The prayers that are normally (when not travelling) 4 rakats, are performed as 2 rakats when on a journey. That means, performing the obligatory prayers as 2 rakats during a journey is not because it is shortened, but because it is obliged as 2 rakats.
It is clearly understood from the following verses that shortening the 2 rakats of obligatory prayer to 1 rakat pertains only to the case of fear:
“When you go forth journeying in the land, there is no blame on you if you shorten the Prayer when you fear that those who conceal (their faith) might attack you. Surely those who conceal are your manifest enemies.
If you are among the believers and rise (in the state of war) to offer the Prayer for them, let a party of them pray with you, keeping their arms. When they have performed their prostration, let them go around, and let another party who have not prayed, pray with you, remaining on guard and keeping their arms, for the concealers love to see you heedless of your arms and your baggage so that they might swoop upon you. But there shall be no blame upon you if you were to lay aside your arms if you are either troubled by rain or are sick; but remain on guard. Surely Allah has prepared a humiliating chastisement for those who conceal.” (An-Nisa/ The Women 4:101-102)
The object of the verb “aqame” (أقام) in Nisa 4:101 is the word “as-salat”. When this expression is used in Quran, it is understood that the prayer must be offered fully. The case when our Prophet (p.b.u.h.) offered the prayer as 2 rakats without shortening, and leading also the congregation is expressed as “فأقمت الهم الصلاة , when you offer the prayer for them”. For those who shorten the prayer and perform it as 1 rakat, the expression “فلتقم طائفة منهم معك let a party of them pray with you” is used on the contrary to our Prophet (p.b.u.h.) who offered the prayer fully as 2 rakats.
A narration from Abdullah Ibn Umar is as follows:
“I have been on journey with the Messenger of Allah (p.b.u.h.). He prayed no more than two rakats. And I have been with Abu Bakr, neither did he pray more than two rakats. I have alse been with ‘Umar and Usman; they also did the same.” (Ibn Majah, Iqamat as-Salat, 75; Nasai, Taqseer as-Salat Fi Safar, 5)
It was narrated that ‘Umar said:
“The prayer while traveling is two Rak’ah, and Friday is two Rak’ah, and ‘Eid is two Rak’ah. They are complete and are not shortened, as told by Muhammad” (Ibn Majah, Iqamat as-Salat, 73; Nasai, Vol. III, 111, 118, 183)
Our Prophet always used to offer the 4-rakat prayers as 2 rakats during all his journeys. He also enjoins: “Offer the prayer the way you see me do.” (Bukhari, Adhan, 18) Therefore, we must always offer the 4-rakat obligatory prayers as 2 rakats when we are on journey.
To summarize: all the prayers, except maghrib, are obliged as 2 rakats when we travel. This is not shortening. The prayers may only be shortened when we fear from enemy; and this is established by performing these 2-rakat obligatory prayers as 1-rakat.