from sicknesses that similarly, 'all roads are established to be dangerous' refers to ha'Gomel, since the Yerushalmi used the same wording for them.

Bach (4): The end of the Rambam's words 'when he reaches a settled area' prove that he discusses one who traveled through an unsettled area. There is no source to say that he obligates one who went from city to city through a settled area, or for every sickness. The Rambam wrote Stam, without bringing a proof for his words. It is difficult to say that the Tur brings a proof for him from the Yerushalmi, since he brought that Tosfos explains that it discusses Tefilas ha'Derech! Rather, he cites the Ramban, who learns from the Yerushalmi that every Choleh must bless. The scribe omitted 'all roads are b'Chezkas Sakanah.' We hold like this, unlike the Rema (in Sa'if 8, who obligates only one who was dangerously sick). 'All Cholim are b'Chezkas Sakanah', i.e. those who have dangerous sicknesses.

Shulchan Aruch (ibid.): However, one does not bless for less than a Parsah (about four kilometers). If it is established to be an overly dangerous place, one blesses for even less than a Parsah.

Beis Yosef (DH Kasav ha'Rav): The Avudraham and Ramban say that the custom is to bless even for going from one city to another. Bahag says that Tefilas ha'Derech is only for a trip at least a Parsah. This shows that this is the Shi'ur of 'on the road.' The Ramban say that if it is established to be an overly dangerous place, one blesses for even less than a Parsah. Orchos Chayim says that this is not the custom. The Halachah follows Bahag.

Taz (4): All the more so one must say Tefilas ha'Derech for a Parsah. It is Tefilah, which is needed more. Ha'Gomel is in place of a Korban Todah.

Mishnah Berurah (22): He equates this to Tefilas ha'Derech.

Teshuvos v'Hanhagos (3:81): One who flew in a plane may say 'Baruch Rachmana Malka d'Alma ha'Gomel...', for there is no concern for a Brachah Levatalah when saying a Brachah of thanks (with Shem and Malchus) in Arame'ic.